Change-of-state 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 example, with steam traps; when a trap exceeds a certain temperature, a white dot on a sensor label attached to the trap turns 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. Furthermore, the change in state is irreversible, except in the case of liquid-crystal displays. Even so, change-of-state sensors can be handy when one needs confirmation that the temperature of a piece of equipment or a material has not exceeded a certain level, for example, for technical or legal reasons during product shipment.

Infrared probe devices

One of the most effective methods of making non-contact, high-temperature measurements in industrial applications is with a fiber optic infrared sensing device. This typically consists of a lens probe assembly that is aimed at the object to be measured, and a fiber optic interconnecting cable connected to an electronics package and the transmitter, which makes the temperature measurement and converts it into a useable output signal. Let's examine each of these components:

The probe assembly consists of a housing specifically designed for the conditions to which it will be exposed, a lens or an optical rod to collect infrared radiation from the target, and an optical fiber interface for connection to the fiber optic cable. The probe is usually placed within a few inches of the object being measured. Because of this, construction of the probe assembly can vary dramatically. For measurements in an open-air environment, it can be a simple metal cylinder. However, it is not uncommon for these devices to be used in very harsh environments, such as high-temperature chambers, under vacuum, in corrosive atmospheres, or immersed in molten plastic. These applications require specialized probes with threaded housings, ceramic or other durable construction, non-glass lenses, and even glass or quartz optical rods (tips).

The fiber optic interconnecting cable acts as a waveguide to bring the radiation to the infrared detector assembly in the electronics package. The quality of the fiber optic interfaces at each end is critical to overall system accuracy and repeatability. Because the signal is transmitted optically, it is immune to the often-substantial electrical and magnetic interference found in industrial settings.

The electronics package does the work of converting the infrared radiation delivered by the fiber optic cable into a temperature reading or a signal proportional to the temperature. It may include enhancements such as high and low temperature alarms, various output options, and even a computer interface connection.

Applications for non-contact temperature measurement

Although thermocouples are the most common temperature measurement devices in process control, they have limitations. They must be in contact with the measured object, have a slow response time, and are subject to electrical and magnetic interference. Fiber-optic infrared transmitters overcome these issues, but are generally limited to reading temperatures above 100° C. Why? Fiber optic cable cannot transmit infrared energy below a certain wavelength; this depends on the cross-section of the fiber optic strands and their optical properties. Following are some typical applications:

Annealing processes

The critical surface temperature of the metal can be monitored directly while it is inside an oven, rather than indirectly by measuring the ambient oven temperature.