4.11 - Answers and Explanations

Module: Refrigeration Cycle Components Gauges and Pressure-Temperature Relationships
Covers: All concept-check questions in Sections 4.1–4.8 and all module practice questions in Section 4.10
Total questions answered: 99
Technical verification basis: Module 4 content current through August 9, 2026
Purpose: Provide the correct answer, explain the governing technical concept, address every distractor, show fully checked calculations where needed, and direct the learner to the appropriate remediation section

How to Use This File

1. Complete the relevant concept checks or the full Module 4 practice set before reading these answers.
2. Score the attempt using the answer key below.
3. Review every incorrect response.
4. Review every response marked C0 or C1, even when the selected answer was correct.
5. Enter unresolved items in the project error log.
6. For calculation errors, identify whether the error involved the equation, pressure reference, unit conversion, P-T lookup, bubble/dew selection, or subtraction direction.
7. Explain the corrected concept without looking at this file.
8. Retest the concept later using a different question.

Mastery rule: A correct guess remains unresolved until the learner can explain why the correct answer is right and why every distractor is wrong.

Complete Module 4 Answer Key

• Section 4.1: 4.1-1: B | 4.1-2: A | 4.1-3: D | 4.1-4: C | 4.1-5: B | 4.1-6: A | 4.1-7: D | 4.1-8: C
• Section 4.2: 4.2-1: B | 4.2-2: C | 4.2-3: D | 4.2-4: A | 4.2-5: C | 4.2-6: B | 4.2-7: A | 4.2-8: D
• Section 4.3: 4.3-1: B | 4.3-2: C | 4.3-3: A | 4.3-4: D | 4.3-5: B | 4.3-6: C | 4.3-7: A | 4.3-8: D
• Section 4.4: 4.4-1: C | 4.4-2: A | 4.4-3: B | 4.4-4: D | 4.4-5: B | 4.4-6: C | 4.4-7: B | 4.4-8: D
• Section 4.5: 4.5-1: B | 4.5-2: C | 4.5-3: A | 4.5-4: D | 4.5-5: A | 4.5-6: C | 4.5-7: B | 4.5-8: D
• Section 4.6: 4.6-1: C | 4.6-2: D | 4.6-3: B | 4.6-4: A | 4.6-5: C | 4.6-6: D | 4.6-7: A | 4.6-8: B
• Section 4.7: 4.7-1: B | 4.7-2: A | 4.7-3: C | 4.7-4: D | 4.7-5: A | 4.7-6: C | 4.7-7: B | 4.7-8: C
• Section 4.8: 4.8-1: B | 4.8-2: A | 4.8-3: C | 4.8-4: B | 4.8-5: D | 4.8-6: A | 4.8-7: C | 4.8-8: A
• Section 4.10: 4.10-1: B | 4.10-2: C | 4.10-3: A | 4.10-4: D | 4.10-5: B | 4.10-6: C | 4.10-7: A | 4.10-8: B | 4.10-9: D | 4.10-10: C | 4.10-11: B | 4.10-12: A | 4.10-13: D | 4.10-14: B | 4.10-15: C | 4.10-16: A | 4.10-17: D | 4.10-18: B | 4.10-19: C | 4.10-20: A | 4.10-21: D | 4.10-22: B | 4.10-23: C | 4.10-24: A | 4.10-25: B | 4.10-26: D | 4.10-27: A | 4.10-28: C | 4.10-29: B | 4.10-30: D | 4.10-31: A | 4.10-32: C | 4.10-33: B | 4.10-34: D | 4.10-35: A

Answer-File Organization

• Part A: Section 4.1 concept-check answers
• Part B: Section 4.2 concept-check answers
• Part C: Section 4.3 concept-check answers
• Part D: Section 4.4 concept-check answers
• Part E: Section 4.5 concept-check answers
• Part F: Section 4.6 concept-check answers
• Part G: Section 4.7 concept-check answers
• Part H: Section 4.8 concept-check answers
• Part I: Section 4.10 module practice-question answers

Part A - Section 4.1 Concept-Check Answers

Source section: Section 4.1 - Vapor-Compression Refrigeration Cycle

Answer key: B A D C B A D C

Question 4.1-1

Correct answer: B. Compressor → Condenser → Metering Device → Evaporator → Compressor

The four major components are traversed in the order compressor → condenser → metering device → evaporator → compressor. This sequence follows the refrigerant from compression, through heat rejection, through the major pressure drop, and then through heat absorption.

• A is incorrect: This sends refrigerant from the compressor directly to the evaporator and reverses the normal locations of the condenser and metering device.

• C is incorrect: The metering device is downstream of the condenser, not immediately downstream of the compressor.

• D is incorrect: The evaporator is downstream of the metering device, not directly downstream of the condenser.

Topic: Cycle sequence

Suggested error code: PRO

Review: Section 4.1 - Vapor-Compression Refrigeration Cycle

Question 4.1-2

Correct answer: A. Raise the pressure of refrigerant vapor and provide the major work input to the cycle

The compressor receives work input, draws in low-pressure vapor, and raises the refrigerant pressure so that heat can later be rejected on the high side.

• B is incorrect: The metering device, not the compressor, creates the major pressure reduction before the evaporator.

• C is incorrect: Heat rejection to air or water is the condenser’s primary heat-transfer role.

• D is incorrect: Heat absorption from the cooled space or process is the evaporator’s role.

Topic: Compressor function

Suggested error code: DEF

Review: Section 4.1 - Vapor-Compression Refrigeration Cycle

Question 4.1-3

Correct answer: D. Reject heat from the refrigerant to a cooler external medium

The condenser rejects heat from the refrigerant to an external sink such as outdoor air or cooling water. As heat is rejected, refrigerant moves from vapor toward liquid.

• A is incorrect: Heat absorption from the conditioned space occurs in the evaporator.

• B is incorrect: The major high-to-low pressure reduction occurs across the metering device.

• C is incorrect: Mechanical work is supplied to the compressor, not the condenser.

Topic: Condenser function

Suggested error code: DEF

Review: Section 4.1 - Vapor-Compression Refrigeration Cycle

Question 4.1-4

Correct answer: C. Metering device

The metering device produces the major pressure drop between the high-pressure liquid side and the low-pressure evaporator side while controlling refrigerant flow.

• A is incorrect: The compressor raises pressure rather than creating the high-to-low pressure drop.

• B is incorrect: The condenser primarily rejects heat; it is not the main pressure-reducing device.

• D is incorrect: The evaporator absorbs heat after the refrigerant has already been reduced to low pressure.

Topic: Metering-device function

Suggested error code: DEF

Review: Section 4.1 - Vapor-Compression Refrigeration Cycle

Question 4.1-5

Correct answer: B. Absorb heat from the space, product, or process being cooled

The evaporator absorbs heat from the space, product, or process being cooled. That heat drives refrigerant evaporation on the low-pressure side.

• A is incorrect: Heat rejection to the surroundings is the condenser’s function.

• C is incorrect: Pressure is raised by the compressor.

• D is incorrect: Liquid storage, when used, is associated with a receiver rather than the evaporator.

Topic: Evaporator function

Suggested error code: DEF

Review: Section 4.1 - Vapor-Compression Refrigeration Cycle

Question 4.1-6

Correct answer: A. Condenser heat rejection equals evaporator heat absorption plus compressor work input.

For the simplified cycle, conservation of energy gives condenser heat rejection equal to evaporator heat absorption plus compressor work input: $Q_H=Q_L+W_{\text{in}}$.

Relationship:

$$Q_H=Q_L+W_{\text{in}}$$

The condenser must reject both the heat absorbed in the evaporator and the compressor work supplied to the cycle.

• B is incorrect: This omits the heat absorbed in the evaporator, which must also be rejected at the condenser.

• C is incorrect: With positive compressor work input, condenser heat rejection is greater than evaporator heat absorption in the simplified cycle.

• D is incorrect: Condensation requires heat rejection; condenser heat rejection is not zero during normal operation.

Topic: Cycle energy balance

Suggested error code: CAL

Review: Section 4.1 - Vapor-Compression Refrigeration Cycle

Question 4.1-7

Correct answer: D. Low-pressure vapor

A conventional vapor-compression compressor should normally receive low-pressure vapor. Liquid entering the compressor can cause liquid floodback or slugging and can dilute lubricant.

• A is incorrect: High-pressure liquid belongs on the high-side liquid circuit, not at the compressor suction.

• B is incorrect: Liquid-only refrigerant is not the normal desired compressor inlet condition.

• C is incorrect: A high-pressure liquid-vapor mixture is inconsistent with the normal low-pressure suction condition and also introduces liquid to the compressor.

Topic: Compressor inlet state

Suggested error code: PRO

Review: Section 4.1 - Vapor-Compression Refrigeration Cycle

Question 4.1-8

Correct answer: C. It uses work input to move heat from a lower-temperature region to a higher-temperature region.

A refrigeration system uses work input to transfer heat from a lower-temperature region to a higher-temperature region. It moves heat; it does not manufacture or destroy heat.

• A is incorrect: Cold is not a form of energy manufactured by the evaporator; the evaporator absorbs heat.

