Leak rate qL (mbar · l · s–1)
A leak rate of 1 mbar.l/s is present when in an enclosed, evacuated vessel with a volume of 1 liter, the pressure rises by 1 mbar per second or, where there is positive pressure in the container, pressure drops by 1 mbar.
According to the definition formulated above it is easy to understand that the size of a gas leak, i.e. movement through undesired passages or “pipe” elements, will also be given in mbar · l · s^–1). A leak rate is often measured or indicated with atmospheric pressure prevailing on the one side of the barrier and a vacuum at the other side (p < 1 mbar). If helium (which may be used as a tracer gas, for example) is passed through the leak under exactly these conditions, then one refers to “standard helium conditions”.