BTPS calibration of heated Fleisch pneumotachometer.

The Fleisch pneumotachometer (PTM) gives the instantaneous respiratory gas flow, and when integrated provides the volume of gas displaced. Its output signal is proportional to the product of the flow and the viscosity of the gas. The calibration factor is thus different for inspiration and expiration. Since the exact value of the viscosity is unknown, accurate figures for flow cannot be obtained. This study examines the use of a precisely sinusoidal pump with adjustable speed and capacity, displacing air in a thermostatically controlled chamber containing water at 37 degrees C stirred by a propellor. Thus air at ambient temperature flows through the PTM on inspiration, and on expiration the gas exactly simulates in temperature and humidity that normally expired by a human subject. This makes the output signal asymmetrical, with the expired volume VTE being greater than the inspired value, the ratio VTE/VTI = 1.035. Other sources of error, notably temperature and pressure changes in the chamber and differences in the proportions of O2 and CO2 in the expired gas, have been considered from both a theoretical and experimental standpoint. Their combined effects produce a less than 0.5% error. Using this pump, the Fleisch PTM can be calibrated empirically without making any assumptions about the temperature and viscosity of the expired gas mixture.