11.2 - Acronyms Symbols Units and Conversions

Module: Standalone Reference Appendices and Instructor Resources
Purpose: Fast reference for the abbreviations, symbols, pressure/vacuum scales, units, and conversions used throughout the EPA Section 608 course
Project integration review date: August 14, 2026
Source-status note: This file consolidates terminology and numerical relationships already used in Modules 1–10. It does not replace the controlling regulatory tables in the applicable instructional sections.

How to Use This Reference

Use this file when an abbreviation, symbol, or unit appears unfamiliar, or when two pressure or vacuum scales seem to conflict.

For exam preparation, remember:

NUMBER
+
UNIT
+
REFERENCE
+
CONDITION

A numerical value without its unit and reference can be misleading.

Examples:

25 mm Hg absolute
≠
25 in. Hg vacuum

0 psig
≠
0 psia

4 in. Hg vacuum
≠
4 in. Hg absolute

The most important pressure/vacuum rule is:

Always identify what the zero point of the scale represents before comparing two readings.

1. Master Acronym and Abbreviation Reference

1.1 Federal Regulatory and Program Terms

High-Priority Distinction

SECTION 608
→ primarily stationary refrigeration and air conditioning

SECTION 609
→ MVAC service for consideration

1.2 Refrigerant-Family and Environmental Terms

Family Memory Aid

CFC / HCFC
→ chlorine present
→ nonzero ODP

HFC / HFO
→ no chlorine
→ zero ODP

Zero ODP does not mean zero GWP, zero flammability, zero toxicity, or permission to vent.

1.3 HVAC/R Organizations, Standards, and Safety Terms

Historical terminology warning: Older EPA 608 materials may use ARI where current material uses AHRI. Treat this as a terminology change, not as two unrelated organizations.

1.4 Common Equipment and Application Abbreviations

2. Symbols Used Throughout the Course

2.1 Pressure Symbols

The fundamental relationship is:

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

At approximately standard atmospheric pressure:

$$P_{\text{abs}}\approx P_{\text{gauge}}+14.7\text{psi}$$

The 14.7 psi value is an approximation for standard atmospheric pressure, not a universal local atmospheric pressure.

2.2 Temperature Symbols

For zeotropic blends:

SUPERHEAT
→ use DEW temperature

SUBCOOLING
→ use BUBBLE temperature

2.3 Superheat and Subcooling Shorthand

Superheat:

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

For a zeotropic blend:

$$SH=T_{\text{actual vapor}}-T_{\text{dew}}$$

Subcooling:

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

For a zeotropic blend:

$$SC=T_{\text{bubble}}-T_{\text{actual liquid}}$$

2.4 Recovery-Cylinder Symbols

For the standard project 80% training calculation:

$$W_{\text{refrigerant,max}}=0.80\cdot WC$$

$$W_{\text{gross,max}}=TW+0.80\cdot WC$$

$$W_{\text{available}}=W_{\text{gross,max}}-W_{\text{gross,current}}$$

A lower applicable manufacturer or transportation filling limit controls if it is more restrictive than the project’s 80% training calculation.

2.5 Comparison and Mathematical Symbols

3. Master Unit Reference

3.1 Pressure and Vacuum Units

Direction Memory Aid

in. Hg VACUUM
DEEPER VACUUM
→ NUMBER GOES UP

psia / mm Hg absolute / microns
DEEPER VACUUM
→ NUMBER GOES DOWN

3.2 Mass and Weight Units

Exact course conversion:

$$1\text{lb}=16\text{oz}$$

Therefore:

$$\text{ounces}=\text{pounds}\cdot 16$$

$$\text{pounds}=\frac{\text{ounces}}{16}$$

3.3 Temperature Units

Temperature conversion:

$$T_{{}^{\circ }\text{C}}=\left(T_{{}^{\circ }\text{F}}-32\right)\cdot \frac{5}{9}$$

$$T_{{}^{\circ }\text{F}}=T_{{}^{\circ }\text{C}}\cdot \frac{9}{5}+32$$

For temperature differences, do not add or subtract 32:

$$\Delta T_{{}^{\circ }\text{C}}=\Delta T_{{}^{\circ }\text{F}}\cdot \frac{5}{9}$$

$$\Delta T_{{}^{\circ }\text{F}}=\Delta T_{{}^{\circ }\text{C}}\cdot \frac{9}{5}$$

3.4 Time and Area Units Used in Regulatory Conditions

No additional area conversion is required for the EPA 608 questions in this course.

