Guidance for selecting temperature controllers

General considerations

All ovens and furnaces need some sort of control to ensure that they do not get too hot and cause damage to themselves, the items inside or the surrounding environment. On this page are some pointers to the types of thermostats, controllers and recorders available.

Energy regulators

The most primitive form of temperature limiting device is the energy regulator. These devices turn the power supply on and off at specified time intervals from 0% “Always off” to 100% “Always on”. They do not control temperature but limit the average power input so that heating rate and maximum temperature obtainable are turned up and down by virtue of the energy regulator setting.

A typical energy regulator is called a Simmerstat.

Thermostats

In broad terms thermostats are used in low temperature ovens where a temperature is selected and the oven powers up at maximum rate until the chosen temperature is reached. The thermostat then cuts off power. There will be a slight over-run of the chamber temperature until the heat around the elements is dissipated. When the thermostat detects that the temperature has dropped below the chosen temperature the power is again switched on. Again there will be a slight delay as the heat from the elements makes its way to the working chamber. These delays cause the chamber temperature to fluctuate by a greater amount than the accuracy of the temperature sensing of the thermostat.

Thermostats may either rely on the expansion of fluid or the change in dimensions of a bi-metallic strip expose to the chamber temperature.

Further information on thermostats may be obtained from the manufacturer.

Temperature controllers

When a closer control of the over-runs is needed it is necessary to have a “thinking” device to anticipate the proximity to the set temperature. These devices require a thermocouple to sense the chamber temperature and send an electrical signal to the controller circuitry. The circuit can be tuned so that it anticipates the time required for a power input change to affect the chamber temperature and so is able to reduce the power as the set point is being reached from a lower temperature or increasing if it is in a cooling cycle.

Simple controllers only allow one temperature to be fixed at a time and they do not have a built-in timer. A typical controller of this type is the VT 26 made by Vertex.

Program controllers

So that a time limit can be set on a heating program a more complex temperature controller is required. These are available from simple systems that just allow temperature and time at temperature to be set to complex controllers allowing a pause before starting, several temperature stages to be set along with the rate of temperature change between the stages and the length of time (or “dwell”) to be allowed at each stage.

The manufacturer of the most popular multiple program controller is Eurotherm.

Recorders

In order to have a record of the temperature treatment obtained in the furnace or oven it is necessary to have some recording device. This could be a paper chart or a digital record. Again Eurotherm have the answers.

Any particular features of heating and cooling rates will affect the type of controller to be used and as the requirements become more complicated the cost of the controller can become a significant part of the furnace package.

If you have any queries on these items please contact us and we will respond as quickly as possible.

Please E-mail us directly from here.