• B is incorrect: The metering device lowers pressure but does not destroy heat.

• D is incorrect: Absorbed heat is not converted entirely into work; the condenser rejects the absorbed heat plus compressor work.

Topic: Purpose of refrigeration cycle

Suggested error code: DEF

Review: Section 4.1 - Vapor-Compression Refrigeration Cycle

Part B - Section 4.2 Concept-Check Answers

Source section: Section 4.2 - Refrigerant State Through the Cycle

Answer key: B C D A C B A D

Question 4.2-1

Correct answer: B. Low-pressure vapor

In the simplified cycle, low-pressure vapor leaves the evaporator and enters the compressor. Real systems often provide some superheat at this point.

• A is incorrect: High-pressure liquid belongs on the condenser/liquid-line side of the cycle.

• C is incorrect: Liquid-only refrigerant is not the desired compressor suction condition.

• D is incorrect: A high-pressure two-phase mixture is inconsistent with the low-pressure suction side and would introduce liquid to the compressor.

Topic: Compressor inlet refrigerant state

Suggested error code: DEF

Review: Section 4.2 - Refrigerant State Through the Cycle

Question 4.2-2

Correct answer: C. High-pressure high-temperature vapor

Compression raises both pressure and, in normal operation, vapor temperature. The compressor therefore discharges high-pressure, high-temperature vapor.

• A is incorrect: Compression does not produce low-pressure saturated liquid.

• B is incorrect: The compressor inlet is low pressure; the outlet is high pressure and normally vapor.

• D is incorrect: Condensation occurs in the condenser after compressor discharge, so liquid is not the normal immediate compressor outlet state.

Topic: Compressor outlet refrigerant state

Suggested error code: DEF

Review: Section 4.2 - Refrigerant State Through the Cycle

Question 4.2-3

Correct answer: D. It rejects heat and changes from vapor toward liquid.

In the condenser, high-pressure refrigerant rejects heat and changes from vapor toward liquid. It may enter superheated, pass through condensation, and leave as saturated or subcooled liquid depending on design and conditions.

• A is incorrect: Absorbing heat while moving from liquid toward vapor describes the evaporator.

• B is incorrect: The condenser does not compress refrigerant; pressure is raised by the compressor.

• C is incorrect: Normal condenser operation includes condensation, so refrigerant does not remain entirely superheated vapor from inlet to outlet.

Topic: Condenser state change

Suggested error code: PRO

Review: Section 4.2 - Refrigerant State Through the Cycle

Question 4.2-4

Correct answer: A. High-pressure liquid

The simplified condenser outlet state is high-pressure liquid. In real operation the liquid may also be subcooled.

• B is incorrect: Low-pressure vapor belongs near the evaporator outlet and compressor inlet.

• C is incorrect: A low-pressure liquid-vapor mixture is typical after the metering device.

• D is incorrect: The condenser is intended to condense vapor toward liquid, so vapor-only is not the simplified outlet state.

Topic: Condenser outlet refrigerant state

Suggested error code: DEF

Review: Section 4.2 - Refrigerant State Through the Cycle

Question 4.2-5

Correct answer: C. Low-pressure liquid-vapor mixture

The metering device drops the refrigerant pressure. Part of the liquid flashes, so a low-pressure liquid-vapor mixture normally enters the evaporator.

• A is incorrect: The metering device reduces pressure rather than producing high-pressure superheated vapor.

• B is incorrect: A pressure drop causes some flashing, so high-pressure liquid-only is not the normal downstream state.

• D is incorrect: The outlet is normally a two-phase mixture rather than vapor only.

Topic: Metering-device outlet refrigerant state

Suggested error code: PRO

Review: Section 4.2 - Refrigerant State Through the Cycle

Question 4.2-6

Correct answer: B. The large pressure reduction causes part of the liquid refrigerant to vaporize.

The large pressure reduction through the metering device lowers the saturation temperature and causes part of the liquid refrigerant to flash into vapor. The energy for this flashing comes from the refrigerant itself.

• A is incorrect: The compressor is not adding heat directly to the metering device.

• C is incorrect: The condenser does not normally drive the refrigerant above the critical point as the explanation for metering-device flash gas.

• D is incorrect: The evaporator is downstream of the metering device and does not force all refrigerant to condense before entry.

Topic: Flash gas formation

Suggested error code: PRO

Review: Section 4.2 - Refrigerant State Through the Cycle

Question 4.2-7

Correct answer: A. A low-pressure liquid-vapor mixture absorbs heat and changes toward vapor.

The evaporator receives a low-pressure liquid-vapor mixture. As it absorbs heat, remaining liquid evaporates and the refrigerant moves toward saturated and then possibly superheated vapor.

• B is incorrect: High-pressure vapor rejecting heat toward liquid describes the condenser.

• C is incorrect: Compression from low-pressure vapor to high-pressure vapor occurs in the compressor.

• D is incorrect: The compressor does not subcool high-pressure liquid.

Topic: Evaporator state change

Suggested error code: PRO

Review: Section 4.2 - Refrigerant State Through the Cycle

Question 4.2-8

Correct answer: D. Superheated vapor can occur near the evaporator outlet and compressor inlet, while subcooled liquid can occur near the condenser outlet and liquid line.

Superheat applies to vapor above its saturation temperature, commonly near the evaporator outlet and suction line. Subcooling applies to liquid below its saturation temperature, commonly near the condenser outlet and liquid line.

• A is incorrect: The phase assignments are reversed: superheat is a vapor condition and subcooling is a liquid condition.

• B is incorrect: Required superheat and subcooling values are system-specific, not universal constants.

• C is incorrect: Subcooling describes liquid below the saturated-liquid temperature, not vapor above saturation.

Topic: Superheat and subcooling locations

Suggested error code: DEF

Review: Section 4.2 - Refrigerant State Through the Cycle

Part C - Section 4.3 Concept-Check Answers

Source section: Section 4.3 - High Side Low Side and Refrigerant Lines

Answer key: B C A D B C A D

Question 4.3-1

Correct answer: B. Compressor outlet through condenser to metering-device inlet

The high side begins at the compressor outlet and continues through the condenser and liquid line to the metering-device inlet.

• A is incorrect: The metering-device outlet through the evaporator to the compressor inlet is the low side.

• C is incorrect: The evaporator outlet and suction line are part of the low side.

• D is incorrect: The compressor inlet is on the low side; the compressor itself is a boundary between low-side inlet and high-side outlet.

Topic: High-side boundary

Suggested error code: CLS

Review: Section 4.3 - High Side Low Side and Refrigerant Lines

Question 4.3-2

Correct answer: C. Discharge line

The discharge line carries high-pressure, high-temperature vapor from the compressor outlet to the condenser inlet.

• A is incorrect: The liquid line carries high-pressure liquid from the condenser toward the metering device.

• B is incorrect: The suction line carries low-pressure vapor from the evaporator toward the compressor.

• D is incorrect: An evaporator feed line is downstream of the metering device and is not the compressor-to-condenser vapor line.

Topic: Discharge line

Suggested error code: DEF

Review: Section 4.3 - High Side Low Side and Refrigerant Lines

Question 4.3-3

Correct answer: A. Liquid line

The liquid line carries high-pressure liquid from the condenser toward the metering device.

• B is incorrect: The suction line is a low-pressure vapor line from evaporator to compressor.

• C is incorrect: The discharge line carries high-pressure vapor from compressor to condenser.

• D is incorrect: A low-side access line or port is not the main condenser-to-metering-device liquid path.

Topic: Liquid line

Suggested error code: DEF

Review: Section 4.3 - High Side Low Side and Refrigerant Lines

Question 4.3-4

Correct answer: D. It carries low-pressure vapor from the evaporator toward the compressor.

The suction line connects the evaporator outlet to the compressor inlet and normally carries low-pressure vapor.

• A is incorrect: High-pressure liquid travels through the liquid line from condenser toward the metering device.

• B is incorrect: High-pressure vapor leaves the compressor through the discharge line.

• C is incorrect: The low-pressure liquid-vapor mixture normally travels from the metering device into and through the evaporator, not through the suction line as the normal outlet condition.

Topic: Suction line

Suggested error code: DEF

Review: Section 4.3 - High Side Low Side and Refrigerant Lines

Question 4.3-5

Correct answer: B. Across the metering device

The metering device is the major high-side-to-low-side pressure boundary. Refrigerant enters it at high pressure and leaves at much lower pressure.

• A is incorrect: The condenser mainly rejects heat on the high side and does not create the major system pressure drop.

• C is incorrect: The evaporator operates on the low side after the major pressure drop.

• D is incorrect: The suction line is already on the low-pressure side.

Topic: High-to-low pressure transition

Suggested error code: PRO

Review: Section 4.3 - High Side Low Side and Refrigerant Lines

Question 4.3-6

Correct answer: C. Access-point locations vary with equipment design, so the technician must know where the port connects before interpreting its pressure.

Service-port locations vary by equipment design. A technician must know where a port connects in the refrigerant circuit before interpreting the pressure reading.