4. Gauge Pressure, Absolute Pressure, and Atmospheric Pressure

4.1 Gauge Pressure

Gauge pressure uses the surrounding atmosphere as zero.

Therefore:

0 psig
→ pressure equal to local atmosphere

It does not mean there is no pressure.

At standard atmospheric conditions:

0 psig
≈ 14.7 psia

A pressure gauge attached to an appliance and open to atmosphere normally reads approximately:

0 psig

because the pressure inside and outside the gauge sensing element is the same.

4.2 Absolute Pressure

Absolute pressure uses perfect vacuum as zero.

Therefore:

0 psia
→ theoretical perfect vacuum

Absolute pressure cannot be negative.

The relationship is:

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

At standard atmosphere, a gauge reading of $20\text{psig}$ corresponds approximately to:

$$P_{\text{abs}}=20+14.7$$

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

4.3 Atmospheric Pressure

Atmospheric pressure is the pressure exerted by the surrounding atmosphere.

A commonly used standard-atmosphere approximation is:

1 standard atmosphere
≈ 14.696 psia
≈ 760 mm Hg absolute
≈ 29.92 in. Hg absolute
≈ 760,000 microns

For routine EPA 608 calculations, the course commonly rounds:

14.696 psia
→ 14.7 psia

Actual local atmospheric pressure changes with:

• Elevation.
• Weather.
• Barometric conditions.

Therefore, atmospheric-referenced vacuum values should not be treated as exact absolute-pressure conversions unless the atmospheric pressure is known or a standard-atmosphere approximation is explicitly being used.

5. Vacuum Scales

5.1 Inches of Mercury Vacuum

An in. Hg vacuum reading normally tells how far pressure has fallen below local atmospheric pressure.

At local atmospheric pressure:

0 in. Hg vacuum

As the vacuum becomes deeper:

4 in. Hg vacuum
10 in. Hg vacuum
15 in. Hg vacuum
...

the number increases.

Under a standard atmosphere, the theoretical limit is approximately:

29.92 in. Hg vacuum

Real field readings are affected by local barometric pressure.

5.2 Millimeters of Mercury Absolute

mm Hg absolute is referenced to perfect vacuum.

Therefore:

760 mm Hg absolute
≈ standard atmosphere

and:

25 mm Hg absolute
→ much lower absolute pressure
→ deeper vacuum than atmospheric pressure

As the vacuum becomes deeper:

mm Hg absolute goes DOWN

The current Type III Table 1 endpoint emphasized throughout this course is:

25 mm Hg absolute

5.3 Microns

A micron in HVAC vacuum work means one micrometer of mercury column on an absolute-pressure basis.

The course relationship is:

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

Therefore:

$$25\text{mm Hg absolute}=25,000\text{microns}$$

and:

$$500\text{microns}=0.5\text{mm Hg absolute}$$

Why This Matters

The regulatory low-pressure refrigerant-removal endpoint:

25 mm Hg absolute
=
25,000 microns

is not the same as a deep-dehydration target such as:

500 microns

The two values represent very different absolute pressures and different purposes.

6. Why Unlike Vacuum Units Cannot Be Compared Directly

Consider the values:

10 in. Hg vacuum

and:

10 mm Hg absolute

The fact that both numbers are 10 does not make the pressures similar.

The scales use different:

• Units.
• Zero points.
• Directions.

Inches Hg Vacuum

ZERO
→ atmosphere

DEEPER VACUUM
→ larger number

Millimeters Hg Absolute

ZERO
→ perfect vacuum

DEEPER VACUUM
→ smaller number

Therefore:

Never compare vacuum readings by the numerical value alone. First identify the unit and whether the scale is atmospheric-referenced or absolute.

7. Standard-Atmosphere Vacuum Interpretation Table

The following table is a teaching conversion based on a standard atmosphere of approximately 29.92 in. Hg absolute and 14.696 psia.

It is useful for understanding the scales. It should not be treated as an exact local field conversion when barometric pressure differs from standard atmosphere.

For a standard-atmosphere approximation:

$$P_{\text{abs,in.Hg}}=29.92-V_{\text{in.Hg vacuum}}$$

For actual field conversion, use the actual local atmospheric/barometric pressure instead of automatically using 29.92.

