4.6 - Gauge Pressure Absolute Pressure and Vacuum

Module: Refrigeration Cycle Components Gauges and Pressure-Temperature Relationships
Course role: Explains the pressure reference systems and vacuum units used in refrigeration service, including psig, psia, atmospheric pressure, inches of mercury vacuum, millimeters of mercury absolute, and microns

Learning Objectives

After completing this section, a student should be able to:

1. Distinguish gauge pressure (psig) from absolute pressure (psia).
2. Explain the role of atmospheric pressure as the reference point for an ordinary pressure gauge.
3. Convert between gauge pressure and absolute pressure when atmospheric pressure is known.
4. Interpret inches of mercury vacuum as pressure below atmospheric pressure.
5. Interpret millimeters of mercury absolute and microns as absolute-pressure units measured from perfect vacuum.
6. Explain why 0 psig is not a perfect vacuum and why a micron gauge is required for deep-vacuum measurement.

Introduction

Pressure can be confusing because the same physical condition can be described using different reference points.

In refrigeration work, technicians commonly encounter:

• psig — pounds per square inch gauge.
• psia — pounds per square inch absolute.
• in. Hg vacuum — inches of mercury below atmospheric pressure.
• mm Hg absolute — millimeters of mercury measured from perfect vacuum.
• microns — a very small absolute-pressure unit used for deep vacuum.

The most important idea is that pressure numbers are meaningless unless the reference point and unit are understood.

For example:

0 psig

does not mean that no pressure exists.

It means:

System pressure = surrounding atmospheric pressure

At standard sea-level atmospheric conditions:

0 psig
≈ 14.7 psia

A perfect vacuum is:

0 psia

not:

0 psig

This distinction is essential for:

• Reading manifold gauges.
• Understanding low-pressure systems.
• Interpreting recovery requirements.
• Evaluating evacuation.
• Using micron gauges.
• Reading pressure-temperature data correctly.

Key Concepts

1. Pressure Needs a Reference

A pressure measurement compares one pressure with a reference.

Two common reference systems are:

Gauge pressure
→ referenced to local atmospheric pressure

Absolute pressure
→ referenced to perfect vacuum

This difference produces two different zero points.

2. Two Different Zeros

For gauge pressure:

0 psig
= local atmospheric pressure

For absolute pressure:

0 psia
= perfect vacuum

Therefore:

0 psig and 0 psia are completely different physical conditions.

3. Vacuum Units May Increase or Decrease as Vacuum Deepens

Some vacuum scales become numerically larger as vacuum increases.

Example:

0 in. Hg vacuum
→ atmospheric pressure

29+ in. Hg vacuum
→ much deeper vacuum

Other vacuum scales become numerically smaller as vacuum increases.

Example:

760,000 microns
→ approximately atmospheric pressure at standard sea level

500 microns
→ deep vacuum

0 microns
→ perfect vacuum

This opposite behavior is a common source of mistakes.

Gauge Pressure - psig

1. Definition

Gauge pressure is pressure measured relative to the surrounding atmosphere.

The unit is:

psig
= pounds per square inch gauge

An ordinary refrigeration pressure gauge is commonly referenced to atmospheric pressure.

When exposed only to the surrounding atmosphere, it normally indicates:

0 psig

2. Positive Gauge Pressure

If the pressure inside the system is greater than atmospheric pressure:

System pressure > Atmospheric pressure
→ Positive psig

For example:

100 psig

means the system pressure is 100 psi above atmospheric pressure.

It does not mean the absolute pressure is 100 psia.

3. Pressure Below Atmosphere

When system pressure falls below atmospheric pressure, a compound gauge enters its vacuum range.

A traditional refrigeration compound gauge often shows this condition in:

inches of mercury vacuum

rather than negative psig.

4. Why Gauge Pressure Is Convenient

Gauge pressure is convenient because technicians are usually interested in the pressure difference between the refrigerant system and the surrounding atmosphere.

Common refrigeration measurements such as:

• Suction pressure.
• Discharge pressure.
• Liquid-line service pressure.

are often reported in psig.

5. Atmospheric Pressure Changes

The atmosphere is not exactly the same everywhere.

Atmospheric pressure changes with:

• Elevation.
• Weather.
• Barometric conditions.

