Long-term flow measurement: an assessment of the Siemens Servo 900B expiratory flowmeter

The accuracy and linearity of three Siemens Servo 900B expiratory flowmeters have been assessed by comparison with a dry gas meter. Their performance was reassessed after 792,966 and 950 h respectively. The effects of changes in respiratory rate, relative humidity and oxygen concentration on the meters were studied. It was found that the meters were accurate and linear and that reliable measurements could still be made after long periods of continuous use. The meters were found to be less accurate at low respiratory rates. Relative humidity had little effect on flow measurement. Changes in oxygen concentration, and hence viscosity, led to errors of up to 16% in flow measurement.