Gas Laws

Pressure test today. Check it tomorrow. See how much of the pressure change is explained by temperature, and compare it with your actual gauge reading.

Calculate the expected follow-up pressure first. Add the actual gauge reading when you have it to compare. You can also select any one unknown to solve for it.

What do you want to calculate? Fixed vessel — volume stays the same

For the same vessel, volume cancels out. Uncheck this to enter different volumes; selecting a volume to solve also enables them.

01 / Test startGauge pressurepsi Gas temperature°F Barometric pressureinHg

02 / Follow-up readingGauge pressure (optional)psi Gas temperature°F Barometric pressureinHg

Expected follow-up gauge pressure

—psiExpected if temperature and atmospheric pressure explain the change

Actual follow-up gauge pressure: — psi
Actual minus expected: — psi
Actual change from test start: — psi
Apparent change in gas amount: — %

How does this check work?

For a fixed vessel, expected absolute pressure follows P₂ = P₁ × T₂ / T₁. Gauge readings are converted to absolute pressure by adding the local barometric pressure. Temperatures are converted to an absolute scale before calculating.

For example, a lower gauge reading the next morning can be explained partly or entirely by cooler gas. Compare that expected reading with what the gauge actually shows. A negative actual-minus-expected result means the measured pressure is below the prediction.

If volume changes, the calculation also includes V₁ / V₂. The variable solver rearranges this same relationship for one unknown. Apparent gas amount change uses both measured readings: n₂ / n₁ = P₂ V₂ T₁ / (P₁ V₁ T₂).

Use the gas temperature. Ambient temperature is an approximation after the gas has equilibrated with its surroundings. These ideal-gas calculations do not establish a leak cause or a pass/fail limit.

Start the test. Check it later.