Throughput, leak rate, and the rate-of-rise method
Using throughput units and the pressure rise test to quantify leaks and gas loads.
Throughput Q is defined as pressure times volumetric flow. In SI units it is expressed in Pa·m³/s, while traditional vacuum units use mbar·L/s or Torr·L/s. These units are all equivalent ways of quantifying gas load or leak rate.
Standard flow units such as sccm (standard cubic centimeters per minute) can be converted to throughput using NIST factors. Under a common STP definition, 1 sccm ≈ 0.0168875 mbar·L/s ≈ 0.001689 Pa·m³/s. This allows process recipes written in sccm to be interpreted directly as vacuum gas loads.
In a pressure rise (rate-of-rise) test, the chamber is isolated from the pump and the pressure increase from p₁ to p₂ over Δt is measured. With known volume V, the total gas load or leak rate is q_L = V·(Δp/Δt). Expressed in mbar and liters, q_L has units of mbar·L/s. This simple method is widely used to quantify leaks and outgassing without specialised leak detectors.
If the effective pumping speed S_eff of the system is known, an approximate ultimate pressure can be estimated as p_ult ≈ q_L / S_eff. This relationship helps distinguish between problems dominated by insufficient S_eff (piping or pump issues) and those dominated by excessive gas load (leaks, outgassing).
Our Throughput & Leak Rate Converter implements these conversions and the rate-of-rise calculation, making it easy to move between sccm, mbar·L/s, Torr·L/s, and Pa·m³/s.