8. High-Priority Course Conversions

8.1 Pounds and Ounces

The 5 lb value is important because the complete small-appliance definition includes a charge of 5 lb or less.

Charge alone does not determine Type I classification.

8.2 Fahrenheit and Celsius

104°F = 40°C exactly under the standard conversion equation.

8.3 mm Hg Absolute and Microns

This table is especially useful for separating:

• Regulatory refrigerant-removal vacuum levels.
• Deep system dehydration measurements.

9. Pressure and Vacuum Examples

Example 1 - Gauge to Absolute Pressure

Given:

• Gauge pressure = $35\text{psig}$
• Standard atmospheric pressure approximation = $14.7\text{psi}$

Relationship:

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

Substitute:

$$P_{\text{abs}}=35+14.7$$

Result:

$$P_{\text{abs}}=49.7\text{psia}$$

Exam Concept

Do not treat psig and psia as interchangeable.

Example 2 - 0 psig

A refrigerant section is at:

0 psig

Using standard atmospheric pressure for interpretation:

$$P_{\text{abs}}\approx 0+14.7$$

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

Therefore:

0 psig
≠
perfect vacuum

Example 3 - Type III Endpoint in Microns

Given:

$$25\text{mm Hg absolute}$$

Use:

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

Then:

$$25\cdot 1000=25,000$$

Therefore:

$$25\text{mm Hg absolute}=25,000\text{microns}$$

Exam Concept

25 mm Hg absolute is not a typical deep-dehydration endpoint such as 500 microns.

Example 4 - Temperature Conversion

Convert $104{}^{\circ }\text{F}$ to Celsius.

$$T_{{}^{\circ }\text{C}}=\left(104-32\right)\cdot \frac{5}{9}$$

$$T_{{}^{\circ }\text{C}}=72\cdot \frac{5}{9}$$

$$T_{{}^{\circ }\text{C}}=40{}^{\circ }\text{C}$$

This matters because the refrigerant pressure categories used in the course are defined using saturation pressure at:

104°F
=
40°C

Example 5 - Pounds and Ounces

A small appliance contains:

$$4\text{lb}8\text{oz}$$

Convert the ounces:

$$8\text{oz}=\frac{8}{16}\text{lb}$$

$$8\text{oz}=0.5\text{lb}$$

Total charge:

$$4+0.5=4.5\text{lb}$$

The charge is below the 5 lb Type I charge limit, but the appliance must still satisfy the other factory-manufacture, factory-charge, and factory-hermetic-sealing requirements before it qualifies as a small appliance.

10. Recovery-Cylinder Calculation Example

A recovery cylinder is marked:

TW = 28.0 lb
WC = 47.5 lb

Using the standard project 80% training calculation:

$$W_{\text{refrigerant,max}}=0.80\cdot WC$$

$$W_{\text{refrigerant,max}}=0.80\cdot 47.5$$

$$W_{\text{refrigerant,max}}=38.0\text{lb}$$

Maximum gross scale weight:

$$W_{\text{gross,max}}=TW+W_{\text{refrigerant,max}}$$

$$W_{\text{gross,max}}=28.0+38.0$$

$$W_{\text{gross,max}}=66.0\text{lb}$$

If the current scale reading is $52.0\text{lb}$:

$$W_{\text{available}}=66.0-52.0$$

$$W_{\text{available}}=14.0\text{lb}$$

This is the project’s standard 80% training method. An applicable lower manufacturer or transportation filling limit controls actual use.

11. Pressure-Temperature and Superheat/Subcooling Unit Rules

11.1 Pressure-Temperature Tables

Before using a P-T table, identify:

1. Refrigerant.
2. Pressure unit.
3. Gauge or absolute pressure.
4. Temperature unit.
5. Pure/azeotropic or zeotropic refrigerant.
6. Bubble or dew value when the blend has glide.

A pressure-temperature table must not be used correctly unless its pressure reference is understood.

For technician use:

SUPERHEAT
→ saturated vapor
→ DEW for zeotropic blends

SUBCOOLING
→ saturated liquid
→ BUBBLE for zeotropic blends

11.2 Temperature Difference Versus Temperature

A superheat value of:

12°F

means a temperature difference, not an absolute Fahrenheit temperature.

