Temperature Sensors for Flexible Heaters
Many temperature control methods are available for flexible heaters, each suitable for different control purpose. This section is to give an overview of the options.
Thermal switch, also called thermal cutout switch, is (manually or automatically) resettable thermostat with preconfigured operation temperature. They provide a low cost means of temperature control for electric heaters. Thermal switches are normally wired directly into the heater circuit. If the current draw of the heater exceeds the rating of the switch, additional power relay is needed.
Snap action cutout, often with higher profile, can be enclosed in a molded silicone rubber housing and permanently attached to the heater, while creep action thermostat (thermal protector) is usually covered by a silicone rubber patch.
Due to the limited life cycles (typically 100,000), thermostats are more suitable for over heat protection than constant temperature maintenance. For heavy load electric heaters, due to the delay caused by thermal propagation to the sensing element of thermostats, the operation temperature of the thermostats will be a few degrees lower than the actual temperature of the devices. In other words,there is an overshoot of temperature.
A timer-delay relay can be configured to bypass the thermostat in the start of the heating to avoid frequent interruptions.
Adjustable thermostat allows the users to adjust the temperature setting to attain a desired result. The thermostat is encapsulated in a silicone rubber housing. This can often be found in drum heaters and heating blankets.
Two types of temperature sensing elements are available for adjustable thermostats: bimetallic disc or liquid expansion temperature sensing elements. A capillary sensing probe will be required for the later case.
Thermal fuses (cutoffs) are used as high limiting protection devices to guard the thermal load from dangerous temperatures in the event of a primary control device failure.
Please pay attention to the parameter "holding temperature" when selecting a thermal fuse. The holding temperature is the maximum temperature that the device can operate continuously without opening. It is roughly 20 degree lower than the rated functioning temperature of the thermal fuse. The behavior of thermal fuses is unpredictable if the devices operate above the holding temperature constantly.
Temperature sensors are normally mounted over a heated section to sense the temperature of the heating element or optionally over a cold section to indirectly sense the temperature of the load. The sensor signal will be fed to the controllers as input. Several types of temperature sensors are available.
A thermocouple consists of two conductors of different materials (usually metal alloys) that produce a voltage in the vicinity of the point where the two conductors are in contact. The voltage produced is dependent on, but not necessarily proportional to, the temperature difference of the junction to other part of the conductors. Commercial thermocouples are inexpensive, interchangeable, are supplied with standard connectors, and can measure a wide range of temperatures. In contrast to most other temperature measurement method, thermocouples are self powered and require no external excitation.
RTD - Resistance Temperature Detector
RTD is a temperature sensor that contains a resistance element that changes resistance value as its temperature changes. By far the most common devices used in industry have a nominal resistance of 100 ohms at 0 °C, and are called Pt100 sensors. The nominal sensitivity of a standard Pt100 is 0.385 Ω/°C. Compared to thermistor or thermocouples, they are:
- A wide temperature range (-50°C to 400°C for thin film RTD and -200°C to 850°C for wire wound)
- Good accuracy (better than thermocouples)
- Good interchangeability
- Long-term stability
They are slowly replacing the use of thermocouples in many industrial applications below 600 °C.
Depending on the allowed tolerance of the measurement, Pt100 can be further categorized into class A, class B, class B/3, etc. Temperature controllers can require 2 3 or 4 wire input from Pt100. Please specify if you need more than 2 wires.
Thermistor is also a resistance-based temperature sensor. It does not generally respond in a linear style and is used in a limited temperature range or at a specific single temperature.
Thermistor can be classified into two types, depending on the sign of temperature coefficient: PTC and NTC. NTC are normally used as temperature sensor while PTC are more used for circuit protection.
Thermistor differs from resistance temperature detectors (RTD) in that the material used in thermistor is generally ceramic or polymer, while RTDs use pure metals. The temperature range is also different. RTDs are useful over larger temperature ranges, while thermistors typically achieve a high precision within a limited temperature range.
PID Temperature Controller
If without the knowledge of the underlying process, a PID controller has historically been considered to be the best controller. By tuning the three parameters in the PID controller algorithm, the controller can provide satisfactory control action for most process.
PID temperature controllers read the sensor signal, normally from thermocouple or RTD, and convert the measurement to temperature units e.g. °C. It then subtracts the measurement from a desired setpoint to determine an error. The error is acted upon by the three terms (P, I & D) simultaneously.