The operating principle of the SKC film flowmeter is simple: the sampling pump to be calibrated is attached to the top of a volumetric glass tube containing a small amount of liquid soap. As the pump draws air through the tube, a flat soap bubble is interposed in the flow path. While the air flow causes the soap film to move from one volume mark to another, the travel time is measured with a stopwatch. The flow rate can then be directly calculated using the travel time and the known tube volume, or it can be read from the flow chart. The test is quick and easy to perform. With a soap film flowmeter, flow rates are based upon the inherent, unchanging properties of the measuring apparatus.