The validity of wireless iButtons® and thermistors for human skin temperature measurement

Skin temperature is a fundamental variable in human thermo-physiology, and yet skin temperature measurement remains impractical in most free-living, exercise and clinical settings, using currently available hard-wired methods. The purpose of this study was to compare wireless iButtons and hard-wired thermistors for human skin temperature measurement. In the first of two investigations, iButtons and thermistors monitored temperature in a controlled water bath (range: 10-40 degrees C) and were referenced against a certified, mercury thermometer. In the second investigation, eight healthy males completed three randomized trials (ambient temperature = 10 degrees C, 20 degrees C and 30 degrees C) while both devices recorded skin temperature at rest (in low and high wind velocities) and during cycle-ergometry exercise. The results are as follows. Investigation 1: both devices displayed very high validity correlation with the reference thermometer (r > 0.999). Prior to correction, the mean bias was +0.121 degrees C for iButtons and +0.045 degrees C for thermistors. Upon calibration correction the mean bias for iButtons and thermistors was not significantly different from zero bias. Interestingly, a typical error of the estimate of iButtons (0.043 degrees C) was 1.5 times less than that of thermistors (0.062 degrees C), demonstrating iButtons' lower random error. Investigation 2: the offset between iButton and thermistor readings was generally consistent across conditions; however, thermistor responses gave readings that were always closer to ambient temperature than those given by iButtons, suggesting potential thermistor drift towards environmental conditions. Mean temperature differences between iButtons and thermistors during resting trials ranged from 0.261 degrees C to 1.356 degrees C. Mean temperature differences between iButtons and thermistors during exercise were 0.989 degrees C (ambient temperature = 10 degrees C), 0.415 degrees C (ambient temperature = 20 degrees C) and 0.318 degrees C (ambient temperature = 30 degrees C). Observed error estimates were within the acceptable limits for the skin temperature method comparison, with typical errors 0.9 and CV <1% under all conditions. These findings indicate that wireless iButtons provide a valid alternative for human skin temperature measurement during laboratory and field investigations particularly when skin temperature measurement using other currently available methods may prove problematic.