• A is incorrect: Appliances do not all use exactly the same number or location of service ports.

• B is incorrect: A high-side access point may be located elsewhere in the high-side circuit; it is not universally at the compressor discharge valve.

• D is incorrect: The low side can operate above atmospheric pressure, so a low-side port does not always indicate vacuum.

Topic: Access-point interpretation

Suggested error code: PRO

Review: Section 4.3 - High Side Low Side and Refrigerant Lines

Question 4.3-7

Correct answer: A. Trace its connection from the condenser toward the metering device and confirm the operating state rather than relying on diameter alone.

Line identification should be based on circuit location and operating state. Tracing a line from the condenser toward the metering device is a reliable way to identify the liquid line.

• B is incorrect: Tube diameter alone is not a universal identifier across equipment designs.

• C is incorrect: Touch temperature alone is unreliable and can vary with operating conditions and insulation.

• D is incorrect: Ignoring the system diagram and relying only on diameter discards the most useful functional information.

Topic: Refrigerant-line identification

Suggested error code: PRO

Review: Section 4.3 - High Side Low Side and Refrigerant Lines

Question 4.3-8

Correct answer: D. High side and low side describe regions within a refrigeration cycle, while EPA pressure classifications describe categories of appliances or refrigerants.

High side and low side identify pressure regions within a vapor-compression cycle. EPA appliance pressure classifications are separate equipment/refrigerant categories and should not be treated as the same terminology.

• A is incorrect: A high-pressure appliance still has both high- and low-side regions within its refrigeration cycle.

• B is incorrect: A low-pressure appliance can still contain different pressure regions; appliance classification is not identical to cycle-side terminology.

• C is incorrect: The two terms serve different classification purposes and are not interchangeable.

Topic: Cycle pressure regions versus appliance classification

Suggested error code: CLS

Review: Section 4.3 - High Side Low Side and Refrigerant Lines

Part D - Section 4.4 Concept-Check Answers

Source section: Section 4.4 - Receivers Accumulators and Filter-Driers

Answer key: C A B D B C B D

Question 4.4-1

Correct answer: C. After the condenser and before the metering device on the high-pressure liquid side

A liquid receiver is normally located downstream of the condenser and upstream of the metering device on the high-pressure liquid side.

• A is incorrect: The evaporator-to-compressor suction line is the normal accumulator region, not the receiver location.

• B is incorrect: The discharge line before the condenser carries high-pressure vapor, not stored condensed liquid.

• D is incorrect: Downstream of the metering device is the low-pressure evaporator feed region.

Topic: Receiver location

Suggested error code: CLS

Review: Section 4.4 - Receivers Accumulators and Filter-Driers

Question 4.4-2

Correct answer: A. Store and manage high-pressure liquid refrigerant for the downstream liquid circuit

A receiver stores and manages high-pressure liquid refrigerant and helps provide liquid to the downstream liquid circuit and metering device.

• B is incorrect: Compression of low-pressure vapor is the compressor’s job.

• C is incorrect: Separating excess liquid from suction vapor before the compressor is the accumulator’s job.

• D is incorrect: A receiver is not an air/noncondensable-removal device.

Topic: Receiver function

Suggested error code: DEF

Review: Section 4.4 - Receivers Accumulators and Filter-Driers

Question 4.4-3

Correct answer: B. Between the evaporator outlet and compressor inlet

A suction accumulator is normally installed between the evaporator outlet and compressor inlet on the low-pressure suction side.

• A is incorrect: The condenser-to-receiver region is high-side liquid service, not suction protection.

• C is incorrect: The compressor discharge-to-condenser region is high-pressure vapor.

• D is incorrect: Immediately upstream of the metering device is the high-pressure liquid line.

Topic: Accumulator location

Suggested error code: CLS

Review: Section 4.4 - Receivers Accumulators and Filter-Driers

Question 4.4-4

Correct answer: D. To separate excess liquid from suction vapor and help protect the compressor from liquid return

The accumulator separates excess liquid from suction vapor and meters or returns retained liquid gradually, helping prevent uncontrolled liquid return to the compressor.

• A is incorrect: The accumulator is not installed to raise condenser pressure.

• B is incorrect: Metering high-pressure liquid into the evaporator is the metering device’s function.

• C is incorrect: Moisture removal is a filter-drier function; an accumulator is not a system dehydrator.

Topic: Accumulator function

Suggested error code: DEF

Review: Section 4.4 - Receivers Accumulators and Filter-Driers

Question 4.4-5

Correct answer: B. Filter solid contamination and remove moisture

A filter-drier combines filtration of solid contamination with a desiccant function that removes a limited amount of moisture from the refrigerant circuit.

• A is incorrect: A filter-drier is not a compressor or pressure-raising device.

• C is incorrect: Liquid storage and flow metering are receiver and metering-device functions.

• D is incorrect: Suction-liquid separation and vapor compression are functions of the accumulator and compressor, not the filter-drier.

Topic: Filter-drier function

Suggested error code: DEF

Review: Section 4.4 - Receivers Accumulators and Filter-Driers

Question 4.4-6

Correct answer: C. It can indicate moisture condition, but the manufacturer’s legend must be used and the indicator itself does not dry the system.

A moisture-indicating sight glass can show moisture condition according to its specific indicator legend, but the indicator itself does not remove moisture. Manufacturer color interpretation must be followed.

• A is incorrect: The indicator senses or displays moisture condition; the filter-drier is the component intended to remove moisture.

• B is incorrect: Indicator colors are not universal across all manufacturers and refrigerants.

• D is incorrect: A moisture indicator does not remove air or nitrogen from the refrigeration system.

Topic: Moisture indicator

Suggested error code: PRO

Review: Section 4.4 - Receivers Accumulators and Filter-Driers

Question 4.4-7

Correct answer: B. The bubbles require further diagnosis because several operating conditions can produce flashing or bubbles.

Bubbles in a sight glass are an observation, not a complete diagnosis. Flashing or bubbles can result from several conditions, so other pressures, temperatures, load conditions, and system design must be evaluated.

• A is incorrect: Bubbles do not by themselves prove undercharge, so adding refrigerant immediately is unsupported.

• C is incorrect: A liquid-line sight-glass observation does not prove liquid is entering the compressor.

• D is incorrect: Bubbles do not specifically prove that the filter-drier is saturated with moisture.

Topic: Sight-glass interpretation

Suggested error code: DIST

Review: Section 4.4 - Receivers Accumulators and Filter-Driers

Question 4.4-8

Correct answer: D. The receiver is normally a high-side liquid-storage component, while the accumulator is normally a low-side compressor-protection component.

The receiver is normally a high-side liquid-storage/management component. The accumulator is normally a low-side suction component that helps protect the compressor from excess liquid return.

• A is incorrect: They are installed in different regions and serve different purposes.

• B is incorrect: This reverses the normal functions and locations of the two components.

• C is incorrect: Neither component is the primary refrigerant metering device.

Topic: Receiver versus accumulator

Suggested error code: CLS

Review: Section 4.4 - Receivers Accumulators and Filter-Driers

Part E - Section 4.5 Concept-Check Answers

Source section: Section 4.5 - Manifold Gauge Set and Service Hoses

Answer key: B C A D A C B D

Question 4.5-1

Correct answer: B. Measure low-side positive pressure and indicate vacuum below atmospheric pressure

The compound gauge is the low-side gauge. It reads positive low-side pressure and also extends below atmospheric pressure to provide a rough vacuum indication.

• A is incorrect: High-side discharge pressure is read with the high-pressure gauge.

• C is incorrect: Refrigerant mass is measured with a scale, not the compound gauge.

• D is incorrect: Deep vacuum should be verified with a micron gauge; an ordinary compound gauge lacks adequate resolution.

Topic: Compound gauge function

Suggested error code: DEF

Review: Section 4.5 - Manifold Gauge Set and Service Hoses

Question 4.5-2

Correct answer: C. The gauges can normally read their connected side pressures with both manifold valves closed.

With hoses connected, each gauge normally has a direct pressure path to its respective system side. Closing both manifold valves blocks flow through the manifold toward the center port but does not prevent the gauges from reading connected-side pressure.

• A is incorrect: Opening both valves can create unwanted flow paths and is not required merely to read pressures.

• B is incorrect: The high-side valve does not need to be opened just to read high-side pressure.

• D is incorrect: A vacuum pump connection is unrelated to ordinary operating-pressure readings.

Topic: Reading manifold gauges

Suggested error code: PRO

Review: Section 4.5 - Manifold Gauge Set and Service Hoses

Question 4.5-3

Correct answer: A. Provide a common connection for service equipment such as a refrigerant source, recovery machine, or vacuum pump

The center service port is the common service connection used for equipment such as a refrigerant source, recovery machine, or vacuum pump, depending on the task.

• B is incorrect: Compressor discharge temperature is measured with temperature instrumentation, not the center manifold port.

• C is incorrect: The center port does not permanently connect the high and low sides; manifold valves control the flow paths.

• D is incorrect: The center port is part of the manifold and does not replace the appliance’s low-side access fitting.