Therefore:

A gauge reading does not by itself provide exact absolute pressure unless the atmospheric pressure is known or appropriately approximated.

For most basic exam problems, standard sea-level atmospheric pressure is used unless another value is given.

Absolute Pressure - psia

1. Definition

Absolute pressure is measured relative to a perfect vacuum.

The unit is:

psia
= pounds per square inch absolute

The zero point is:

0 psia
= perfect vacuum

Absolute pressure cannot be less than zero.

2. Standard Atmospheric Pressure

A commonly used standard sea-level atmospheric pressure is approximately:

14.7 psia

More precisely, one standard atmosphere is approximately:

14.696 psia

For EPA 608 exam-level calculations:

14.7 psia

is normally sufficient unless a problem provides another atmospheric pressure.

3. Gauge-to-Absolute Relationship

The basic relationship is:

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

where:

• $P_{\text{abs}}$ = absolute pressure.
• $P_{\text{gauge}}$ = gauge pressure.
• $P_{\text{atm}}$ = atmospheric pressure.

The reverse relationship is:

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

4. Example - Positive Gauge Pressure

A pressure gauge reads:

18 psig

Assume atmospheric pressure is:

14.7 psia

Then:

$$P_{\text{abs}}=18+14.7$$

$$P_{\text{abs}}=32.7\text{psia}$$

Therefore:

18 psig corresponds to approximately 32.7 psia at standard sea-level atmospheric pressure.

5. Why Absolute Pressure Matters

Absolute pressure is important when working with:

• Gas laws.
• Thermodynamic properties.
• Low-pressure refrigerant systems.
• Vacuum measurements.
• Deep evacuation.
• Certain regulatory evacuation units.

Using gauge pressure where absolute pressure is required can produce incorrect results.

Atmospheric Pressure

1. What Atmospheric Pressure Is

The air surrounding Earth has weight.

That air exerts pressure on:

• People.
• Equipment.
• Refrigeration systems.
• Pressure gauges.

At standard sea-level conditions:

Atmospheric pressure
≈ 14.7 psia
≈ 29.92 in. Hg absolute
≈ 760 mm Hg absolute

These are different units describing approximately the same standard atmospheric pressure.

2. A Gauge Uses Atmosphere as Its Reference

An ordinary gauge exposed to atmosphere on its reference side is effectively comparing:

System pressure
minus
Atmospheric pressure

That is why:

System open to atmosphere
→ 0 psig

even though the absolute pressure is approximately:

14.7 psia

3. Atmospheric Pressure at Higher Elevation

Atmospheric pressure decreases as elevation increases.

Therefore, the exact relationship:

0 psig = 14.7 psia

is only an approximation for standard sea-level conditions.

At higher elevation:

0 psig
= local atmospheric pressure
< 14.7 psia

This matters in precision vacuum work and some diagnostic situations.

4. Barometric Pressure

Atmospheric pressure can be measured with a barometer.

A mercury barometer historically expresses atmospheric pressure as the height of a mercury column.

At standard atmospheric pressure:

29.92 in. Hg
≈ 760 mm Hg

Why 0 psig Is Not a Perfect Vacuum

This is one of the most important concepts in this section.

1. What 0 psig Means

If an ordinary gauge reads:

0 psig

then the pressure inside the system is approximately equal to the surrounding atmospheric pressure.

At standard sea-level conditions:

0 psig
≈ 14.7 psia

The system still contains gas pressure.

2. What Perfect Vacuum Means

A perfect vacuum would contain no gas molecules exerting pressure.

The absolute pressure would be:

0 psia

At standard sea-level conditions, reaching perfect vacuum would require reducing pressure from approximately:

14.7 psia

down to:

0 psia

3. Pressure Reference Diagram

A useful conceptual scale is:

Higher absolute pressure
        ↑
        │
Positive psig
        │
-------------------------------
Atmospheric pressure
0 psig ≈ 14.7 psia
-------------------------------
        │
Vacuum region
        │
        ↓
Perfect vacuum
0 psia

Therefore:

0 psig
≠
0 psia

and:

0 psig
≠
perfect vacuum

Vacuum Pressure

1. What Vacuum Means in HVAC Service

A vacuum exists when system pressure is below the surrounding atmospheric pressure.

Conceptually:

P_abs < P_atm
→ system is under vacuum

Vacuum does not mean that pressure is zero.