If converting that difference to Celsius:

$$\Delta T_{{}^{\circ }\text{C}}=12\cdot \frac{5}{9}$$

$$\Delta T_{{}^{\circ }\text{C}}\approx 6.7{}^{\circ }\text{C}$$

Do not subtract 32 when converting a temperature difference.

12. EPA 608 High-Priority Unit Associations

The following table is not a substitute for the full regulatory tables. Its purpose is to prevent unit mistakes.

Unit-Association Rule

Do not memorize:

15

Memorize:

15 lb
→ system-dependent recovery-equipment threshold context

or:

15 in. Hg vacuum
→ specified evacuation-table context

or:

15 psig
→ Type III recovery-vessel rupture-disc exam context

The number is the same.

The concept is not.

13. Common Unit and Symbol Mistakes

Mistake 1 - Treating 0 psig as Perfect Vacuum

Incorrect:

0 psig = 0 psia

Correct:

0 psig
→ local atmospheric pressure

At standard atmosphere:

0 psig
≈ 14.7 psia

Mistake 2 - Comparing 25 mm Hg Absolute With 25 in. Hg Vacuum

These are different scales with different zero references.

25 mm Hg absolute
→ absolute pressure

25 in. Hg vacuum
→ pressure reduction below atmosphere

Mistake 3 - Assuming a Larger Absolute-Pressure Number Means a Deeper Vacuum

For:

• psia.
• mm Hg absolute.
• microns.

deeper vacuum means a smaller number.

Mistake 4 - Assuming a Smaller in. Hg Vacuum Number Means a Deeper Vacuum

For in. Hg vacuum, the direction is reversed:

larger reading
→ deeper vacuum

Mistake 5 - Forgetting Local Atmospheric Pressure

The common relationship:

29.92 in. Hg

represents a standard atmosphere.

Actual local barometric pressure can differ.

Do not use 29.92 as an exact field value without understanding that assumption.

Mistake 6 - Converting a Temperature Difference Like an Absolute Temperature

For an absolute temperature:

°F ↔ °C
→ includes the 32°F offset

For a temperature difference:

Δ°F ↔ Δ°C
→ scale factor only

Mistake 7 - Confusing Pounds With Ounces

1 lb = 16 oz

not 10 oz.

Mistake 8 - Treating TW and WC as the Same Cylinder Quantity

TW
→ empty cylinder weight

WC
→ water capacity by weight

For the standard project 80% calculation, use:

0.80 × WC

not:

0.80 × TW

Mistake 9 - Comparing Refrigerant Pressure Categories in psig

The pressure classifications taught in this course use:

psia at 104°F

not psig.

Mistake 10 - Reading a Blend P-T Table Without Bubble/Dew Labels

For a zeotropic blend:

bubble
≠
dew

Use the phase-appropriate value.

14. Fast Exam Memory Sheet

Pressure

psig
→ zero at atmosphere

psia
→ zero at perfect vacuum

Pabs = Pgauge + Patm

standard atmosphere
≈ 14.7 psia

Vacuum

in. Hg vacuum
→ deeper = larger

mm Hg absolute
→ deeper = smaller

microns
→ deeper = smaller

1 mm Hg
=
1000 microns

25 mm Hg absolute
=
25,000 microns

Mass

1 lb
=
16 oz

Temperature

°C = (°F − 32) × 5/9

°F = °C × 9/5 + 32

104°F
=
40°C

Blend P-T Use

SUPERHEAT
→ DEW

SUBCOOLING
→ BUBBLE

Recovery Cylinder

TW
→ tare / empty weight

WC
→ water capacity

maximum refrigerant training weight
→ 0.80 × WC

15. Cross-Reference Guide

For more detail, review:

References

Course Technical and Regulatory Foundations

1. Section 4.6 - Gauge Pressure Absolute Pressure and Vacuum

2. Section 4.7 - Pressure-Temperature Relationships

3. Section 4.8 - Superheat and Subcooling

4. Section 5.6 - Recovery Cylinders

5. Section 5.7 - Evacuation and Dehydration

6. Section 5.9 - Service-Practice Requirements and Exceptions

7. Section 8.2 - Refrigerant Pressure Classifications

8. Section 8.8 - Type II Evacuation Requirements

9. Section 9.9 - Type III Evacuation Requirements

10. Section 10.2 - High-Priority Numbers and Thresholds

11. Section 11.1 - Master EPA 608 Glossary