Topic: Center service port

Suggested error code: DEF

Review: Section 4.5 - Manifold Gauge Set and Service Hoses

Question 4.5-4

Correct answer: D. To the recovery-machine inlet, with the recovery-machine outlet connected to the recovery cylinder

In a common manifold-based recovery arrangement, the manifold center hose connects to the recovery-machine inlet, and the recovery-machine outlet connects to an appropriate recovery cylinder.

• A is incorrect: Directly routing system refrigerant to a recovery cylinder without a recovery machine is not the described manifold recovery arrangement.

• B is incorrect: The center hose is not restricted to the compressor discharge line.

• C is incorrect: A recovery machine is not connected through a vacuum-pump exhaust.

Topic: Recovery connection

Suggested error code: PRO

Review: Section 4.5 - Manifold Gauge Set and Service Hoses

Question 4.5-5

Correct answer: A. Air can introduce noncondensables and moisture into the system.

Air introduced through a charging hose adds noncondensables and may also introduce moisture. The hose should be prepared/purged according to the applicable procedure while minimizing refrigerant release.

• B is incorrect: Air does not convert every refrigerant into an azeotrope.

• C is incorrect: Noncondensable air tends to create undesirable pressure/heat-transfer effects rather than automatically lowering discharge pressure.

• D is incorrect: Air in a hose is a contamination concern; it is not primarily a cause of inability to read positive pressure.

Topic: Charging-hose air removal

Suggested error code: SAF

Review: Section 4.5 - Manifold Gauge Set and Service Hoses

Question 4.5-6

Correct answer: C. Minimize refrigerant release when hoses or service equipment are disconnected

Low-loss fittings are designed to reduce refrigerant release when service hoses are connected or disconnected.

• A is incorrect: They do not increase compressor capacity.

• B is incorrect: Pressure-reference conversion is a calculation, not a function of a hose fitting.

• D is incorrect: Moisture removal is a filter-drier function.

Topic: Low-loss fitting

Suggested error code: SAF

Review: Section 4.5 - Manifold Gauge Set and Service Hoses

Question 4.5-7

Correct answer: B. Micron gauge

A micron gauge measures low absolute pressure with the sensitivity needed to verify deep evacuation. A compound gauge is suitable only for rough vacuum indication.

• A is incorrect: A high-pressure gauge is designed for positive high-side pressure, not deep vacuum.

• C is incorrect: An ordinary compound gauge does not have adequate resolution in the deep-vacuum range.

• D is incorrect: A cylinder scale measures mass, not vacuum.

Topic: Deep-vacuum instrumentation

Suggested error code: SAF

Review: Section 4.5 - Manifold Gauge Set and Service Hoses

Question 4.5-8

Correct answer: D. Hoses and fittings must be suitable for the refrigerant and pressure, inspected before use, and handled to minimize releases and contamination.

Service hoses and fittings must be compatible with the refrigerant, rated for expected pressure, inspected for condition, and handled to minimize releases and cross-contamination.

• A is incorrect: Color is a convention and does not prove refrigerant compatibility or pressure rating.

• B is incorrect: A fitting that threads together is not proof that the hose is chemically or pressure compatible.

• C is incorrect: Residual refrigerant in hoses can cause cross-contamination if service practices do not control it.

Topic: Service-hose selection and handling

Suggested error code: SAF

Review: Section 4.5 - Manifold Gauge Set and Service Hoses

Part F - Section 4.6 Concept-Check Answers

Source section: Section 4.6 - Gauge Pressure Absolute Pressure and Vacuum

Answer key: C D B A C D A B

Question 4.6-1

Correct answer: C. Pressure equal to the surrounding atmospheric pressure

Gauge pressure is referenced to the surrounding atmosphere. Therefore 0 psig means the measured pressure equals atmospheric pressure, not that absolute pressure is zero.

• A is incorrect: Perfect vacuum corresponds to 0 psia, not 0 psig.

• B is incorrect: At 0 psig, absolute pressure is approximately local atmospheric pressure.

• D is incorrect: 760 microns is a low absolute pressure, far below atmospheric pressure.

Topic: Gauge pressure reference

Suggested error code: CAL

Review: Section 4.6 - Gauge Pressure Absolute Pressure and Vacuum

Question 4.6-2

Correct answer: D. 34.7 psia

At standard sea-level conditions, $P_{\text{abs}}=P_{\text{gauge}}+P_{\text{atm}}=20+14.7=34.7\text{psia}$.

Given:

• Gauge pressure = $20\text{psig}$
• Standard atmospheric pressure = $14.7\text{psia}$

Relationship:

$$P_{\text{abs}}=P_{\text{gauge}}+P_{\text{atm}}$$

Numerically, $20+14.7=34.7$.

$$P_{\text{abs}}=34.7\text{psia}$$

• A is incorrect: Subtracting atmospheric pressure from 20 psig gives the wrong pressure reference.

• B is incorrect: This incorrectly treats gauge pressure as absolute pressure.

• C is incorrect: 29.92 is commonly associated with inches of mercury for standard atmospheric pressure, not the requested psia result.

Topic: Gauge-to-absolute conversion

Suggested error code: CAL

Review: Section 4.6 - Gauge Pressure Absolute Pressure and Vacuum

Question 4.6-3

Correct answer: B. 0 psia

The theoretical perfect-vacuum limit is zero absolute pressure, or 0 psia.

• A is incorrect: 0 psig is atmospheric pressure on a gauge reference.

• C is incorrect: 14.7 psia is approximately standard atmospheric pressure, not vacuum.

• D is incorrect: 760 mm Hg absolute is approximately one standard atmosphere.

Topic: Perfect vacuum

Suggested error code: DEF

Review: Section 4.6 - Gauge Pressure Absolute Pressure and Vacuum

Question 4.6-4

Correct answer: A. A larger in. Hg vacuum reading generally indicates a deeper vacuum.

An inches-of-mercury vacuum scale is referenced to atmospheric pressure; as pressure drops farther below atmosphere, the indicated in. Hg vacuum value generally becomes larger.

• B is incorrect: On this vacuum scale, a smaller value is closer to atmospheric pressure, not deeper vacuum.

• C is incorrect: This reverses the reference; in. Hg vacuum is ordinarily measured below atmospheric pressure rather than upward from perfect vacuum.

• D is incorrect: 0 in. Hg vacuum corresponds approximately to atmospheric pressure, not 0 psia.

Topic: Inches Hg vacuum

Suggested error code: CAL

Review: Section 4.6 - Gauge Pressure Absolute Pressure and Vacuum

Question 4.6-5

Correct answer: C. It is referenced to perfect vacuum, so lower values indicate deeper vacuum.

Millimeters of mercury absolute is an absolute-pressure scale referenced to perfect vacuum. Lower mm Hg absolute means lower pressure and a deeper vacuum.

• A is incorrect: This describes an atmospheric-referenced vacuum scale rather than an absolute-pressure scale.

• B is incorrect: psig and mm Hg absolute use different reference points and units.

• D is incorrect: mm Hg absolute can be converted to microns because 1 mm Hg equals 1,000 microns.

Topic: mm Hg absolute

Suggested error code: CAL

Review: Section 4.6 - Gauge Pressure Absolute Pressure and Vacuum

Question 4.6-6

Correct answer: D. 2,500 microns

Because 1 mm Hg equals 1,000 microns, $2.5\text{mm Hg}=2.5\cdot 1000=2500\text{microns}$.

Given:

• Vacuum pressure = $2.5\text{mm Hg absolute}$
• $1\text{mm Hg}=1000\text{microns}$

Calculation:

$$2.5\cdot 1000=2500$$

Therefore, the pressure is $2500\text{microns}$.

• A is incorrect: This omits the factor of 1,000.

• B is incorrect: This uses a factor of 100 instead of 1,000.

• C is incorrect: 1,000 microns corresponds to 1 mm Hg, not 2.5 mm Hg.

Topic: Vacuum unit conversion

Suggested error code: CAL

Review: Section 4.6 - Gauge Pressure Absolute Pressure and Vacuum

Question 4.6-7

Correct answer: A. Micron gauge

A micron gauge is specifically designed to measure the low absolute pressures encountered during deep evacuation.

• B is incorrect: A high-side gauge is intended for positive pressure, not deep vacuum.

• C is incorrect: A compound gauge is useful for rough vacuum indication but lacks adequate deep-vacuum resolution.

• D is incorrect: A refrigerant cylinder scale measures mass.

Topic: Deep-vacuum measurement

Suggested error code: SAF

Review: Section 4.6 - Gauge Pressure Absolute Pressure and Vacuum

Question 4.6-8

Correct answer: B. Gauge pressure is referenced to atmospheric pressure, so 0 psig still represents approximately atmospheric absolute pressure.

Gauge pressure uses atmospheric pressure as its zero reference. Thus 0 psig still corresponds to approximately atmospheric absolute pressure rather than perfect vacuum.

• A is incorrect: Gauge pressure is not referenced to absolute-zero temperature.

• C is incorrect: Gauge instruments can include negative gauge/vacuum ranges; this statement does not explain the pressure reference.

