Can pump-down time be calculated? The short answer is yes. But the longer answer is: this is an iterative process involving exponential decay and manual calculation can be involved and tedious, even using a spreadsheet program.
Over the years, computer algorithms have been developed for iterative techniques needed to calculate conductances, throughputs, gas loads, effective pumping speeds, and pump-down times across continuum, transitional, and molecular flow regimes using the available formulas from vacuum technology. Typically, the user keys in the pump's pumping speed curve, chamber dimensions, surface outgassing rates, start/finish pressure, etc., and the calculations are done automatically with the program choosing the appropriate formula at each iteration.
We have used successive updates of the VacTran® program (from Professional Engineering Computations) for more than 17 years. We find it invaluable and, of the programs we have examined, the easiest to use and most versatile.
VacTran is particularly valuable during system re-design. The existing system's measured pump-down characteristics determine the real gas load which is used for the model. The designer then makes the proposed design changes on the model and re-runs pump-down iterations. Since the model's gas load reflects reality, the calculated results closely parallel real-world experience when the re-design change are made.
If you have a one-time question to which VacTran can be applied, try our TechInfo Service request page and we may be able to help. If you are facing vacuum design issues that involve a number of “what if” conditions, we urge you to buy this program. But note, successful computer modeling depends on the nature of the problem and the operator's understanding of vacuum technology.