Vacuum Pump Evacuation (Pump-Down) Time

Each decade of pressure costs the same time as the last, so getting from 760 torr to 76 takes as long as getting from 76 to 7.6 -- which is why a system that seemed fast in the first minute takes an hour to reach its setpoint, and why the instinct built on the first thirty seconds is always wrong. A 15 cu ft chamber on a 25 cfm pump reaches 1 torr from atmosphere in about 4 minutes, spread evenly across 2.88 decades at 1.38 minutes each, and going one decade further to 0.1 torr costs another 1.38 -- the same as the first decade, which took the pressure from 760 down to 76. The pump's rated speed is not the speed at the chamber. The connecting line and its fittings have a finite conductance, and on a long or narrow line the effective speed can be a fraction of the rating, so the conductance efficiency is entered here rather than assumed at one. LEAKAGE SETS AN ULTIMATE PRESSURE that no amount of pumping time will beat: the leak rate divided by the effective speed. If that ultimate sits BELOW the target the target is reachable and the leak only costs time near the end; if it sits ABOVE the target, the system will never get there and more time is wasted time. A pump-down curve that flattens out short of the target is therefore a leak MEASUREMENT, and the flattening pressure times the pumping speed is the leak rate. This is the ideal isothermal volume relation. It assumes the pump holds its rated speed across the whole pressure range, which no real pump does -- speed falls off near the ultimate, so real pump-downs run longer than this at the low end. It does not model outgassing from chamber walls and elastomer seals, which dominates below roughly 1e-3 torr and which no volume calculation captures; it does not handle water vapour load, which is the usual reason a chamber that pumped down fine yesterday is slow today; and it does not size a pump, select a pump type for a pressure range, or evaluate a trap or foreline. The pump manufacturer's speed curve and the system designer govern.

evacuation time t = (V / S) ln(p1 / p2), so each decade costs 2.303 V / S and the total is spread evenly across log10(p1/p2) decades; the leak-limited ultimate pressure = leak rate / effective pumping speed, where effective speed = rated speed x the line's conductance efficiency.

The isothermal volume pump-down relation as standard vacuum practice, by name. Real pumps lose speed near their ultimate, so this runs optimistic at the low end. The pump manufacturer's speed curve and the system designer govern.

A logarithm on a volume and a pumping speed; no pump curve is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: chamber_volume_ft3, pump_speed_cfm, start_pressure_torr, target_pressure_torr, leak_rate_torr_cfm, conductance_efficiency, evacuation_minutes, minutes_per_decade, decades, ultimate_pressure_torr, blocking_leak_torr_cfm

• Rated speed is assumed constant real pumps lose speed near the ultimate, so this runs optimistic low downthe pump manufacturer's speed curve
• Outgassing is not modelled it dominates below roughly 1e-3 torrvacuum practice
• Water vapour load is not modelled it is the usual reason a chamber slower today than yesterdayvacuum practice