The thermopile: An anisotropic temperature sensor

This article discusses the operation of thermopiles when used under non-isothermal baseline conditions. Although isothermal baselines are the common thread among existing thermopile applications, this article demonstrates that these sensors can also be used to detect subtle thermal events that are superimposed over much larger steady-state or transient temperature gradients. The larger temperature variation will not contribute to the transduced voltage as long as specific thermal symmetry conditions are matched with a specific thermopile design. This article discusses this sensing anisotropy that corresponds to certain thermopile designs, and experimental results are presented which demonstrate the relative performance under several thermal conditions. Thermopile performance characteristics of baseline voltage, baseline drift and noise, and signal sensitivity have been compared experimentally for isothermal and high gradient conditions. When symmetry conditions were met, certain thermopile geometries exhibited a sensing anisotropy of more than 1000:1.