The Conductance Bottleneck
A vacuum pump's nameplate speed tells only half the story. The pipe connecting pump to chamber has a finite conductance — its ability to pass gas molecules — and the effective pumping speed at the chamber is always less than the rated pump speed. This series combination, Seff = Sp × C / (Sp + C), is the most important equation in practical vacuum system design. Doubling the pump speed helps little if the pipe remains the bottleneck.
Throughput and Gas Load
Throughput Q = Seff × P represents the volumetric flow rate of gas at a given pressure. In steady state, throughput equals the total gas load from outgassing, leaks, and process gas. Understanding throughput helps size pumps correctly — you need enough throughput to handle all gas sources while maintaining the target pressure.
Pumpdown Dynamics
Evacuating a chamber from atmospheric pressure follows an exponential decay: pressure drops as P(t) = P₀ × exp(−Seff × t / V). The time constant V/Seff determines how fast pressure falls. Below about 0.01 mbar, outgassing from chamber walls typically becomes the dominant gas source, and pumpdown slows dramatically compared to the ideal exponential model.
Practical System Design
Experienced vacuum engineers follow the rule: make the pipe as short and wide as possible. Conductance scales as the cube of diameter in molecular flow, so doubling pipe diameter increases conductance eightfold. This simulation visualizes the pump-pipe-chamber system and shows how conductance limitation affects your actual pumping performance in real time.