The percent difference in FEVi, obtained using room vs heated-humidified air (proportional to the magnitude of BTPS correction factor needed), ranged from 0.3 percent to 6.2 percent and varied with the number of maneuvers previously performed, the time interval between maneuvers, the volume of the current and previous maneuvers, and the starting temperature of the sensor there health and care mall. Correspondingly, the temperature of the air leaving the sensor (exit) temperature) showed a steady rise with each successive maneuver using heated air. When six subjects performed repeated tests over several days, a maneuver order effect was observed similar to the results obtained using the mechanical pump.
One possible explanation for the larger FEVi and FEVe values, with subsequent FVC maneuvers, is the buildup of water condensation within the sensor, rather than a warming of the sensor’s surface or change in BTPS correction factor. We considered, but rejected, this explanation for several reasons. In experiment 4, we cooled a sensor to below 0°C overnight and the following day injected 20 consecutive FVC maneuvers using heated-humidified air. At the completion of these maneuvers, we detected some water condensation on the aluminum housing but none within the ceramic element of the sensor. Additionally, there is a rise in exit air temperature and a corresponding decrease in the percent difference with each consecutive FVC maneuver, until a plateau is reached. This initial rise and plateau (shown in Fig 3) suggest that the sensor has reached an equilibrium in temperature approximately equal to the air passing through. Water condensation should not be significant since the sensor and air temperatures are approximately equal.
Our results using room air vs heated-humidified air are best explained by a transfer of heat from the air to the ceramic element of the flow sensor as the air passes through the sensor. The amount of heat transferred to the sensor is proportional to the volume of air that passes through the sensor.