A method of determining the accuracy and repeatability of leak testing instrumentation comprises the following steps: providing a two chamber vessel having an access port and a flow controlling reference orifice associated with each chamber and a third reference orifice communicating between the two chambers, providing a leak testing device and connecting such leak testing device first to one of such ports, pressurizing the associated chamber and, with the associated orifice open, observing and recording the pressure measured by the leak testing device under test as a function of time. The second test repeats this activity with the other chamber and the other orifice. A third test is undertaken with the third orifice open. One of the chambers is smaller and incorporates a smaller orifice and the other chamber is larger and incorporates a larger orifice thus achieving leak testing under different conditions.