Temperature control products span a range from simple manual devices to more advanced digital controllers with sensor feedback. In one application, the simple manual controller may be the best choice whereas another application may require a more advanced controller.

Temperature control can be either manual or automatic. The most basic control type—manual—consists of a heating element connected to a variable voltage output controller or variable time power controller.

A variable voltage output controller allows the user to adjust the voltage level supplied to a resistive load to achieve a desired temperature. A heating mantle or heating tape is connected to the unit; the control knob is set to the desired level. A high setting provides a higher voltage output and thus a higher temperature.

A variable time power controller is used in a similar fashion to the variable voltage controller above. The difference is the variable time power controller varies the “on” time of the heating element to achieve the desired temperature. The controller has a cycle time. The control setting will determine what percentage of the cycle is spent on and what percentage off. For example, if the cycle time is 60 seconds and the control setting is 40%, the controller will be on for 24 seconds and off for 36 seconds. The on/off cycle repeats continuously.

An automatic temperature control system consists of three components: a temperature controller, temperature sensor, and one or more relays to control devices such as a heating element, pump, fan, or alarm.

The controller compares the temperature sensor input to a temperature set point. The relay output of the controller turns on or off the device depending on the measured temperature in relation to the set point.

Choose from three common types of automatic temperature controllers: on\off, proportional, and PID. Cole-Parmer offers a wide selection of both on/off and PID controllers.

The on\off controller is the most simple and economical. As the name implies, the device will turn on or off when a set point temperature is reached. In some cases, such as a small volume or low load system, an on/off controller will rapidly turn the heating element or pump on and off as the temperature continuously hits the set point and drops. To prevent this rapid cycling, a hysteresis or dead band can be programmed into the controller. Hysteresis is the distance from the set point that the temperature must travel before the controller turns the device on again. An on/off controller is best suited to applications in which precise temperature control is not needed or the temperature change is very slow. This devise is also a good choice for a temperature alarm.

A proportional controller has a programmable proportional band in place of a hysteresis band. When the temperature is far from the set point, the proportional controller acts like an on/off controller. As the temperature gets closer to the set point it enters the proportional band. Within the proportional band, the controller varies the on and off time of a device, such as a heating element, based on how far the temperature is from the set point. As the temperature nears the set point, the controller turns off the heating element for proportionally longer times until the set point is reached. This eliminates the oscillations associated with a strict on\off controller.

A PID controller is a proportional controller with integral and derivative—a means of selecting the response speed or sensitivity of a proportional controller to achieve stability in a system. A PID controller takes into account the rate at which the temperature of a system changes. A PID controller needs to be tuned to the system. Tuning can be done manually, but many controllers feature an auto-tune cycle that will tune it automatically. Cole-Parmer carries a number of PID controllers in both benchtop and panel-mount configurations. Some are available with multiple control outputs and RS-232 of RS-485 communication. These controllers can also be programmed for ramp and soak applications.