# What equations can help me understand Convectron gauges?

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In summary, the conversation discusses the differences in pressure readings from a Convectron gauge when measuring nitrogen gas (calibrated) and argon gas (requires correction). At 10 torr indicated pressure, the true pressure of argon should be around 200, which is a factor of 20 higher than nitrogen. The thermal conductivity and heat capacity of the gases may contribute to this difference, with nitrogen having a higher heat capacity due to its diatomic nature. Other factors such as gas density may also play a role. However, the speaker acknowledges that they are not an expert in this area and their explanation may not be entirely accurate.

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?

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.

## 1. What is the equation for convectron gauges?

The equation for convectron gauges is P = K * (P1 - P2) / (P1 + P2), where P is the pressure measured by the gauge, K is a constant determined by the gauge's design, P1 is the pressure inside the gauge, and P2 is the pressure outside the gauge.

## 2. How do I calculate pressure using a convectron gauge?

To calculate pressure using a convectron gauge, use the equation P = K * (P1 - P2) / (P1 + P2), where P is the pressure measured by the gauge, K is a constant determined by the gauge's design, P1 is the pressure inside the gauge, and P2 is the pressure outside the gauge.

## 3. What is the purpose of the constant K in the convectron gauge equation?

The constant K in the convectron gauge equation is a calibration factor that takes into account the specific design and sensitivity of the gauge. It allows for accurate and consistent pressure measurements.

## 4. How do I determine the constant K for my convectron gauge?

The constant K for a convectron gauge can be determined through calibration. This involves comparing the pressure measurements of the gauge to a known reference pressure and adjusting the K value until the gauge reads the correct pressure. This process can be repeated multiple times at different pressure levels to ensure accuracy.

## 5. Can the convectron gauge equation be used for all pressure ranges?

The convectron gauge equation can be used for a wide range of pressures, typically from about 10^-3 to 10^3 mbar. However, it may not be accurate for extremely high or low pressures, as the sensitivity and behavior of the gauge may change outside of this range.