Is Evaporative Cooling Causing Pressure Drop in Vacuum Chamber?

[kawikdx225]

I am having a problem at work and I'm hoping some of the brains on this board can help.

I'm using a Focused Ion beam to induce insulator deposition using a siloxane liquid. The liquid is stored in a small container with a valve to let the gas escape the container. This small container is located inside a vacuum chamber.

The vacuum chamber has a pressure of 1.5e-6 millibar when the siloxane valve is closed.

When the valve is opened I see the pressure rise to 8.0e-6 which is where I want to perform deposition.

As I watch the pressure I notice a slow linear drop with time. This effect causes the deposition process to fail.

As far as I know the chamber pressure is determined by the vapor pressure of the liquid and the size of the valve(which also has a limiting aperture in it)

My questions are:
1. Is this pressure drop due to the temperature of the liquid dropping due to evaporative cooling? This drop in temperature would also drop the vapor pressure right?

2. Is there a formula to calculate the vapor pressure vs temperature, or to calculate how fast the liquid will cool in the presence of a vacuum? or some other formula to help me characterize this effect.

3. Could the gauge used to measure the pressure level become inaccurate in the presence of an insulator like siloxane? It's a hot cathode Ionization gauge.

4. Is there something else that could be going on that I haven't mentioned.

Thanks for any help

 

[S = k log w]

Would Boyle's law's be of any help?

 

[kawikdx225]

I don't know, where do I find information on this law?

Thanks

 

[S = k log w]

I had found several citations when I ran a google search.