• D is incorrect: Perfect vacuum is 0 psia, not 14.7 psia.

Topic: Why 0 psig is not vacuum

Suggested error code: DEF

Review: Section 4.6 - Gauge Pressure Absolute Pressure and Vacuum

Part G - Section 4.7 Concept-Check Answers

Source section: Section 4.7 - Pressure-Temperature Relationships

Answer key: B A C D A C B C

Question 4.7-1

Correct answer: B. Saturation pressure and saturation temperature

A refrigerant P-T chart relates saturation pressure and saturation temperature for a specified refrigerant. At saturation, knowing one allows the corresponding other value to be found.

• A is incorrect: Compressor amperage and refrigerant mass are not the variables related by a saturation P-T chart.

• C is incorrect: Airflow and pressure drop are operating-system quantities, not the direct saturation relationship.

• D is incorrect: Oil viscosity is not the companion variable in a refrigerant saturation P-T chart.

Topic: P-T relationship

Suggested error code: DEF

Review: Section 4.7 - Pressure-Temperature Relationships

Question 4.7-2

Correct answer: A. 35.0 psig

Gauge pressure is absolute pressure minus atmospheric pressure. Using standard atmosphere, $49.7-14.7=35.0\text{psig}$.

Given:

• Absolute saturation pressure = $49.7\text{psia}$
• Standard atmospheric pressure = $14.7\text{psia}$

Relationship:

$$P_{\text{gauge}}=P_{\text{abs}}-P_{\text{atm}}$$

Numerically, $49.7-14.7=35.0$.

$$P_{\text{gauge}}=35.0\text{psig}$$

• B is incorrect: 49.7 is the given absolute pressure, not the gauge pressure.

• C is incorrect: Adding atmospheric pressure converts in the wrong direction.

• D is incorrect: The number 14.7 is the standard atmospheric-pressure value in psia used in the conversion; it is not the resulting gauge pressure.

Topic: P-T pressure conversion

Suggested error code: CAL

Review: Section 4.7 - Pressure-Temperature Relationships

Question 4.7-3

Correct answer: C. Temperature and pressure should approach a representative equilibrium condition before comparison.

A static P-T comparison is meaningful only after the cylinder approaches thermal equilibrium. Stabilization allows the measured temperature and pressure to represent the same two-phase equilibrium condition more closely.

• A is incorrect: No required chemical reaction with the cylinder wall establishes P-T equilibrium.

• B is incorrect: A meaningful two-phase cylinder check requires both liquid and vapor, not complete removal of vapor.

• D is incorrect: Thermal stabilization does not change a zeotropic blend into an azeotrope.

Topic: Cylinder stabilization

Suggested error code: PRO

Review: Section 4.7 - Pressure-Temperature Relationships

Question 4.7-4

Correct answer: D. The mismatch requires investigation for causes such as noncondensables, wrong or mixed refrigerant, or measurement error.

A significantly high pressure is a screening result, not proof of one cause. Possible causes include noncondensables, wrong or mixed refrigerant, measurement error, or failure to reach the intended equilibrium condition.

• A is incorrect: Noncondensables are one possibility, but the mismatch alone does not prove they are present.

• B is incorrect: Cylinder fill level alone is not established from the pressure mismatch.

• C is incorrect: A higher-pressure refrigerant is one possible wrong-refrigerant scenario, but it is not uniquely proven.

Topic: P-T consistency screening

Suggested error code: DIST

Review: Section 4.7 - Pressure-Temperature Relationships

Question 4.7-5

Correct answer: A. A running system contains pressure differences, superheat, subcooling, and active heat transfer rather than one static equilibrium condition.

A running refrigeration system is not one static equilibrium container. It contains high- and low-pressure regions, superheated and subcooled states, pressure drops, and ongoing heat transfer, so compressor discharge pressure cannot be treated like stabilized cylinder saturation pressure.

• B is incorrect: Discharge pressure is normally above atmospheric pressure, not always below it.

• C is incorrect: Pressure can and routinely is measured while compressors operate.

• D is incorrect: Refrigerant P-T data applies to refrigerants at saturation, not only to water.

Topic: Static versus operating P-T conditions

Suggested error code: PRO

Review: Section 4.7 - Pressure-Temperature Relationships

Question 4.7-6

Correct answer: C. Dew point

For a zeotropic blend, the dew point is the saturated-vapor boundary at a specified pressure.

• A is incorrect: Bubble point is the saturated-liquid boundary.

• B is incorrect: Freezing point describes solid-liquid phase behavior, not the saturated-vapor boundary used here.

• D is incorrect: Critical point is a different thermodynamic limit and is not the vapor saturation reference for ordinary superheat calculations.

Topic: Dew point

Suggested error code: DEF

Review: Section 4.7 - Pressure-Temperature Relationships

Question 4.7-7

Correct answer: B. Bubble point

For a zeotropic blend, subcooling is referenced to the saturated-liquid boundary, so the bubble-point temperature is used.

• A is incorrect: Dew point is the saturated-vapor reference used for superheat.

• C is incorrect: Critical temperature is not the ordinary saturation reference for liquid-line subcooling.

• D is incorrect: Averaging bubble and dew is not the standard technician rule for subcooling.

Topic: Bubble point for subcooling

Suggested error code: CAL

Review: Section 4.7 - Pressure-Temperature Relationships

Question 4.7-8

Correct answer: C. P-T comparison is a consistency check that should be used with refrigerant identification, temperature, pressure, and system-condition information.

P-T comparison is a consistency check. Reliable identification also requires correct refrigerant labeling/identity information, appropriate temperature and pressure measurements, and awareness of the system or cylinder condition.

• A is incorrect: Many refrigerants can have overlapping pressures, so one pressure reading cannot uniquely identify refrigerant.

• B is incorrect: Agreement with a P-T value does not prove laboratory purity or rule out all mixtures.

• D is incorrect: Cylinder color alone is not a reliable modern refrigerant-identification method.

Topic: P-T identification limitations

Suggested error code: DIST

Review: Section 4.7 - Pressure-Temperature Relationships

Part H - Section 4.8 Concept-Check Answers

Source section: Section 4.8 - Superheat and Subcooling

Answer key: B A C B D A C A

Question 4.8-1

Correct answer: B. Actual vapor temperature minus vapor saturation temperature

Superheat is the actual vapor temperature minus the corresponding saturated-vapor temperature at the same pressure.

• A is incorrect: This is the basic direction used for subcooling, not superheat.

• C is incorrect: This subtracts the wrong phase and reverses the relevant relationship.

• D is incorrect: Superheat is a temperature difference, not a pressure difference.

Topic: Superheat calculation

Suggested error code: CAL

Review: Section 4.8 - Superheat and Subcooling

Question 4.8-2

Correct answer: A. 14°F

Using $\text{Superheat}=T_{\text{actual vapor}}-T_{\text{sat}}$, $56-42=14{}^{\circ }\text{F}$.

Given:

• Vapor saturation temperature = $42{}^{\circ }\text{F}$
• Actual vapor-line temperature = $56{}^{\circ }\text{F}$

Relationship:

$$\text{Superheat}=T_{\text{actual vapor}}-T_{\text{vapor saturation}}$$

Numerically, $56-42=14$.

$$\text{Superheat}=14{}^{\circ }\text{F}$$

• B is incorrect: 42°F is the saturation temperature, not the temperature difference.

• C is incorrect: 56°F is the actual line temperature, not the superheat.

• D is incorrect: 98°F adds the two temperatures instead of subtracting them.

Topic: Superheat calculation

Suggested error code: CAL

Review: Section 4.8 - Superheat and Subcooling

Question 4.8-3

Correct answer: C. Liquid saturation temperature minus actual liquid temperature

Subcooling is the saturated-liquid temperature minus the actual liquid temperature at the same pressure.

• A is incorrect: This uses the actual liquid temperature first and references vapor saturation, so both the phase reference and subtraction direction are wrong.

• B is incorrect: This mixes vapor temperature with the liquid saturation reference.

• D is incorrect: Subcooling is a temperature difference, not a pressure difference.

Topic: Subcooling calculation

Suggested error code: CAL

Review: Section 4.8 - Superheat and Subcooling

Question 4.8-4

Correct answer: B. 11°F

Using $\text{Subcooling}=T_{\text{liquid saturation}}-T_{\text{actual liquid}}$, $105-94=11{}^{\circ }\text{F}$.

Given:

• Liquid saturation temperature = $105{}^{\circ }\text{F}$
• Actual liquid-line temperature = $94{}^{\circ }\text{F}$

Relationship:

$$\text{Subcooling}=T_{\text{liquid saturation}}-T_{\text{actual liquid}}$$

Numerically, $105-94=11$.

$$\text{Subcooling}=11{}^{\circ }\text{F}$$

• A is incorrect: 94°F is the measured liquid-line temperature, not the subcooling.

• C is incorrect: 105°F is the saturation temperature, not the temperature difference.

• D is incorrect: 199°F adds the temperatures rather than subtracting them.