There can be many different vacuum levels.

2. Vacuum Pressure as a Difference

If vacuum pressure is expressed as the amount by which system pressure is below atmosphere:

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

where:

• $P_{\text{vac}}$ = vacuum pressure relative to atmosphere.
• $P_{\text{atm}}$ = atmospheric pressure.
• $P_{\text{abs}}$ = absolute system pressure.

The equation is a reference relationship.

In field work, vacuum is often read directly using inches of mercury or an absolute-pressure instrument.

Inches of Mercury Vacuum

1. Meaning

A compound refrigeration gauge commonly indicates vacuum in:

in. Hg vacuum

or:

inches of mercury vacuum

This scale measures how far the pressure is below atmospheric pressure.

At standard sea-level atmospheric conditions:

0 in. Hg vacuum
→ atmospheric pressure

approximately 29.92 in. Hg vacuum
→ perfect-vacuum limit

2. Increasing Number Means Deeper Vacuum

On an inches-of-mercury vacuum scale:

0 in. Hg vacuum
→ no vacuum relative to atmosphere

10 in. Hg vacuum
→ deeper vacuum

20 in. Hg vacuum
→ still deeper vacuum

29+ in. Hg vacuum
→ very deep vacuum relative to atmosphere

Therefore:

For in. Hg vacuum, a larger number generally means a deeper vacuum.

3. Example - Converting an in. Hg Vacuum Reading

Assume standard atmospheric pressure:

29.92 in. Hg absolute

If a compound gauge indicates:

10 in. Hg vacuum

the remaining absolute pressure is approximately:

$$P_{\text{abs}}=29.92-10$$

$$P_{\text{abs}}=19.92\text{in. Hg absolute}$$

This example shows the different directions of the two scales:

Vacuum reading increases
→ Absolute pressure decreases

4. Maximum Vacuum Reading Depends on Local Atmosphere

Because the compound gauge is referenced to local atmospheric pressure, the theoretical maximum in. Hg vacuum reading changes with:

• Elevation.
• Weather.
• Local atmospheric pressure.

Therefore:

Do not treat 29.92 in. Hg vacuum as a universal field maximum at every location.

It represents standard sea-level atmospheric pressure.

Millimeters of Mercury Absolute

1. Meaning

Some refrigeration and regulatory vacuum measurements use:

mm Hg absolute

This means:

millimeters of mercury
measured from perfect vacuum

The zero point is:

0 mm Hg absolute
= perfect vacuum

At standard atmospheric pressure:

760 mm Hg absolute
≈ atmosphere

2. Smaller Number Means Deeper Vacuum

Unlike the in. Hg vacuum scale:

760 mm Hg absolute
→ atmosphere

100 mm Hg absolute
→ vacuum

25 mm Hg absolute
→ deeper vacuum

1 mm Hg absolute
→ very deep vacuum

0 mm Hg absolute
→ perfect vacuum

Therefore:

For mm Hg absolute, a smaller number means a deeper vacuum.

3. Do Not Confuse in. Hg Vacuum With mm Hg Absolute

These are not simply two versions of the same scale.

They use different:

• Units.
• Reference directions.

Compare:

in. Hg vacuum
→ starts at atmosphere
→ increases toward perfect vacuum

mm Hg absolute
→ starts at perfect vacuum
→ increases toward atmosphere

This distinction is especially important in EPA 608 material because some evacuation requirements are expressed using absolute pressure rather than a compound-gauge vacuum reading.

Microns

1. Meaning

In refrigeration vacuum work, the term micron normally means:

micron of mercury

or one micrometer of mercury column absolute pressure.

The basic relationship is:

1 mm Hg
= 1,000 microns

Therefore:

1 micron
= 0.001 mm Hg

2. Microns Are an Absolute-Pressure Scale

Micron readings are referenced to perfect vacuum.

Therefore:

0 microns
= perfect vacuum

and:

higher micron value
= higher absolute pressure

At standard atmospheric pressure:

760 mm Hg
= 760,000 microns

3. Smaller Micron Number Means Deeper Vacuum

Examples:

100,000 microns
→ relatively poor vacuum

10,000 microns
→ deeper vacuum

1,000 microns
→ much deeper vacuum

500 microns
→ deep vacuum

100 microns
→ deeper still

0 microns
→ perfect-vacuum limit

Therefore:

For microns, lower is deeper.

