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A gas load is the rate at which gas enters or is released into a vacuum system, expressed as pressure-volume per time at a stated temperature. Contributions include leaks, outgassing and intentional gas injection.

For a chamber at fixed volume $V$ and temperature, $V\dot{p}=Q-Sp$ balances a constant load against pumping. The steady pressure is $p=Q/S$. Reducing the load can therefore improve the pressure as effectively as increasing the pumping speed.^[ref]

Outgassing is gas release from materials or their surfaces into the surrounding space. In a vacuum chamber it contributes to the gas load, even when the seals have no leak. Adsorbed water and dissolved gases can have very different release times.

The relation $p\approx Q/S$ shows why surface preparation and material choice matter: increasing pumping speed addresses removal, while reducing outgassing addresses the source. A pressure-rise measurement with pumping isolated estimates total release only after accounting for leaks and temperature changes.^[ref]

The pumping speed at a specified inlet is the gas throughput divided by the inlet pressure: $S=Q/p$. Its units are volume per time, commonly litres per second. A pump's speed depends on gas species and operating conditions.

Connecting a pump of speed $S_p$ through a line of vacuum conductance $C$ gives $1/S_{\mathrm{eff}}=1/S_p+1/C$ in the linear flow regime. Thus a large pump cannot compensate for an arbitrarily restrictive connection.^[ref]

A laboratory vacuum is a region with substantially reduced gas pressure, rather than an experimentally perfect absence of matter. Lowering the number density increases the mean free path, allowing a molecular beam to travel with fewer collisions. In molecular flow, the mean free path exceeds the apparatus dimensions, so wall encounters dominate.

A vacuum pump competes with leaks and outgassing. For an isothermal vessel of volume $V$, constant gas load $Q$ in pressure-volume per unit time, and effective pumping speed $S$, gas balance givesThis ideal model predicts a pump-down time constant $V/S$ and a limiting pressure $Q/S$. A restricted connecting pipe reduces the effective pumping speed; a larger pump alone need not fix it. Measuring the pressure transient tests the model, while deviations can reveal changing outgassing or pumping speed.^[ref]

A vacuum pump lowers gas pressure by removing gas from a vessel or trapping it so that it no longer contributes to the free gas population. Mechanical displacement, momentum transfer and capture are distinct operating mechanisms.

Its relevant performance includes pumping speed, attainable pressure and gas dependence. The pressure achieved in an apparatus also depends on connecting vacuum conductance, leaks and outgassing; the pump's advertised limit is therefore not a complete system specification.^[ref]