I am trying to use Molflow to calculate the ultimate pressure of a future chamber. While also using molflow to match the data from an existing chamber as proof of me doing it right.
I don’t really know what outgassing rate to give the existing chamber, it’s been baked at an unknown temperature (less than 300 C), multiple times. With just a turbo on it, the cold cathode is at low -10 Torr. It is made out of 304 stainless. The textbook I have gives 304 stainless steel baked for 30h at 250C an outgassing rate of 400x10^11 pa*m/s.
The plan for the new chamber is for it to be made out of 316L and Vacuum fired at 800C. In my textbook, the outgassing for that is listed as 46x10^-11 Pa*m/s. But unless I’m wrong, that sounds too high?
In articles I see 316L baked at 400C have an outgassing rate from anywhere between 5e-14 Torr to 8e-12 Torr L/s cm^2. I understand length of the bake would cause it to differ, but it seems all over the place. Even when accounting for time.
I looked through the forum a bit and saw a recommendation for the CERN accelerator school event page for outgassing data but the link wasn’t working for me. Is there a better source I should be looking at? Apologies that my units are all over the place as well.
I have texture on some facets for the pressure scale, and I have my pumping speed set up, so assuming I finally understand outgassing, Is there anything else that you can recommend me to do to figure out the ultimate pressure?
Hello: unless you have measured the outgassing of a sample of your steel to which you have applied exactly the bakeout temperature and duration of your complete system there’s not much use of trying to find the “right” value for the specific outgassing rate. The one you cite and call it too high is very high indeed for baked steel, so do not use it.
If you can assume that the outgassing is the same everywehere, in mbar*l/s/cm2, then you can use any value you like in your simulation, if later you compare it to measurements.
This is so because in molecular flow pressures are linear with outgassing, you double the latter and you double the former.
So just run the simulation with a “reasonable” value for the outgassing rate, then compare the result of the simulation with the measurement, putting representative facet in your Molflow+ model where the gauges are, and then you should be able to find out a scaling factor that makes the two match, i.e. calculated to measured. After that, you can derive the outgassing rate of your complete system, by multiplying the value you set in the Molflow+ simulation times the factor you just determined.
Try doing that and, if you don’t mind, come back here and tell us whether it worked or not, it’s an interesting thing.
There is one more bit of the equation that I left out. The cold cathode is at its max. So, the pressure in the existing chamber, isn’t known. Apologies.
I’d assume, that due to the chamber never getting vacuum fired (or treated in a different way), and that it is just a turbo on it, it wouldn’t be that much lower…
Doing what you described did give me a much clearer picture of what the outgassing could be. I have the outgassing at 8e-12 (mbar*l/s/cm^2) and it is slightly below where the gauge stopped.