4. Converting mm Hg Absolute to Microns

Because:

1 mm Hg = 1,000 microns

then:

0.5 mm Hg
= 500 microns

and:

25 mm Hg
= 25,000 microns

5. Converting Inches of Mercury Absolute to Microns

Since:

1 in. = 25.4 mm

then:

1 in. Hg absolute
= 25.4 mm Hg absolute
= 25,400 microns

This relationship is useful when comparing vacuum scales.

Standard Pressure Reference Table

The following values use standard sea-level atmospheric pressure as an instructional reference.

The values involving atmospheric pressure are approximate and depend on local barometric pressure. A conventional compound gauge normally expresses below-atmospheric pressure in inches of mercury vacuum rather than negative psig.

Comparing Vacuum Scales

Inches of Mercury Vacuum

Reference:

Atmosphere = zero

Direction:

Higher number
→ deeper vacuum

Millimeters of Mercury Absolute

Reference:

Perfect vacuum = zero

Direction:

Lower number
→ deeper vacuum

Microns

Reference:

Perfect vacuum = zero

Direction:

Lower number
→ deeper vacuum

Quick Comparison

This comparison is a high-priority exam concept.

Compound Gauge Versus Micron Gauge

1. Compound Gauge

A compound gauge can indicate:

• Positive low-side pressure.
• Vacuum below atmospheric pressure.

Its vacuum portion is commonly calibrated in:

in. Hg vacuum

It is useful for:

• General low-side service.
• Observing whether a system has entered vacuum.
• Rough vacuum indication during recovery or evacuation.

2. Limitation Near Deep Vacuum

Near the deep-vacuum region, many different absolute pressures are compressed into a very small portion of the compound-gauge scale.

For example, at standard sea-level reference:

1,000 microns
and
500 microns

both correspond to a compound-gauge indication extremely close to:

29.9 in. Hg vacuum

The mechanical gauge cannot resolve the difference accurately.

3. Micron Gauge

A micron gauge is designed specifically for deep-vacuum measurement.

It provides much greater resolution at very low absolute pressures.

Therefore:

Compound gauge
→ rough vacuum indication

Micron gauge
→ deep-vacuum measurement

4. 500 Microns as a Study Example

A value of:

500 microns

is commonly used in refrigeration training as a deep-vacuum example.

It equals:

0.5 mm Hg absolute

It should not be interpreted as one universal EPA-required evacuation level for every appliance.

EPA recovery and evacuation requirements vary with appliance category and are developed in later modules.

Pressure Reference and Refrigeration Gauges

1. High-Side Gauge

A high-side manifold gauge normally reads:

psig

because the operating high-side pressure is usually above atmosphere.

2. Low-Side Compound Gauge

The low-side compound gauge can read:

Positive psig

when suction pressure is above atmospheric pressure.

If suction pressure falls below atmosphere, the same gauge can enter its:

in. Hg vacuum

range.

3. Low Side Does Not Automatically Mean Vacuum

As discussed in Section 4.3:

Low side
≠
vacuum

The low side simply means the lower-pressure region of the cycle.

Depending on the refrigerant and application, the low side can operate:

• Above atmospheric pressure.
• Near atmospheric pressure.
• Below atmospheric pressure.

4. Pressure-Temperature Charts Usually Use Gauge Pressure

Many refrigeration service P-T charts used with manifold gauges display saturation pressure in:

psig

The technician must confirm the unit and reference used by the specific chart.

Pressure-temperature relationships are developed in Section 4.7 - Pressure-Temperature Relationships.

Technician-Level Conversion Process

When a pressure problem appears on an exam, use this sequence.

Step 1 - Identify the Unit

Ask:

psig?
psia?
in. Hg vacuum?
mm Hg absolute?
microns?

Step 2 - Identify the Reference

Ask:

Referenced to atmosphere?
or
Referenced to perfect vacuum?

Step 3 - Identify Atmospheric Pressure

If converting between psig and psia:

• Use the atmospheric pressure provided.
• If none is provided and the problem clearly assumes standard sea-level conditions, use approximately:

14.7 psia

Step 4 - Apply the Correct Relationship

For gauge and absolute pressure:

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

For a vacuum difference:

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

Step 5 - Check Whether the Result Makes Physical Sense

Ask:

• Is absolute pressure nonnegative?
• If the system is below atmosphere, is absolute pressure less than atmospheric pressure?
• If vacuum becomes deeper, does the absolute-pressure value decrease?
• Did I confuse inches with millimeters?
• Did I confuse vacuum pressure with absolute pressure?