Topic: Subcooling calculation

Suggested error code: CAL

Review: Section 4.8 - Superheat and Subcooling

Question 4.8-5

Correct answer: D. Dew point

Superheat is a vapor-side comparison, so a zeotropic blend uses the dew-point temperature, which is the saturated-vapor boundary.

• A is incorrect: Bubble point is the saturated-liquid boundary used for subcooling.

• B is incorrect: Freezing point is unrelated to ordinary vapor superheat calculations.

• C is incorrect: Critical temperature is not the saturation reference used for superheat.

Topic: Dew point for superheat

Suggested error code: CAL

Review: Section 4.8 - Superheat and Subcooling

Question 4.8-6

Correct answer: A. Bubble point

Subcooling is a liquid-side comparison, so a zeotropic blend uses the bubble-point temperature, which is the saturated-liquid boundary.

• B is incorrect: Dew point is the saturated-vapor boundary used for superheat.

• C is incorrect: Critical temperature is not the saturation reference for ordinary liquid-line subcooling.

• D is incorrect: The midpoint of bubble and dew is not automatically used for technician subcooling calculations.

Topic: Bubble point for subcooling

Suggested error code: CAL

Review: Section 4.8 - Superheat and Subcooling

Question 4.8-7

Correct answer: C. The reading can have several causes and should be interpreted with system design, load, flow conditions, and other measurements.

Higher-than-expected superheat can result from several conditions, including evaporator underfeeding, restrictions, load changes, suction-line heat gain, or measurement problems. It must be interpreted with the system design and other measurements.

• A is incorrect: High superheat can be associated with undercharge in some systems, but it does not prove undercharge by itself.

• B is incorrect: High superheat does not prove overcharge either.

• D is incorrect: A superheat reading does not establish that the condenser contains only saturated liquid.

Topic: Superheat interpretation

Suggested error code: DIST

Review: Section 4.8 - Superheat and Subcooling

Question 4.8-8

Correct answer: A. Refrigerant pressure can change through piping and components, changing the corresponding saturation temperature.

Saturation temperature depends on pressure. If pressure changes between the pressure measurement and temperature measurement locations, the derived saturation temperature may no longer represent the line temperature location accurately.

• B is incorrect: Pressure and saturation temperature are directly related at saturation.

• C is incorrect: Line temperature changes around the refrigeration circuit and even along individual lines.

• D is incorrect: Both pressure-derived saturation temperature and actual line temperature are required for superheat or subcooling.

Topic: Measurement-location consistency

Suggested error code: CAL

Review: Section 4.8 - Superheat and Subcooling

Part I - Section 4.10 Module Practice-Question Answers

Source section: Section 4.10 - Practice Questions

Answer key: B C A D B C A B D C B A D B C A D B C A D B C A B D A C B D A C B D A

Question 4.10-1

Correct answer: B. Compressor → Condenser → Metering Device → Evaporator → Compressor

The four major components are traversed in the order compressor → condenser → metering device → evaporator → compressor. This sequence follows the refrigerant from compression, through heat rejection, through the major pressure drop, and then through heat absorption.

• A is incorrect: This incorrectly routes compressor discharge to the evaporator before the condenser and metering device.

• C is incorrect: This sequence places the compressor after the condenser and does not follow the normal closed-loop order.

• D is incorrect: This begins at the metering device and then sends low-side refrigerant directly to the compressor before the evaporator.

Topic: Cycle sequence

Suggested error code: PRO

Review: Section 4.1 - Vapor-Compression Refrigeration Cycle

Question 4.10-2

Correct answer: C. Compressor

The compressor receives work input, draws in low-pressure vapor, and raises the refrigerant pressure so that heat can later be rejected on the high side.

• A is incorrect: The evaporator absorbs heat; it does not raise refrigerant pressure.

• B is incorrect: The condenser rejects heat and condenses refrigerant; it is not the work-input pressure-raising component.

• D is incorrect: The metering device lowers pressure rather than raises it.

Topic: Compressor function

Suggested error code: DEF

Review: Section 4.1 - Vapor-Compression Refrigeration Cycle

Question 4.10-3

Correct answer: A. Evaporator

The evaporator absorbs heat from the space, product, or process being cooled. That heat drives refrigerant evaporation on the low-pressure side.

• B is incorrect: The compressor supplies work and raises pressure.

• C is incorrect: The condenser rejects heat to the surroundings.

• D is incorrect: A receiver stores/manages liquid refrigerant when the system uses one.

Topic: Evaporator function

Suggested error code: DEF

Review: Section 4.1 - Vapor-Compression Refrigeration Cycle

Question 4.10-4

Correct answer: D. Metering device

The metering device produces the major pressure drop between the high-pressure liquid side and the low-pressure evaporator side while controlling refrigerant flow.

• A is incorrect: The compressor raises pressure.

• B is incorrect: The condenser rejects heat on the high side.

• C is incorrect: The accumulator protects the compressor from excess liquid return; it is not the major pressure-drop device.

Topic: Metering-device function

Suggested error code: DEF

Review: Section 4.1 - Vapor-Compression Refrigeration Cycle

Question 4.10-5

Correct answer: B. High-pressure high-temperature vapor

Compression raises both pressure and, in normal operation, vapor temperature. The compressor therefore discharges high-pressure, high-temperature vapor.

• A is incorrect: Low-pressure liquid is not the compressor discharge state.

• C is incorrect: A low-pressure mixture belongs downstream of the metering device, not compressor discharge.

• D is incorrect: High-pressure subcooled liquid is normally associated with the condenser outlet/liquid line, not immediately after compression.

Topic: Compressor outlet state

Suggested error code: DEF

Review: Section 4.2 - Refrigerant State Through the Cycle

Question 4.10-6

Correct answer: C. Low-pressure liquid-vapor mixture

The metering device drops the refrigerant pressure. Part of the liquid flashes, so a low-pressure liquid-vapor mixture normally enters the evaporator.

• A is incorrect: The metering device lowers pressure, so high-pressure vapor is not the normal outlet state.

• B is incorrect: The pressure drop causes flashing, so high-pressure liquid is not the downstream state.

• D is incorrect: A two-phase mixture normally enters the evaporator; it is not vapor only.

Topic: Metering-device outlet state

Suggested error code: DEF

Review: Section 4.2 - Refrigerant State Through the Cycle

Question 4.10-7

Correct answer: A. Discharge line

The discharge line carries high-pressure, high-temperature vapor from the compressor outlet to the condenser inlet.

• B is incorrect: The liquid line carries high-pressure liquid after the condenser.

• C is incorrect: The suction line carries low-pressure vapor toward the compressor.

• D is incorrect: An equalizer line has a control/sensing function and is not the main compressor discharge path.

Topic: Discharge line

Suggested error code: DEF

Review: Section 4.3 - High Side Low Side and Refrigerant Lines

Question 4.10-8

Correct answer: B. Liquid line

The liquid line carries high-pressure liquid from the condenser toward the metering device.

• A is incorrect: The suction line is on the low-pressure vapor side.

• C is incorrect: The discharge line carries compressor outlet vapor to the condenser.

• D is incorrect: The evaporator return line is part of the low-side return toward the compressor.

Topic: Liquid line

Suggested error code: DEF

Review: Section 4.3 - High Side Low Side and Refrigerant Lines

Question 4.10-9

Correct answer: D. Suction line

The suction line connects the evaporator outlet to the compressor inlet and normally carries low-pressure vapor.

• A is incorrect: The discharge line carries high-pressure vapor from compressor to condenser.

• B is incorrect: The liquid line carries high-pressure liquid toward the metering device.

• C is incorrect: A receiver line is associated with high-side liquid storage, not the evaporator-to-compressor vapor return.

Topic: Suction line

Suggested error code: DEF

Review: Section 4.3 - High Side Low Side and Refrigerant Lines

Question 4.10-10

Correct answer: C. After the condenser and before the metering device on the high-pressure liquid side

A liquid receiver is normally located downstream of the condenser and upstream of the metering device on the high-pressure liquid side.

• A is incorrect: That is the suction-side accumulator region.

• B is incorrect: The compressor-to-condenser discharge line carries high-pressure vapor and is upstream of condensation.

• D is incorrect: Immediately downstream of the metering device is the low-pressure evaporator feed region.

Topic: Receiver location

Suggested error code: CLS

Review: Section 4.4 - Receivers Accumulators and Filter-Driers

Question 4.10-11

Correct answer: B. To separate excess liquid from suction vapor and help protect the compressor

The accumulator separates excess liquid from suction vapor and meters or returns retained liquid gradually, helping prevent uncontrolled liquid return to the compressor.

• A is incorrect: High-pressure liquid storage before the metering device is the receiver’s role.

• C is incorrect: An accumulator does not remove noncondensables from the condenser.

• D is incorrect: The main high-to-low pressure drop is created by the metering device.

Topic: Accumulator function

Suggested error code: DEF

Review: Section 4.4 - Receivers Accumulators and Filter-Driers

Question 4.10-12

Correct answer: A. To remove moisture and particles from the refrigerant circuit

A filter-drier combines filtration of solid contamination with a desiccant function that removes a limited amount of moisture from the refrigerant circuit.

