What equations can help me understand Convectron gauges?

[madcow]

Hello,

I am currently trying to understand the physics behind the vastly different pressures read from a Convectron gauge depending on the type of gas.

At the moment I am looking at Nitrogen gas (which the gauge is calibrated to read) and Argon which requires a significant correction once pressures go above approximately 1 torr and convection comes more into play.

At approximately 10 torr indicated pressure, the true pressure of Argon should be approximately 200 according to the indicated vs. true pressure curve given in the GP275 manual. Where does this factor of 20 difference come from between the two gasses?

Looking at the thermal conductivity of the two gasses, at 300K there is approximately difference of 8 between them. This would explain some of the difference but not everything I am looking for.

Does the fact that Nitrogen is diatomic come into play?

Thank you for any help you can provide.

 

[Drakkith]

Good question. From what I've looked up, a Convectron is a type of Pirani gauge, wherein the pressure measurement depends on the cooling effect of the gas on a heated wire. The different thermal conductivity between nitrogen and argon should indeed be a partial explanation. I believe the heat capacity of the gases also plays a part. Nitrogen has about twice the heat capacity of argon, probably due to its diatomic nature, which may explain some of the difference. Other things like the density of the gases probably has an effect as well.

Note that I am not an expert in this area, so I could be incorrect.