Worked Examples

Example 1 - Gauge Pressure to Absolute Pressure

A system pressure is:

125 psig

Assume:

P_atm = 14.7 psia

Then:

$$P_{\text{abs}}=125+14.7$$

$$P_{\text{abs}}=139.7\text{psia}$$

Example 2 - Absolute Pressure to Gauge Pressure

A vessel pressure is:

40 psia

Assume:

P_atm = 14.7 psia

Then:

$$P_{\text{gauge}}=40-14.7$$

$$P_{\text{gauge}}=25.3\text{psig}$$

Example 3 - mm Hg Absolute to Microns

A vacuum reading is:

2.0 mm Hg absolute

Since:

1 mm Hg = 1,000 microns

then:

$$2.0\times 1,000=2,000\text{microns}$$

Example 4 - Microns to mm Hg Absolute

A micron gauge reads:

750 microns

Then:

$$750÷1,000=0.750\text{mm Hg absolute}$$

Example 5 - Why 0 psig Is Not Vacuum

A system open to the surrounding atmosphere reads:

0 psig

At standard sea-level pressure:

P_abs ≈ 14.7 psia

Therefore, the system is not at perfect vacuum.

Perfect vacuum would require:

P_abs = 0 psia

Technical Details

1. Absolute Pressure Cannot Be Negative

Perfect vacuum is the lower physical limit:

P_abs = 0

Therefore:

P_abs < 0

is not a physically meaningful pressure for ordinary thermodynamic calculations.

2. Gauge Pressure Can Be Mathematically Negative

If absolute pressure is below atmospheric pressure:

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

becomes negative.

In HVAC practice, a compound gauge commonly displays this region as inches of mercury vacuum rather than as negative psi.

3. Standard Atmosphere Is a Reference, Not a Universal Field Condition

The commonly memorized values:

14.7 psia
29.92 in. Hg
760 mm Hg

describe approximately one standard atmosphere.

Actual atmospheric pressure changes with location and weather.

4. Absolute Mercury Units and Vacuum Mercury Units Must Be Labeled

A value written only as:

25 Hg

is incomplete.

The technician needs to know:

• Inches or millimeters?
• Absolute or vacuum?

For example:

25 mm Hg absolute

and:

25 in. Hg vacuum

describe very different pressures.

5. Microns Are Especially Useful Near Perfect Vacuum

At deep vacuum, absolute pressure is only a tiny fraction of atmospheric pressure.

The micron scale provides useful resolution where:

• psia values become very small.
• in. Hg vacuum readings bunch close to the maximum possible indication.

6. Gauge Zero Can Shift

Mechanical gauges can develop:

• Zero error.
• Calibration error.
• Mechanical damage.

Before using a gauge for important service decisions:

• Check its condition.
• Verify zero as appropriate.
• Follow manufacturer calibration procedures.

A gauge that does not return properly to zero can produce misleading pressure readings.

Important Terms

Absolute Pressure

Absolute pressure is pressure referenced to perfect vacuum. It is commonly expressed as psia, mm Hg absolute, or microns, depending on the pressure range.

Atmospheric Pressure

Atmospheric pressure is the pressure exerted by the surrounding atmosphere. Standard sea-level atmospheric pressure is approximately 14.7 psia, 29.92 in. Hg absolute, or 760 mm Hg absolute.

Barometer

A barometer is an instrument used to measure atmospheric pressure.

Gauge Pressure

Gauge pressure is pressure referenced to surrounding atmospheric pressure and is commonly expressed as psig.

Inches of Mercury Vacuum

Inches of mercury vacuum is a vacuum scale indicating how far pressure is below atmospheric pressure. A larger reading generally indicates a deeper vacuum.

Micron

A micron, in refrigeration vacuum work, is one micrometer of mercury absolute pressure. One millimeter of mercury equals 1,000 microns.

Millimeters of Mercury Absolute

Millimeters of mercury absolute is an absolute-pressure scale referenced to perfect vacuum. Lower values indicate deeper vacuum.

