Temperature control of flexible heaters

Temperature control of flexible heaters-sensor&control
Different temperature control methods are available for flexible heaters, each suitable for different control purpose. Fullchance has extensive knowledge and experience with thermal control system configuration. 

 

Many temperature control methods are available for flexible heaters, each suitable for different control purpose. Fullchance has extensive knowledge and experience with thermal control system configuration.

 

Preset Built-in Thermostat

Preset thermostatPre-set built-in thermostats (thermal cutout) provide a low cost means of temperature control for heaters. The thermostat is normally wired directly into the heater circuit. If the current draw of the heater exceeds the rating of the thermostat, external leads are provided.

 

The thermostats are normally mounted over a heated section to sense the heater’s temperature or optionally over a cold section to indirectly sense the temperature of the load. The thermostat is enclosed in a molded silicone rubber housing and permanently attached to the heater.

 

Due to the limited life cycles (no more than 100,000), thermostats are only suitable for over temperature protection instead of constant temperature maintenance. For high watt density heaters, due to the delay caused by thermal propagation to the sensing element of thermostats, the operation temperature of the thermostats might be a few degrees lower than the devices to be protected. In other words,there might be an overshoot of temperature.

 

Adjustable Thermostat

Adjustable Thermostat for silicone rubber heaterAdjustable thermostats allow the users to set a specific temperature and attain a desired result. The thermostat is encapsulated in a silicone rubber housing. This can often be found in large size regular shaped drum heaters and heating blankets.

Current rating: 12.5A @ 125V, 6.5 A @ 250V

Temperature range: 0 -150°C. Consult Fullchance for more option.

This adjustable thermostats are often equipped with liquid expansion temperature sensing elements.

 

Built-in Thermal Fusing

thermal fuseThermal fuses (cutoffs) are used as high limit protection devices to guard the object being heated from dangerous temperatures in the event of a primary control device failure. The thermal fuse can be mounted using various methods depending on other options. If the heater does not have a thermostat, the thermal fuse would be mounted under the lead exit patch. If used in conjunction with a thermostat, it could be mounted under the thermostat cover.

 

Temperature sensors can be mounted on the heater to sense the temperature of the part being heated or the heater surface temperature itself. Then the output signal will be fed to the controllers for different purpose. Several types of temperature sensors are available.

 

Thermocouples

temperature sensorA 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 difference of temperature of the junction to other parts of those conductors. Commercial thermocouples are inexpensive, interchangeable, are supplied with standard connectors, and can measure a wide range of temperatures. In contrast to most other methods of temperature measurement, 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 sensitivity of a standard 100 ohm sensor is a nominal 0.385 ohm/°C. Compared to thermistor or thermocouples, they are:

A wide temperature range (-50 to 500°C for thin-film and -200 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. 

 

Thermistors 

respirator heater with controlThermistors are also a resistive-based temperature sensor. They do not generally respond in a linear style and are used in a limited temperature range or at a specific single temperature. Small bead style thermistors can be mounted directly on the heater. The thermistor’s response is generally designed directly into the customer’s electronic control system. Therefore if a thermistor is required, specify manufacturer, specific model number, type and specifications when requesting a quote.

 

Thermistors can be classified into two types, depending on the sign of temperature coefficient: PTC and NTC. Thermistors differ from resistance temperature detectors (RTD) in that the material used in a thermistor is generally a ceramic or polymer, while RTDs use pure metals. The temperature response is also different; RTDs are useful over larger temperature ranges, while thermistors typically achieve a higher precision within a limited temperature range, typically −90 °C to 130 °C. 

 

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 control action designed for specific process requirements.

 

PID temperature controllers reads the sensor signal, normally from a thermocouple or RTD, and converts the measurement to temperature units e.g. Degrees 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.