These are also referred to as change-of-state temperature measurement devices. These are the semi invasive means to sense temperature. In industries, various types of phase change devices are available. Some of the most commonly used devices include:
- Liquid crystals
All these devices undergo a change in color or appearance depending upon the temperature variations. “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.”
Following are the key features of state-change temperature sensors:
- They give poor accuracy as compared to other types of temperature sensors.
- The change in state experienced by these devices is generally permanent. However, in the case of liquid-crystal displays and crayons, this change can be reversed.
- Liquid crystals are the state change devices which turn from transparent to opaque state whereas crayons transforms to liquid state after reaching the transition temperature.
- These temperature measurement devices proves to be very helpful in situations where “one needs confirmation that the temperature of a piece of equipment or a material has not exceeded a certain level, for instance for technical or legal reasons during product shipment.”
- Most of these devices are meant for one time use only. However, they can be applied at several points on a particular label.
- These are the simplest devices employed for fast measurement of temperature.
- They are available in variety of shapes, sizes and forms.
- They are comparatively cheaper than other temperature sensors.
- They can measure a wide range of temperature.
Thermal paints generally result in change of color after exceeding their particular temperature levels. The thermal paints are available in both reversible and irreversible forms. Reversible thermal paints can get back to their original color after getting cooled whereas irreversible paints can never regain their previous color. The change of color in thermochromic paints occurs in relation to the heating time. “Lower color transition temperatures correspond to longer heating times. For heating times between some seconds to some hours the temperatures may differ significantly.”
The major problem faced with the use of these temperature sensors is that we require an entire range of diverse phase change materials for measuring temperature over a wide span. Moreover, applying thermochromic paint to an object can be risky at times.