Direct reading flowmeters , even those obtained with a direct gas calibration , need to be checked when initially installed and also at regular intervals during testing by a method capable of accurately measuring the absolute gas flow at the burner tube . One way of doing this is to connect the burner tube with a short length of tubing ( about 7 mm inside diameter 〕 to a soap bubble flowmeter , such that the upward passage of a soap film meniscus in a glass tube of calibrated volume ( e.g. a burette ) over a known period of time gives an absolute measurement of the flow