• B is incorrect: Air and nitrogen are not removed by a filter-drier; evacuation is used to remove noncondensables.

• C is incorrect: A filter-drier is not primarily a pressure-measuring device.

• D is incorrect: Liquid refrigerant storage is the receiver’s function.

Topic: Filter-drier function

Suggested error code: DEF

Review: Section 4.4 - Receivers Accumulators and Filter-Driers

Question 4.10-13

Correct answer: D. The observation must be interpreted with other system information and does not by itself prove undercharge.

Bubbles in a sight glass are an observation, not a complete diagnosis. Flashing or bubbles can result from several conditions, so other pressures, temperatures, load conditions, and system design must be evaluated.

• A is incorrect: Bubbles can arise from several conditions; they do not prove undercharge.

• B is incorrect: A sight-glass observation does not diagnose compressor damage.

• C is incorrect: Bubbles do not by themselves prove filter-drier failure.

Topic: Sight-glass interpretation

Suggested error code: DIST

Review: Section 4.4 - Receivers Accumulators and Filter-Driers

Question 4.10-14

Correct answer: B. It measures low-side positive pressure and provides a rough vacuum indication.

The compound gauge is the low-side gauge. It reads positive low-side pressure and also extends below atmospheric pressure to provide a rough vacuum indication.

• A is incorrect: High-side discharge pressure is measured by the high-pressure gauge.

• C is incorrect: A compound gauge is not precise enough to verify deep vacuum in microns.

• D is incorrect: Flow through the center hose is controlled by manifold valves, not by the gauge itself.

Topic: Compound gauge

Suggested error code: DEF

Review: Section 4.5 - Manifold Gauge Set and Service Hoses

Question 4.10-15

Correct answer: C. High-pressure gauge

The high-pressure gauge is connected through the high-side hose to a system high-pressure access point, where it indicates high-side pressure.

• A is incorrect: A micron gauge is used for deep-vacuum measurement, not routine high-side pressure.

• B is incorrect: The compound gauge is the low-side gauge.

• D is incorrect: A vacuum gauge is not the normal instrument for system high-side positive pressure.

Topic: High-pressure gauge connection

Suggested error code: PRO

Review: Section 4.5 - Manifold Gauge Set and Service Hoses

Question 4.10-16

Correct answer: A. Center service hose

The center service port is the common service connection used for equipment such as a refrigerant source, recovery machine, or vacuum pump, depending on the task.

• B is incorrect: The red high-side hose belongs at the system high-pressure access point.

• C is incorrect: The blue low-side hose belongs at the system low-pressure access point.

• D is incorrect: The gauge face has no service connection function.

Topic: Center service hose

Suggested error code: DEF

Review: Section 4.5 - Manifold Gauge Set and Service Hoses

Question 4.10-17

Correct answer: D. System high-pressure access point

The red high-side hose should connect to the system high-pressure access point. The refrigerant source, recovery machine, or vacuum pump uses the center service connection as appropriate.

• A is incorrect: The refrigerant supply cylinder normally connects through the center service hose, not the red high-side hose.

• B is incorrect: The vacuum pump normally connects through the center service hose.

• C is incorrect: A recovery cylinder is downstream of the recovery machine in the normal recovery arrangement, not the system high-side hose connection.

Topic: High-side hose connection

Suggested error code: PRO

Review: Section 4.5 - Manifold Gauge Set and Service Hoses

Question 4.10-18

Correct answer: B. The gauges can still indicate their respective system pressures while the valves block flow through the manifold toward the center port.

With hoses connected, each gauge normally has a direct pressure path to its respective system side. Closing both manifold valves blocks flow through the manifold toward the center port but does not prevent the gauges from reading connected-side pressure.

• A is incorrect: Gauge pressure paths remain open to their connected system sides even when the manifold valves are closed.

• C is incorrect: The center port is isolated from each side when the corresponding manifold valve is closed; it does not automatically connect to both sides.

• D is incorrect: Closing manifold valves does not change the gauges into deep-vacuum instruments.

Topic: Manifold valve operation

Suggested error code: PRO

Review: Section 4.5 - Manifold Gauge Set and Service Hoses

Question 4.10-19

Correct answer: C. 14.7 psia

Gauge pressure is referenced to the surrounding atmosphere. Therefore 0 psig means the measured pressure equals atmospheric pressure, not that absolute pressure is zero.

At standard sea-level conditions:

$$P_{\text{atm}}\approx 14.7\text{psia}$$

Because $0\text{psig}$ means the pressure equals the surrounding atmosphere, the corresponding absolute pressure is approximately $14.7\text{psia}$.

• A is incorrect: 0 psia, not 0 psig, represents perfect vacuum.

• B is incorrect: 1 psia is a low absolute pressure and is not equivalent to atmospheric pressure.

• D is incorrect: 29.92 is approximately standard atmospheric pressure in inches of mercury, not psia.

Topic: Gauge versus absolute pressure

Suggested error code: CAL

Review: Section 4.6 - Gauge Pressure Absolute Pressure and Vacuum

Question 4.10-20

Correct answer: A. Perfect vacuum

Absolute pressure is referenced to perfect vacuum, where pressure is defined as zero absolute.

• B is incorrect: Atmospheric pressure is the zero reference for gauge pressure, not absolute pressure.

• C is incorrect: Water saturation pressure is not the reference for absolute pressure.

• D is incorrect: A low-side gauge zero is an instrument gauge reference tied to atmosphere, not the absolute-pressure origin.

Topic: Absolute-pressure reference

Suggested error code: DEF

Review: Section 4.6 - Gauge Pressure Absolute Pressure and Vacuum

Question 4.10-21

Correct answer: D. 1,000 microns

One millimeter of mercury is equal to 1,000 microns, so 1 mm Hg absolute corresponds to 1,000 microns.

Relationship:

$$1\text{mm Hg}=1000\text{microns}$$

Therefore, $1\text{mm Hg absolute}$ corresponds to $1000\text{microns}$.

• A is incorrect: 25.4 relates inches and millimeters, not millimeters and microns.

• B is incorrect: 100 microns is one-tenth of a millimeter of mercury.

• C is incorrect: 760 microns is 0.760 mm Hg, not 1 mm Hg.

Topic: Vacuum unit conversion

Suggested error code: CAL

Review: Section 4.6 - Gauge Pressure Absolute Pressure and Vacuum

Question 4.10-22

Correct answer: B. A smaller micron value means a lower absolute pressure and therefore a deeper vacuum.

Microns are an absolute-pressure unit. As the micron reading decreases, absolute pressure decreases and the vacuum becomes deeper.

• A is incorrect: A larger micron value means higher absolute pressure and therefore a shallower vacuum.

• C is incorrect: Microns are not gauge pressure above atmosphere.

• D is incorrect: Micron gauges are specifically used to evaluate evacuation.

Topic: Vacuum interpretation

Suggested error code: CAL

Review: Section 4.6 - Gauge Pressure Absolute Pressure and Vacuum

Question 4.10-23

Correct answer: C. Micron gauge

A micron gauge is specifically designed to measure the low absolute pressures encountered during deep evacuation.

• A is incorrect: A high-pressure gauge is not designed for deep-vacuum verification.

• B is incorrect: A standard compound gauge lacks the resolution needed in the micron range.

• D is incorrect: A sight glass does not measure vacuum.

Topic: Deep-vacuum measurement

Suggested error code: SAF

Review: Section 4.6 - Gauge Pressure Absolute Pressure and Vacuum

Question 4.10-24

Correct answer: A. Saturation pressure and saturation temperature

A refrigerant P-T chart relates saturation pressure and saturation temperature for a specified refrigerant. At saturation, knowing one allows the corresponding other value to be found.

• B is incorrect: Compressor amperage and refrigerant mass are not the saturation variables related by a P-T chart.

• C is incorrect: Airflow and line temperature are system operating quantities, not the chart’s fundamental saturation pair.

• D is incorrect: Oil viscosity and receiver volume are unrelated to a refrigerant saturation P-T table.

Topic: Pressure-temperature relationship

Suggested error code: DEF

Review: Section 4.7 - Pressure-Temperature Relationships

Question 4.10-25

Correct answer: B. 40°F

From the supplied R-134a table, 35.0 psig corresponds directly to a saturation temperature of 40°F.

Lookup:

The supplied R-134a P-T table gives:

35.0 psig → 40°F saturation temperature

No interpolation is required.

• A is incorrect: 25°F corresponds to 22.1 psig in the supplied table, not 35.0 psig.

• C is incorrect: 45.4 is a pressure value in psig in the supplied data, not the requested temperature.

• D is incorrect: 50°F corresponds to 45.4 psig.

Topic: P-T chart reading

Suggested error code: CAL

Review: Section 4.7 - Pressure-Temperature Relationships

Question 4.10-26

Correct answer: D. 50°F

From the supplied R-134a table, 45.4 psig corresponds directly to a saturation temperature of 50°F.

Lookup:

The supplied R-134a P-T table gives:

45.4 psig → 50°F saturation temperature

No interpolation is required.

