Noncontact Temperature Sensing, Part 1
A real case: A heavy, rotating steel disk was used to preheat a stream of aluminum bottle caps riding on a flat surface before insertion of thermoplastic liners. Disk temperature was sensed and controlled by a liquid-filled thermostat attached to the part. The thermostat switched the gas flame under the disk on and off. The bulb followed disk temperature with a lag of 1 or 2 min, time-constant, resulting in large temperature cycles, made worse by the thermostat's dead-band of some 30°F. Seal quality and scrap rates were unacceptable. Possible solutions:
Fast Sensor. Take a light gauge, Type J thermocouple, packed around with fiberglass, and stuff it in a 0.5" open-ended metal tube. Let the exposed thermocouple tip emerge from the fiberglass, flush with the open end of the tube. Fix the thermocouple on the machine frame under the disk so that the tip nearly touches as the disk rotates. This picks up a close approximation of the disk temperature and responds in a few seconds.
Temperature Controller. Run the thermocouple wires to a PID controller with a relay or triac output that controls the gas solenoid valve. This results in tight control and high yield.
Another case: The same idea is used to pick up the temperature of a high speed, heated steel drum, called a godet, used to texturize synthetic fibers wound around it. Here, a deep annular groove is cut in the flat end of the drum. A stationary RTD probe is inserted parallel to the drum's axis so that it senses only the surrounding metal temperature. (The groove was not necessary in the first example.) A temperature controller picks up the signal and drives a stationary induction heater inside the drum.
Later designs use a sensor and analog/digital circuit that rotate with the godet, transmitting the digital temperature signal by fiber optic to the control system. Radio transmission is another common option. The circuit is powered by noncontact electromagnetic coupling.
When you run out of the many ways of sensing the untouchable with a thermocouple or RTD, you can turn to sensing the infrared radiation from your process. Figure 1 shows the radiation intensity of a black-body heat source (emissivity equals one) as a function of wavelength and temperature.
Construction of an Infrared ThermometerTake a metal tube, put a lens in the end and a silicon radiation detector behind it. Point the lens at something hot (called the target) and measure the voltage from the radiation detector. As the target gets hotter, the voltage increases. Not bad so far, but consider what the professionals do to make it usable.
- Arrange the optics to determine the field of view (FOV). The ratio is distance-to-target to target diameter. Typical values are 12:1 to 200:1.
- Determine the minimum target size.
- Choose the radiation detector material: silicon, germanium or lead sulfide, according to temperature range and target surface characteristics.
- Add an optical filter that passes only the parts of the infrared spectrum that give best accuracy and stability for the application.
- Keep the thermometer cool and air purged if threatened by process heat or dirty atmosphere.
- Incorporate ambient temperature correction (analogous to cold junction compensation in thermocouples).
- In hostile or cramped locations, transmit the target radiation to the sensor via fiber optic cable.
- Incorporate laser targeting to line up the target.
Next month, I'll continue my discussion of infrared thermometers by looking at how to convert detector voltage to a temperature display, emissivity compensation and other performance variables.