Figure 30-21 Bourdon tube flow meter. Unlike the Thorpe tube, which has constant pressure but a variable orifice, the Bourdon tube has a constant orifice but variable pressure. The tube uncoils under the high backpressure. This drawback makes it unsuitable for use in low-pressure respiratory systems. However, it receives much use in portable oxygen tanks. Of note, if the orifice is increased in radius, the flow meter will under-read actual flow. If dirty (i.e., decreased radius), the flow will be overestimated. (From Mushin WW, Jones PL: Physics for the Anaesthetist, 4th ed. Oxford, Blackwell, 1987.)