• A is incorrect: 22.1 is a pressure value associated with 25°F, not the temperature corresponding to 45.4 psig.

• B is incorrect: 35°F is not the listed saturation temperature for 45.4 psig.

• C is incorrect: 45.4 is the measured pressure value, not the saturation temperature.

Topic: P-T chart reading

Suggested error code: CAL

Review: Section 4.7 - Pressure-Temperature Relationships

Question 4.10-27

Correct answer: A. Investigate possible causes such as noncondensables, wrong or mixed refrigerant, or measurement error.

A significantly high pressure is a screening result, not proof of one cause. Possible causes include noncondensables, wrong or mixed refrigerant, measurement error, or failure to reach the intended equilibrium condition.

• B is incorrect: A P-T mismatch does not establish cylinder fill level and therefore does not prove overfill.

• C is incorrect: A mismatch is not evidence that the refrigerant is definitely pure.

• D is incorrect: A two-phase cylinder specifically contains both liquid and vapor, so this conclusion contradicts the stated condition.

Topic: Static cylinder P-T screening

Suggested error code: DIST

Review: Section 4.7 - Pressure-Temperature Relationships

Question 4.10-28

Correct answer: C. Dew point

For a zeotropic blend, the dew point is the saturated-vapor boundary at a specified pressure.

• A is incorrect: Bubble point is the saturated-liquid reference.

• B is incorrect: Critical point is a different thermodynamic limit.

• D is incorrect: Triple point describes solid-liquid-vapor equilibrium and is not the superheat reference.

Topic: Dew point

Suggested error code: DEF

Review: Section 4.7 - Pressure-Temperature Relationships

Question 4.10-29

Correct answer: B. Bubble point

For a zeotropic blend, subcooling is referenced to the saturated-liquid boundary, so the bubble-point temperature is used.

• A is incorrect: Dew point is the saturated-vapor reference used for superheat.

• C is incorrect: Critical point is not the ordinary subcooling reference.

• D is incorrect: A bubble/dew average is not automatically used for subcooling.

Topic: Bubble point

Suggested error code: DEF

Review: Section 4.7 - Pressure-Temperature Relationships

Question 4.10-30

Correct answer: D. Actual vapor temperature − vapor saturation temperature

Superheat is the actual vapor temperature minus the corresponding saturated-vapor temperature at the same pressure.

• A is incorrect: This reverses the superheat subtraction.

• B is incorrect: This is a liquid-side relationship and is not the superheat formula.

• C is incorrect: This is the subcooling equation, not the superheat equation.

Topic: Superheat formula

Suggested error code: CAL

Review: Section 4.8 - Superheat and Subcooling

Question 4.10-31

Correct answer: A. 14°F

Using $\text{Superheat}=T_{\text{actual vapor}}-T_{\text{sat}}$, $56-42=14{}^{\circ }\text{F}$.

Given:

• Vapor saturation temperature = $42{}^{\circ }\text{F}$
• Actual vapor-line temperature = $56{}^{\circ }\text{F}$

Relationship:

$$\text{Superheat}=T_{\text{actual vapor}}-T_{\text{vapor saturation}}$$

Numerically, $56-42=14$.

$$\text{Superheat}=14{}^{\circ }\text{F}$$

• B is incorrect: 42°F is the saturation temperature, not the calculated superheat.

• C is incorrect: 56°F is the actual line temperature, not the temperature difference.

• D is incorrect: 98°F results from addition instead of subtraction.

Topic: Superheat calculation

Suggested error code: CAL

Review: Section 4.8 - Superheat and Subcooling

Question 4.10-32

Correct answer: C. Liquid saturation temperature − actual liquid temperature

Subcooling is the saturated-liquid temperature minus the actual liquid temperature at the same pressure.

• A is incorrect: This is the superheat equation.

• B is incorrect: This reverses the subcooling subtraction.

• D is incorrect: This reverses a vapor-side relationship and is not the liquid subcooling formula.

Topic: Subcooling formula

Suggested error code: CAL

Review: Section 4.8 - Superheat and Subcooling

Question 4.10-33

Correct answer: B. 11°F

Using $\text{Subcooling}=T_{\text{liquid saturation}}-T_{\text{actual liquid}}$, $105-94=11{}^{\circ }\text{F}$.

Given:

• Liquid saturation temperature = $105{}^{\circ }\text{F}$
• Actual liquid-line temperature = $94{}^{\circ }\text{F}$

Relationship:

$$\text{Subcooling}=T_{\text{liquid saturation}}-T_{\text{actual liquid}}$$

Numerically, $105-94=11$.

$$\text{Subcooling}=11{}^{\circ }\text{F}$$

• A is incorrect: 9°F is not the result of 105°F minus 94°F.

• C is incorrect: 94°F is the measured liquid temperature, not the subcooling.

• D is incorrect: 199°F is the sum rather than the required difference.

Topic: Subcooling calculation

Suggested error code: CAL

Review: Section 4.8 - Superheat and Subcooling

Question 4.10-34

Correct answer: D. Several conditions can produce high superheat, so the reading must be interpreted with system design, load, refrigerant flow, and other measurements.

Higher-than-expected superheat can result from several conditions, including evaporator underfeeding, restrictions, load changes, suction-line heat gain, or measurement problems. It must be interpreted with the system design and other measurements.

• A is incorrect: High superheat alone does not prove overcharge.

• B is incorrect: A single superheat reading does not prove compressor failure.

• C is incorrect: The reading does not establish that the condenser is flooded with liquid.

Topic: Superheat interpretation

Suggested error code: DIST

Review: Section 4.8 - Superheat and Subcooling

Question 4.10-35

Correct answer: A. Pressure can change through piping and components, which changes the corresponding saturation temperature.

Saturation temperature depends on pressure. If pressure changes between the pressure measurement and temperature measurement locations, the derived saturation temperature may no longer represent the line temperature location accurately.

• B is incorrect: Pressure and saturation temperature are directly related for a given refrigerant at saturation.

• C is incorrect: Line temperature changes through the cycle and can change along piping.

• D is incorrect: Pressure is required to determine the saturation temperature used in the calculation.

Topic: Measurement-location consistency

Suggested error code: CAL

Review: Section 4.8 - Superheat and Subcooling

Calculation and Interpretation Review

The most calculation-sensitive Module 4 relationships are collected here for remediation.

Gauge Pressure and Absolute Pressure

$$P_{\text{abs}}=P_{\text{gauge}}+P_{\text{atm}}$$

At standard sea-level atmospheric pressure:

0 psig ≈ 14.7 psia

A perfect vacuum is:

0 psia

not 0 psig.

Vacuum Units

1 mm Hg = 1,000 microns

Lower absolute pressure in microns means a deeper vacuum.

Pressure-Temperature Lookup

At saturation:

Known pressure
→ use correct refrigerant P-T data
→ find saturation temperature

For zeotropic blends:

Vapor / superheat → Dew point
Liquid / subcooling → Bubble point

Superheat

$$\text{Superheat}=T_{\text{actual vapor}}-T_{\text{vapor saturation}}$$

Subcooling

$$\text{Subcooling}=T_{\text{liquid saturation}}-T_{\text{actual liquid}}$$

Interpretation Guardrail

Do not convert one measurement into an automatic charge diagnosis.

Examples of unsupported conclusions include:

High superheat
→ definitely undercharged

or:

High subcooling
→ definitely overcharged

System design, metering-device behavior, heat load, airflow or water flow, pressure drop, line temperature, and measurement quality can all affect the result.

References

Module 4 Instructional Sources

1. Section 4.1 - Vapor-Compression Refrigeration Cycle.

2. Section 4.2 - Refrigerant State Through the Cycle.

3. Section 4.3 - High Side Low Side and Refrigerant Lines.

4. Section 4.4 - Receivers Accumulators and Filter-Driers.

5. Section 4.5 - Manifold Gauge Set and Service Hoses.

6. Section 4.6 - Gauge Pressure Absolute Pressure and Vacuum.

7. Section 4.7 - Pressure-Temperature Relationships.

8. Section 4.8 - Superheat and Subcooling.

9. Section 4.9 - Quick Reference.

10. Section 4.10 - Practice Questions.

HVAC Technical References

11. Edward G. Pita, Air Conditioning Principles and Systems: An Energy Approach, 4th ed., vapor-compression refrigeration cycle, refrigerant states, metering devices, superheat, and subcooling.

12. Rex Miller and Mark R. Miller, HVAC Licensing Study Guide, 3rd ed., refrigeration-cycle components, service gauges, and refrigeration service fundamentals.

13. Justin Kauwale, Mechanical PE: HVAC & Refrigeration Textbook, refrigeration cycle, pressure-temperature, superheat, and subcooling discussions.

14. NCEES, PE Mechanical Reference Handbook, Version 2.0, refrigeration properties and pressure-temperature data.

15. Arkema, Forane 134a Pressure Temperature Chart, used by the project to verify the R-134a values supplied in Section 4.10; accessed August 9, 2026:
    https://forane.arkema.com/files/live/sites/shared_arkema/files/downloads/products/fluorochemicals/forane-134a-pressure-temperature-chart.pdf