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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]

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]