BTPS Correction for Ceramic Flow Sensor: Conclusion

With FVCs of 5 to 6 L and with an initial sensor temperature of 20°C, the heat transfer after two consecutive trials is sufficient to raise the surface temperature of the sensor to a level at which little additional heat transfer occurs. This heat slowly dissipates with time, with the sensor returning to ambient temperature within approximately 20 min. The fact that the sensor temperature rapidly rises with each successive FVC maneuver probably explains why water condensation has not affected test results.
Our results also suggest that because of this heating of the sensor, a dynamic BTPS correction factor is needed to accurately estimate forced expiratory volumes. The error introduced by using a fixed BTPS correction factor was dependent on the number of maneuvers previously conducted on the sensor, the time interval between the maneuvers, the volume of the current and past maneuvers, and the initial temperature of the sensor. Link Because of the number of different parameters influencing the effective temperature or BTPS correction factor, some additional measure of sensor temperature other than ambient temperature is needed. Use of exit air temperature may provide a means of estimating a dynamic BTPS correction factor necessary to provide an FEVi accuracy of less than ± 3 percent for exit air temperatures from 5 to 28°C. However, the results using human subjects suggest that exit air temperature alone may not completely eliminate the difference due to maneuver order (Fig 1). Specifically, exit air temperature may slightly overestimate the sensor temperature for the first maneuver and slightly underestimate sensor temperature for subsequent maneuvers.
Our results using the mechanical simulator assumed that the temperature of the forced exhaled breath is approximately 37°C. However, the temperature of exhaled air may not be 37°C.

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