Temperature sensing with thermochromic liquid crystals

A review of the most recent developments in the application of thermochromi c liquid crystals to fluid flow temperature measurement is presented. The e xperimental aspects including application, illumination, recording, and cal ibration of liquid crystals on solid surfaces, as well as in fluid suspensi ons, are discussed. Because of the anisotropic optical properties of liquid crystals, on-axis lighting/viewing arrangements, combined with in-situ cal ibration techniques, generally provide the most accurate temperature assess ments. However, where on-axis viewing is not possible, calibration techniqu es can be employed, which reduce the uncertainty associated with off-axis v iewing and lighting arrangements. It has been determined that the use of hu e definitions that display a Linear trend across the color spectrum yield t he most accurate correlation with temperature. The uncertainty of both wide -band and narrow-band thermochromic liquid crystal calibration techniques c an be increased due to hysteresis effects, which occur when the temperature of the liquid crystals exceeds their maximum activation temperature. Altho ugh liquid crystals are commonly used to provide time-mean temperature meas urements, techniques are available which allow the monitoring of temporal c hanges. Selected examples illustrating the use of thermochromic liquid crys tals are shown, and a survey of reported temperature measurement uncertaint ies is presented.