How to size a vacuum pump and estimate pump down time
Two questions come up first on any vacuum chamber project: how big a pump is needed, and how long the chamber takes to reach the target pressure. Enter chamber volume, pump displacement and the starting and target pressures to get the theoretical pump down time, a realistic time with an allowance applied, and the displacement required to hit a time target.
The governing relation is t = (V ÷ S) × ln(P₀ ÷ P). Doubling chamber volume doubles the time, but pressure enters through a logarithm, so pulling ten times deeper adds only about 2.3 times the time. Going from atmospheric pressure at 760 torr down to 1 torr gives a natural log of about 6.6, which is why a 10 ft³ chamber on a 5 CFM pump takes roughly 13 minutes in theory.
Real systems run slower than that. Piping and valve conductance reduce the effective speed at the chamber below the pump nameplate, and small leaks plus outgassing from chamber walls add load. That is why an allowance factor of 1.2 to 2.0 is applied, and why short, large diameter connections matter more than most people expect.
This estimate assumes isothermal conditions and constant pumping speed. In the rough and medium vacuum range, actual speed varies considerably with pressure, so check the manufacturer pumping speed curve, and treat high vacuum processes as a separate analysis.
Frequently asked questions
Generally yes, provided the connecting line has enough conductance. With long, narrow tubing the effective speed is limited by the piping, so a bigger pump alone will not deliver the gain you expect.
A new, tight system with short piping is fine at about 1.2. Chambers with a leak history, moist loads or long runs of tubing are safer at 1.5 to 2.0.