Temperature varying in vacuum chamber during pressure adjust

[Soho]

Hello everybody, I am working on some tests on materials and structures in a vacuum chamber.
I noted that during the final moments of the pressure rising (when I open the valve to let the air in), temperature sensors installed in the chamber record the temperature rising more than ten degrees celsius above the previous constant temperature, before dropping to the previous equilibrium values.
The same, but reverse, applies when the pump is activated to extract the air inside, and it happens in the first minutes of operations, while the pump is extracting the major part of the molecules inside the chamber.
I found a thread here about the Joule-Thompson expansion for real gas. Is this the case?
Can the air extraction from a vacuum chamber be thought as a free expansion of the real gas inside?

 

[Bystander]

Where are the sensors located? And what temperatures are being measured?

 

[Soho]

Oops, the late hour made me forget some useful details about the test:
The temp sensors are fixed to a thin metallized foil that is similar to an aluminum foil.
There are two sensor on each face of the foil sample, plus there are two other sensors which are fixed to a shroud located inside the chamber.
The temperature variation is especially visible for the sensors on the foil, because the chamber itself has an high thermic inertia, so you would need a lot more heat to have the same temperature difference.

 

[Bystander]

I was hoping for a high-low arrangement of sensors to establish existence or absence of temperature gradients. The foil is obviously giving you air T as it circulates in the oven/to the vacuum line. No sensor on the valve for venting? My suspicion is that it runs hotter than the interior of the oven, and you get to see how much hotter when you return the oven to atmospheric P.

 

[sardar]

Hello,

Thank you for sharing your observations with us. It is not uncommon for temperature to vary during pressure adjustments in a vacuum chamber. This is due to the Joule-Thompson effect, which is a phenomenon that occurs when a gas expands into a lower pressure environment.

In a vacuum chamber, when the air is let in or extracted, the pressure inside the chamber changes. This leads to a change in temperature as well. When the air is let in, the pressure increases and the temperature rises due to the compression of the gas molecules. Similarly, when the air is extracted, the pressure decreases and the temperature drops due to the expansion of the gas molecules.

This effect can also be observed in the first few minutes of operation when the majority of the air is being extracted from the chamber. This is because the initial extraction leads to a rapid decrease in pressure, causing a sudden drop in temperature.

In this scenario, it is accurate to think of the air extraction as a free expansion of the real gas inside the chamber. However, it is important to note that the Joule-Thompson effect is only applicable to real gases and not ideal gases.

I hope this helps to clarify your observations. Keep up the good work in your experiments!