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Vacuum Decay Test, Micron and Absolute Pressure Calculator

Three tools in one: check whether an evacuated system held its vacuum, convert vacuum readings between units, and convert gauge to absolute pressure at your elevation.

Method reviewed October 26, 2026. Version 1.0.0. How it works

Your inputs

Unit system

Minutes. New York’s Clean Heat checklist uses at least 15 minutes unless the manufacturer specifies otherwise.

Use the manufacturer’s target if it gives one; the New York Clean Heat checklist uses 500 microns.

The New York Clean Heat checklist calls for investigation if the vacuum rises by more than 200 microns. Use the manufacturer’s figure if it gives one.

Result

Held

Rise during the hold
120microns
Average rate of rise
8microns/min

120 microns rise over 15 minutes — held.

Rise per 10 minutes
80microns
Final absolute pressure
56Pa
Show the working
  1. Rise = final − start
    420 − 300 = 120 microns
  2. Rate = rise ÷ hold time
    120 ÷ 15 min = 8 microns/min
  3. Checks
    final 420 ≤ 500: yes; rise 120 ≤ 200: yes

The decay (standing vacuum) test

After evacuating, the technician isolates the pump and watches the micron gauge. New York’s Clean Heat checklist evacuates to 500 microns or the manufacturer’s target, holds for at least 15 minutes, and calls for investigation if the reading rises by more than 200 microns.

A reading that keeps rising steadily suggests a leak. One that rises and then levels off suggests moisture or refrigerant still boiling off inside the system.

Microns, Torr and inches of mercury

A micron of mercury is a thousandth of a millimetre of mercury, or 0.001 Torr — about 0.133 Pa. Micron, Torr and pascal readings are absolute: zero is a perfect vacuum.

Inches of mercury vacuum on a compound gauge are measured down from the local atmosphere instead, so the same absolute pressure reads differently at different elevations. The converter uses the standard atmosphere at the elevation you enter.

Gauge and absolute pressure at altitude

Absolute pressure is gauge pressure plus the local atmospheric pressure, about 14.7 psia at sea level and 12.2 psia at 5,000 feet. Refrigerant property tables use absolute pressure, which is why altitude shifts the gauge reading for a given saturation temperature.

Worked example

These are the example values the calculator opens with, and the result the engine calculates for them.

Inputs

Calculate
Vacuum decay (standing) test
Reading when the pump was isolated
300 microns
Reading at the end of the hold
420 microns
Hold time
15
Highest acceptable final reading
500 microns
Largest acceptable rise during the hold
200 microns

Result

Rise during the hold
120 microns
Average rate of rise
8 microns/min
Rise per 10 minutes
80 microns
Final absolute pressure
56 Pa

120 microns rise over 15 minutes — held.

Assumptions

  • 1 micron of mercury = 0.001 Torr = 0.1333224 Pa; in. Hg is the conventional inch of mercury (3,386.389 Pa).
  • Local atmospheric pressure from the standard atmosphere at the entered elevation (weather changes it by a few percent).

Limitations

  • Micron gauges are most accurate near the bottom of their range; readings above a few thousand microns are approximate.
  • Evacuation targets and hold times come from the equipment manufacturer. Only certified technicians may open refrigerant circuits (EPA Section 608).

Frequently asked questions

How many microns is a good vacuum?

Follow the manufacturer’s target. New York’s Clean Heat checklist uses 500 microns or the manufacturer’s target, held for at least 15 minutes.

How many Pa is 500 microns?

About 66.7 Pa, or 0.5 Torr.

Why does my compound gauge read less than 29.92 in. Hg at full vacuum?

At altitude the atmosphere is lower, so even a perfect vacuum reads less in inches of mercury below atmosphere.

Sources

  1. Recommended Air Source Heat Pump Commissioning Checklist — NYS Clean Heat Statewide Heat Pump Program (New York). Pressure test to the manufacturer’s specification and check the change against the allowable variance; evacuate to 500 microns or the manufacturer’s target, hold at least 15 minutes, and investigate if the vacuum rises by more than 200 microns.
  2. NIST Guide to the SI (SP 811), Appendix B.8 — factors for units listed alphabetically — National Institute of Standards and Technology. To pascals: inch of water, conventional 249.0889; inch of mercury, conventional 3,386.389; millimeter of mercury, conventional 133.3224; torr 133.3224; psi 6,894.757; bar 1 × 10⁵; millibar 100; standard atmosphere 101,325.
  3. PsychroLib — psychrometric functions after ASHRAE Handbook—Fundamentals (2017) ch. 1 — PsychroLib (open source, MIT licence). Humidity ratio W = 0.621945·pw/(p − pw), wet-bulb, enthalpy and standard-atmosphere equations as published by ASHRAE.

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