There are essentially two types of spirometers: those measuring volume directly and those measuring and integrating flow to determine volume of air. The flow type spirometer, because of its small size, is particularly well suited in situations where portability is important. An unheated ceramic flow sensor, one type of flow sensor frequently used to determine flow and calculate expiratory volume, is particularly well suited for use in battery-operated spirometers. These ceramic flow sensors do not need to be heated-a process that consumes considerable battery power read more . As they become available, these portable devices will likely have wide clinical application in the assessment of asthma, providing much useful information in addition to peak expiratory flow (eg, flow-volume curves and FEVis). With any flow or volume measurement, it is usually necessary to correct values to body temperature pressure saturated (BTPS). Most methods of correcting volumes to BTPS assume that expired air immediately cools to ambient temperature. The BTPS correction factor is based on ambient temperature, and to a lesser extent, barometric pressure. Although some cooling of the air occurs as the air passes through most ceramic flow sensors, cooling is usually not complete, and a BTPS correction factor somewhat less than the factor based on ambient temperature is required. The BTPS correction technique most frequently recommended by ceramic flow sensor manufacturers is to apply a static factor, based on room temperature, approximately equal to 30 percent of the full BTPS correction factor2—assuming only partial cooling of the air. While using a constant BTPS correction factor may be adequate in some situations, we have observed that FVC and FEVi values from the first maneuver are usually lower than those for subsequent maneuvers (Fig 1). This trend was first suspected when an inordinate proportion of tested subjects, using a ceramic sensor, had difficulty satisfying the American Thoracic Society (ATS) FVC and FEVi reproducibility criteria (5 percent), primarily because of differences between the first and second maneuvers. Since one possible explanation for this observation was inappropriate BTPS correction, we investigated BTPS correction factor techniques in unheated ceramic flow sensors, using a mechanical lung simulator filled with either room air or air heated to 37°C and saturated with water vapor. Figure 1. Average difference between first maneuver’s FVC and FEVi values vs subsequent maneuvers for six subjects. Open symbols (UA) represent values obtained with constant BTPS correction factor and filled symbols (AD) represent results using dynamic BTPS correction factor.