Field calculation tool · Gas systems

Gas pressure–temperature correction

What a sealed enclosure reads at one temperature, expressed at another

Gas pressure in a sealed enclosure changes with ambient temperature at constant density. A reading taken on a cold morning is not comparable with a rated filling pressure quoted at 20 °C until it has been corrected. Enter what you measured and the temperature you measured it at.

Correction

Absolute pressure
Gauge = absolute − atmospheric. Site gauges read gauge pressure.
Gas temperature, not necessarily air temperature.
Set either field to 20 °C to work to or from the standard reference.
Standard 1.013 bar at sea level. Lower it for altitude if gauge values matter.
Pressure at 0 °C — absolute
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Pressure at 0 °C — gauge
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01

Across the temperature range

The same gas charge, expressed at each ambient temperature. Useful for setting alarm and lockout thresholds, or for checking whether a reading taken in the field is consistent with the last one.

TemperatureAbsoluteGauge
02

Method and scope

A sealed enclosure holds a fixed mass of gas in a fixed volume, so density is constant and pressure varies with absolute temperature alone:

p₂abs = p₁abs × (T₂ + 273.15) / (T₁ + 273.15)

Temperatures are converted to kelvin before the ratio is taken, and pressures are handled as absolute throughout. Gauge values are derived at the end by subtracting the atmospheric pressure you entered.

What this assumes

  • Sealed, constant volume. No leakage and no top-up between the two conditions. If the enclosure is leaking, the correction is meaningless — that is what it is useful for detecting.
  • Gas temperature, not air temperature. After a load change, or in direct sun, the gas inside can differ from ambient by several degrees. Let the enclosure settle before reading.
  • Ideal gas behaviour. Below about 10 bar absolute, dry air, nitrogen, oxygen and carbon dioxide deviate from the ideal relation by well under one percent — smaller than the uncertainty of a field gauge. The correction is therefore effectively independent of which of those gases is in the enclosure.
  • Single phase. The relation holds only while all of the gas stays gaseous.

This tool does not cover SF6 at service density. SF6 deviates substantially from ideal gas behaviour at circuit breaker filling pressures, and can partially liquefy at low ambient temperature — at which point pressure stops tracking temperature at all. Use the manufacturer's density curve for SF6 equipment.

Using it

  • Field reading to rated value. Enter the measured pressure and the temperature it was taken at, set the target to 20 °C, and compare the result against the nameplate.
  • Rated value to expected field reading. Enter the rated pressure at 20 °C and set the target to today's ambient, to know what the gauge should read before you decide something is wrong.
  • Checking a trend. Correct successive readings to the same reference temperature before comparing them. Uncorrected readings taken in different seasons are not comparable, and a slow leak hides easily inside that variation.

This calculator is provided for reference and cross-checking. It does not replace the equipment manufacturer's filling curves, nameplate data or operating instructions. Confirm any value that will be used to make an operational decision. Version 1.0.

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About HVPACE

HVPACE is the trading name of Power Asset Condition Engineering Limited, a New Zealand-based supplier of test and diagnostic instruments and selected specialty equipment for high-voltage power systems, backed by hands-on engineering expertise in condition assessment and diagnostics.

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HVPACE is the trading name of Power Asset Condition Engineering Limited (NZBN 9422724), Auckland, New Zealand.