I was looking at reading HVAC gauges and I noticed there are rings on the inside that indicate the temperature of the inside gas. These translate directly to the pressure of the gas.

HVAC gauge by JB

So if your gas was R-134a and the temperature inside was 72° F, the pressure would be roughly 74 PSI.

If you were to let out some gas, and allow the chamber to return to the same temperature, would the pressure not drop because you have decreased the quantity of gas?

The ideal gas relation is: $$ PV = nRT $$

But if you let out some gas you've just decreased n. Therefore shouldn't the pressure drop? Is there a point where this stops being true?


This gauge measures refrigerants.

In refrigeration cycles, you have the refrigerant (R-507/404A/134a) in a closed container.

The refrigerant is often in a liquid-vapor mixture in quasi-static equilibrium. There is enough liquid that if you were to lose a bit of it from a leak, more would evaporate to keep the vapour pressure essentially the same, therefore you can use the vapour pressure and the ideal gas law to determine the temperature of the fluid.


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