If we use different gases in constant volume gas thermemetre why does the reading vary plzz explain

The known properties of gases allow for the accurate measurement of temperature using a constant-volume thermometer. Gas is sealed and kept at a constant volume. As temperature changes, the volume or pressure of the gas changes. The amount of pressure required to maintain a constant volume of the gas indicates temperature.

consider ideal gas: PV=nRT,T=PV/nR. n,V and R are constants which is the same for all gases. In addition, the equation is obeyed under all conditions. For a real gas van der Waal's equation states that: (P+(an²/V²)(V-nb)=nRT. each gas has a different value of a and b. This is the reason why the reading varies.