Perfect Vacuum

A perfect vacuum is the theoretical condition of zero absolute pressure.

psia

psia means pounds per square inch absolute.

psig

psig means pounds per square inch gauge.

Vacuum

A vacuum exists when system absolute pressure is below the local atmospheric pressure.

EPA 608 Exam Focus

What Students Must Remember

• psig is referenced to atmospheric pressure.
• psia is referenced to perfect vacuum.
• At standard sea level:

0 psig
≈ 14.7 psia

• 0 psig is not a perfect vacuum.
• 0 psia is the perfect-vacuum limit.
• Basic conversion:

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

• Standard atmospheric reference:

14.7 psia
≈ 29.92 in. Hg absolute
≈ 760 mm Hg absolute
≈ 760,000 microns

• On an in. Hg vacuum scale:
  • Larger number → deeper vacuum.
• On a mm Hg absolute scale:
  • Smaller number → deeper vacuum.
• On a micron scale:
  • Smaller number → deeper vacuum.
• 1 mm Hg = 1,000 microns.
• 1 in. Hg = 25.4 mm Hg = 25,400 microns.
• A compound gauge gives only a rough deep-vacuum indication.
• A micron gauge is used for accurate deep-vacuum measurement.
• Do not confuse:
  • in. Hg vacuum.
  • mm Hg absolute.
• Low side does not automatically mean vacuum.
• Atmospheric pressure changes with elevation and weather.

High-Priority Comparison Table

Typical Exam Question Patterns

Students may be asked to:

• Convert psig to psia.
• Convert psia to psig.
• Identify the reference point for gauge pressure.
• Identify the reference point for absolute pressure.
• Explain why 0 psig is not a vacuum.
• Identify the standard sea-level atmospheric pressure.
• Distinguish in. Hg vacuum from mm Hg absolute.
• Convert mm Hg to microns.
• Determine which direction represents a deeper vacuum on each scale.
• Identify the correct instrument for deep-vacuum measurement.
• Explain why atmospheric pressure affects a vacuum-gauge reading.

Common Mistakes and Confusing Points

Mistake 1: Treating 0 psig as Zero Pressure

Correct:

0 psig
= atmospheric pressure

At standard sea level:

0 psig
≈ 14.7 psia

Mistake 2: Treating 0 psia as Atmospheric Pressure

Correct:

0 psia
= perfect-vacuum limit

Mistake 3: Forgetting to Add Atmospheric Pressure

For positive gauge pressure:

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

Do not simply change the unit label from psig to psia.

Mistake 4: Assuming Atmospheric Pressure Is Always Exactly 14.7 psia

14.7 psia is a standard sea-level approximation.

Actual atmospheric pressure varies.

Mistake 5: Thinking a Smaller in. Hg Vacuum Reading Means Deeper Vacuum

The opposite is true.

For in. Hg vacuum:

larger number
→ deeper vacuum

Mistake 6: Thinking a Larger Micron Reading Means Deeper Vacuum

The opposite is true.

For microns:

smaller number
→ deeper vacuum

Mistake 7: Confusing 25 mm Hg Absolute With 25 in. Hg Vacuum

They use different units and different reference directions.

Always read the full unit.

Mistake 8: Using a Compound Gauge to Verify 500 Microns

A compound gauge does not have enough resolution for accurate deep-vacuum measurement.

Use a micron gauge.

Mistake 9: Assuming the Low Side Must Be Under Vacuum

Many refrigeration systems operate with low-side pressure above atmospheric pressure.

Mistake 10: Forgetting That Microns Are an Absolute-Pressure Unit

Micron readings are referenced to perfect vacuum.

They are not simply another version of psig.

Concept-Check Questions

Question 1

What does a pressure reading of 0 psig indicate?

A. Perfect vacuum

B. Zero absolute pressure

C. Pressure equal to the surrounding atmospheric pressure

D. A pressure of exactly 760 microns

Question 2

At standard sea-level atmospheric pressure, a system gauge reads 20 psig. What is the approximate absolute pressure?

A. 5.3 psia

B. 20 psia

C. 29.92 psia

D. 34.7 psia

Question 3

Which pressure value represents the theoretical perfect-vacuum limit?

