Dear Marton Ady and Pascal Rene Baehr,
I have a question for you.
I want to simulate the effect of the valve disc of the pump on the transmission probability through Molflow. I set the outgassing as 100 mbar l/s and the sticking factor of the pumping facet as 0.9. Stroke denotes the position of the valve disc. I’ve set two positions, 10 cm and 30 cm. But both have the same transmission probability(0.99999). Did I make a mistake? Could you give me some advice?
thank you
Hello:
the formula you’re using is not correct, that’s why you get the same value.
The transmission probability requires that both the outgassing surface and the pumping one have sticking=1, and your formula should read A#/sumdes, where # is the facet number of the pumping facet (the one you have with s=0.9).
sumabs is always equal to sumdes (minus the number of molecules running at any time, which is the number of processes you have on your computer, visible in the “Tools” “Global settings” window), so it is normal that you get 0.999999, i.e. 1 (you have no leaks).
Also, did you set the facet representing the head of the movable valve as 2-sided?
Cheers,
Roberto
Hello Roberto,
Thank you very much. And I have some questions for you:
Is this simulation process correct? why is the transmission probability so small. Could you give me some advice?
Hello, let me chip in with answers:
Yes. In everyday terms, transmission probability is the chance that a molecule gets from A to B in a vacuum system. For this, both A and B must be valid exits for molecules (sticking=1). In your first attempt, since you had sticking=1 at B only, all molecules exited at B (hence the ~100% transmission you got).
2-sided facets reflect hits from both sides, more info here: Documentation: Facet parameters | Molflow+
No, that is the molecules desorbed from 0 to 5E-11s, not relevant for your static simulation. Molflow simulates by default steady state, in your case the field above shows that it simulates 1.68E22 molecules desorbed per second, not to be confused with the virtual molecules that are actually traced in the system.
The simulation is almost correct, but you have to set the disc to two-sided (1st screenshot).
The 4% transmission probability seems more or less correct.
What did you try to achieve with the formula sum(a,s2)? It means “the sum of all absorptions on facets belonging to selection group 2”.
Did you deliberately change the temperature over your system (273K vs 200K)?
Hello, Marton
Thank you for your answer.
Should the sticking factor of the pumping facet be set as 0.9 if the value of sticking coefficient is 0.9 for hydrogen?
Should the temperature of the pumping facet be set as 4.5 K if the temperature of the actual pumping surface is 4.5 K?
Q: What did you try to achieve with the formula sum(a,s2) ?
A:I wanna calculate the transmission probability with multiple adsorption surfaces.
Q: Did you deliberately change the temperature over your system (273K vs 200K)?
A: yes
Hello Roberto,
I have two questions. Would you like to let me know how to deal with them?
Hello again.
Just a side remark that there is a tutorial video about conductance and transmission probability calculation with Molflow: Molflow webinar - Conductance (part 1 - theory and straight pipes) - YouTube
I’m writing this because Molflow will give you a conductance/transmission result, but you have to know what you have just calculated to interpret it. In short:
Transmission probability: mathematical probability that a molecule gets from A to B.
Usually - but not necessarily - we calculate it if A is an emitting surface and both A and B have sticking=1.
Conductance is a vacuum concept, mainly used in the vacuum equation Q=dp*C, where C is conductance. It can be calculated by multiplying the entrance conductance with the transmission probability.
Answering your questions: The temperature has no effect on the transmission probaility, so setting 4.5K is fine. Setting 1 or 0.9 on the sticking facet depends on what you want to answer: the probability of transmission to reach the sticking surface (sticking=1) or the probability of sticking (s=0.9).
Thank you for your answer. I will study hard.
