Abstract: A pneumatic system for comparing air flow characteristics of different unknown flow restrictions. The first head or piece of equipment and a base orifice are installed in turn on the flowbench and air flow rates are measured for each using a critical flow venturi nozzle. The base orifice flow rate is referred to as p.sub.1. The first head or piece of equipment flow rate is referred to as p.sub.2. A small orifice is identified which equates in flow rate to the first head or piece of equipment, p=p.sub.2. A flow coefficient can be calculated were R.sub.1 =p.sub.2 /p.sub.1. A second head or piece of equipment, for comparison, is then flow tested at the same location or at another location under different atmospheric conditions. The ratio of the first condition flow rates p.sub.2 /p.sub.1 multiplied by the base orifice flow rate at condition two, p.sub.3, allows calculation of a theoretical flow rate for the first small orifice (or first head or piece of equipment) at condition two, p.sub.4.