A. 0 psig

B. 0 psia

C. 14.7 psia

D. 760 mm Hg absolute

Question 4

Which statement correctly describes an inches-of-mercury vacuum scale?

A. A larger in. Hg vacuum reading generally indicates a deeper vacuum.

B. A smaller in. Hg vacuum reading always indicates a deeper vacuum.

C. The scale is referenced to perfect vacuum and increases toward atmosphere.

D. 0 in. Hg vacuum is the same as 0 psia.

Question 5

Which statement correctly describes millimeters of mercury absolute?

A. It is referenced to local atmospheric pressure and increases toward perfect vacuum.

B. It is the same as psig.

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

D. It can never be converted to microns.

Question 6

A vacuum gauge reads 2.5 mm Hg absolute. What is the corresponding pressure in microns?

A. 2.5 microns

B. 250 microns

C. 1,000 microns

D. 2,500 microns

Question 7

Which instrument is best suited to verifying a deep refrigeration-system vacuum?

A. Micron gauge

B. High-side pressure gauge

C. Ordinary compound gauge only

D. Refrigerant cylinder scale

Question 8

Which statement best explains why 0 psig is not a perfect vacuum?

A. Gauge pressure is referenced to absolute zero temperature.

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

C. Gauge pressure cannot be measured below 100 psig.

D. Perfect vacuum is defined as 14.7 psia.

Answers and detailed explanations will be provided in 4.11 - Answers and Explanations.md.

Section Summary

Pressure must always be interpreted with its reference point.

Gauge pressure:

psig
→ referenced to atmosphere

Absolute pressure:

psia
→ referenced to perfect vacuum

The fundamental relationship is:

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

At standard sea-level atmospheric pressure:

0 psig
≈ 14.7 psia
≈ atmospheric pressure

Perfect vacuum is:

0 psia

Vacuum units behave differently:

in. Hg vacuum:
larger number → deeper vacuum

mm Hg absolute:
smaller number → deeper vacuum

microns:
smaller number → deeper vacuum

Important conversions include:

1 mm Hg
= 1,000 microns

1 in. Hg
= 25.4 mm Hg
= 25,400 microns

A compound gauge can show that a refrigeration system is below atmospheric pressure, but it cannot accurately verify a deep vacuum.

For deep-vacuum measurement:

Use a micron gauge.

The next section uses pressure measurements together with refrigerant saturation data.

See Section 4.7 - Pressure-Temperature Relationships.

References

Project Source

1. Current EPA Section 608 teaching-material project outline, Module 4 — Refrigeration Cycle Components Gauges and Pressure-Temperature Relationships, Section 4.6. Required scope: psig, psia, atmospheric pressure, inches of mercury vacuum, millimeters of mercury absolute, microns, why zero psig is not a perfect vacuum, and the gauge-to-absolute pressure relationship.

EPA 608 Teaching Reference

2. International Training Institute for the Sheet Metal and Air Conditioning Industry, EPA Section 608 Study Guide. The glossary defines psia as absolute pressure with 0 psia corresponding to a perfect-vacuum condition and identifies 14.7 psia as approximately 0 psig at standard atmospheric pressure. The guide also uses inches of mercury, millimeters of mercury absolute, and microns in refrigeration recovery and evacuation discussions.

HVAC Technical References

3. Edward G. Pita, Air Conditioning Principles and Systems: An Energy Approach, 4th ed., Chapter 2, “Physical Principles.” The text defines absolute, gauge, atmospheric, and vacuum pressure; gives the relationship between absolute, gauge, and atmospheric pressure; and identifies approximately 14.7 psia, 29.92 in. Hg, and 760 mm Hg as standard atmospheric-pressure equivalents.

4. Rex Miller and Mark R. Miller, HVAC Licensing Study Guide, 3rd ed., pressure and psychrometric fundamentals. The text explains that pressure instruments measure a pressure difference, identifies gauge pressure as referenced to atmosphere, identifies psia as including atmospheric pressure, and discusses barometers and mercury pressure measurement.

5. Justin Kauwale, Mechanical PE: HVAC & Refrigeration Textbook, 2025 ed., refrigeration and pressure fundamentals used as a supplemental reference for refrigeration pressure interpretation and vacuum-service concepts.

6. NCEES, PE Mechanical Reference Handbook, Version 2.0, pressure and refrigeration reference material used as a supplemental engineering cross-check.