Change-of-state Temperature Measurement Devices

Change-of-state temperature sensors consist of labels, pellets, crayons, lacquers, or liquid crystals whose appearance changes once a certain temperature is reached. They are used, for instance, with steam traps—when a trap exceeds a certain temperature, a white dot on a sensor label attached to the trap will turn black. Response time typically takes minutes, so these devices often do not respond to transient temperature changes and accuracy is lower than with other types of sensors.