Dear Marton and Roberto,
While comparing a simulation of a chamber with a continuous influx of gas and measurements of the actual chamber, I have encountered a mismatch in pressures.
The chamber in question is the last in a series of three volumes, each of which is equipped with a turbopump and separated from the next by a small aperture. There is a source of gas in the first chamber, and a detector in the last chamber that measures particles in the beam. I have simulated the system with each turbopump modeled as a facet with the correct pumping speed, in this case 350 l/s, at the location of the first rotor blades. I modeled the tubulation on which each pump is mounted and the internal shape of the turbopump as well, so any reduction in pumping speed should be accounted for. In the real setup, all three turbos are connected to a shared forevacuum line which a scroll pump evacuates.
The curious behavior is as follows. When I set a particular flow rate of hydrogen into the first chamber (using a mass flow controller in the lab, or desorption of mass-2 particles from a single facet in the simulation), some pressure arises in chamber #3. That happens to be about 2e-7 hPa as measured by an ion gauge set for nitrogen mounted out of the beam path on the real chamber. I recorded the pressure on the equivalent facet in the simulation and the pressure was the same to within a factor of a few. The base pressure of the real chamber is well below the 5e-10 hPa floor of the ion gauge, and to match that pressure in simulation with zero flow into the gas source, I had to set an outgassing rate of ~ 5e-11 hPa.L/s.cm^2.
Then, in the lab, I added a fourth turbopump of 220 l/s in series with turbopump #3, between it and the forevacuum line. With this additional turbo in place and the same hydrogen flow, the pressure measured by the ion gauge dropped to 1.4e-9 hPa!
My question is how to properly model this behavior. The rated speed of the turbopumps is 350 l/s, which is what I set for each turbopump facet. I could increase the pumping speed of the facet representing turbopump #3 in an ad-hoc way to match the measured pressure, but since the objective of the simulation is to test design options before investing in new hardware, I would like to figure out how to match our measurements from first principles in simulation.
One other possibility is that the shared forevacuum line was applying a high forevacuum pressure to turbopump #3 and decreasing its pumping speed. Unfortunately, the maufacturer does not supply data on the pumping speed vs. forevacuum pressure. However, that would suggest that a simulation with the nominal pumping speed, as obtained with a low forevacuum pressure after adding turbopump #4, should achieve the 1.4e-9 hPa pressure, and that a reduced pumping speed in the simulation should produce the 2e-7 hPa pressure. On the contrary, with a simulated speed of 350 l/s, the simulated pressure more or less matched the single-turbopump measurement of 2e-7 hPa.
This suggests the compression ratio of the turbopumps for hydrogen is playing a role. Turbopump #3 has a compression ratio of 1e6 for hydrogen, and turbopump #4's is 5e5. When the chamber pressure is 1.4e-9 hPa, that suggests a forevacuum pressure after turbopump #4 of 700 hPa, rather too high in comparison with the maximum specified forevacuum pressure of 8.5 hPa for turbopump #4 with H2. On the other hand, the single-turbopump case gives a forevacuum pressure of 1.4e-3 hPa, which is about one-tenth of the rated ultimate pressure for the scroll pump. Clearly there is some interplay among these factors, and I would like your advice on how to set up my simulation to accurately represent the behavior of pumps working with hydrogen at UHV pressures.
I would be happy to share more details on the setup or my simulations, if that would be helpful. Thanks again for building such a useful tool, and I look forward to learning how to use it more effectively.
Best,
Alec