Summary of Flow Measurement

Certain problems are associated with every method of measuring flow. Because not all gases are pure with a known, constant density, flow meters based on the Bernoulli Equation 5 are subject to errors. Furthermore, any device inserted into a fluid flow can disturb the flow by its presence. For example, a rotating vane spirometer may reduce the gas flow at high flow rates because of friction. Several methods of flow or fluid velocity measurement are dependent on a pressure measurement (Pitot tube, Venturi tube, orifice flow meter); hence, the flow measurement is no more accurate than the pressure measurement.

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.)

Figure 30-22 Vane spirometer. Moving gases contain kinetic energy, which can be sampled by a rotating "windmill" in the gas stream.^[13] (Adapted from Ehrenwerth J, Eisenkraft J: Anesthesia Equipment: Principles and Applications. St Louis, Mosby-Year Book, 1993.)