Reducing thermal transient induced errors in thermopile sensors in ear thermometer applications

Abstract

In ear thermometers and similar applications, thermopile sensors have to face a challenging thermal environment. The steady-state sensor temperature as well as the change of sensor temperature, heat flow, and thermal gradients have a significant impact on the sensor's reading. As opposed to the well-known steady-state sensor temperature (so-called “ambient temperature”), the effects of thermal transients have not jet been understood well in the past. They occur when the sensor warms up while the thermometer is inserted into the ear. They have to be treated in addition to the traditional (steady state) ambient temperature compensation. Isothermal packaging of the sensor significantly reduces the error due to thermal transients. This can be combined with mathematical prediction of the remaining error. Based on an analytical thermal model, we show that the error can be represented by a series of derivatives of the sensor housing temperature. Numerical calculation and compensation of the error in only first order combined with an isothermal package allows for an error reduction by the factor of 30.