Temperature Controller

In order to adjust temperature, in other words, to control a temperature, one or more temperature controllers are necessary. This text is supposed to help in making a decision before purchasing a temperature controller. We will clarify here some questions, which always arise during the discussion of temperature controllers.
Which sensor signal is available?
This question is fundamental and should be answered first. If the measuring signal is available as 0 ... 10 V or 4 ... 20 mA, a temperature controller can be used with the process input. The PCE-RE P series are simple controllers that can process these sensor signals as a temperature scaled. The P stands for process signals. If a measuring signal comes from a resistance sensor, i.e. a PT100 or PT1000, or a thermocouple, a temperature controller with a corresponding RTD or TC measuring input has to be selected. In the simplest case, this can be the PCE-RE T series. In these controllers, the T stands for temperature sensors. If it is not clear yet, which sensor signal is ready, or if the sensor can be changed during the application, the following controllers can be used:

- PCE-RE72
- PCE-RE82
- PCE-RE92

All these temperature controllers have one or more universal inputs, so both process and temperature signals can be processed.
If the question about the sensor signal has been clarified, it is necessary to determine which output is to be used to regulate the temperature controllers. For this purpose, it must be known: a) which actuator is connected to the controller and / or b) whether the temperature control should be continuous or discontinuous.
If a continuous temperature control is required, it is recommended to select a temperature controller with an SSR output or a continuous analog output. If only a simple, somewhat inaccurate temperature control is needed, a temperature controller with a relay output is also enough.
When should a continuous temperature control be carried out?
A continuous control of the temperature with a temperature controller should always be carried out if excessive temperature fluctuations cause faults in the process or in the product. With the help of the continuous PID control, the temperature controller manages to accurately approach the setpoint, without overshooting the actual value. Many temperature controllers such as the PCE-RE72 or PCE-RE92 have a so-called Smart-PID function. This means that the controller finds the optimal P, I & D parameters for the process by means of calculations itself.
If a continuous control takes place with the help of a temperature controller, the temperature controller should be equipped with an SSR output or an analog output.

Why cannot a relay be used for continuous temperature control?
If the temperature controller is used for continuous control, no conventional relays should be used. This is so, because then the temperature controller would operate too slowly, and the setpoint would not be reached in time or the actual value fluctuates strongly. Furthermore, relays have a limited number of switching cycles. During a constant temperature control, this would lead to a quick end of the service life of the control relay in the temperature controller. For this reason, wear-free solid-state relays are used in temperature controllers, which are used for continuous control. These SSR semiconductor relays impulsively give out a voltage that is then used by power SSRs to control correspondingly connected heaters or cooling elements with the temperature controller.
Further, there are other criteria that help to select the right temperature controller.
Thus, a simple temperature controller PCE-RE22 can only display the actual value. The PCE-RE 72, which has the same dimensions, shows also the setpoint, besides the actual value.
Various other temperature controllers, such as the PCE-RE 92, allow a program control. This means that different setpoints can be approached by the temperature controller and thus, processed.
PCE Instruments can specifically recommend temperature controllers for different applicationas and develop solutions, so that the correct temperature controller could be selected correctly.

The temperature controller is an important element of control technology. A temperature controller captures the actual value of a medium and changes its temperature until a set point (target value) is reached. The easiest and most common type of temperature controller is a thermostat. This simple type of temperature controller keeps a certain, pre-set target temperature constant. In order to keep temperatures at a constant level in control technology, temperature controllers are used together with different sensors.

For example, a thermocouple can be connected to a simple controller to continuously measure the temperature of a liquid. The temperature controller processes the measured signal – the actual value – and controls the temperature of the water bath by switching, for example a heater, on and off. This makes the actual value approach the target value. Also, various other parameters play an important part in temperature control. The ambient temperature affects the temperature controller. The controller must compensate for this „disturbance variable“ by, for instance, cooling the liquid.

Top-hat rails are not only used as mounting rails in control cabinets and switch boxes for electrical and network installations. The profile rails, which resemble a hat in cross-section, are also suitable, among other things, for the simple and clear mounting of measuring devices for process monitoring. A top-hat rail (DIN-rail) measuring device is already prepared for mounting on the top-hat rail by its design. Thanks to suitably shaped recesses and resilient components on the rear of the housing, the devices can be conveniently hooked onto the top-hat rail, moved as desired and locked into the selected position. If necessary, the units can also be easily removed again.

Because of the large number of possible uses, there are different variants of mounting and fixing rails. DIN EN 60715 (2018/07) or VDE standard 0660 Part 520 (2018-07) specify the standardised mounting rails for the mechanical fastening of electrical devices in switchgear. This includes top-hat rails in various dimensions and made of various materials. These top-hat rails can be used for the installation of timers, relays, load switches, circuit-breakers and contactors, as well as for the fastening of electricity meters, transducers and measuring transducers, which are suitable as top-hat rail measuring devices due to their housing design.

Important dimensions for a DIN-rail measuring device
Since there are different requirements for the quantities to be measured and the display and transmission of the measured data, the devices to be installed differ in many respects, including the dimensions. Almost always, the intended space on the control panel, the wall or in the control cabinet should be used as effectively as possible and the devices should be arranged there in a way that is as clear and easy to control as possible. Therefore, the following dimensions should be clarified first:

- the top-hat rail width,
- the unit dimensions: width, height and depth,
- the dimensions of the display or operating surface

In the case of a DIN-rail measuring device that is to be positioned together with many other components on a limited surface, attention must therefore be paid not only to the DIN-rail width for which it is intended, but also to the width, height and depth of the device. If devices with direct display or manual operation are to be mounted in switch boxes or enclosures and still be accessible at all times, it is also important which opening in the cover must be provided for reading and operating the device.

Top-hat rail measuring device with or without display option
Depending on the intended use and installation location, you can choose between models with or without direct display on the unit. The devices without a display option either pass on the measured data immediately so that they can be displayed or evaluated elsewhere, store them for later queries or activate the switching signals when the defined limit values have been reached. For the top-hat rail measuring device with display option, a distinction must be made between:

- coloured signal LED,
- illuminated info fields,
- numeric display field and
- alphanumeric display field.

Coloured LEDs on the DIN-rail measuring device can indicate, for example, that the device is recording or transmitting the measured values. However, colour displays and illuminated info fields are particularly suitable for signalling in noisy environments that set alarm limits for the measured values have been reached or that there are malfunctions. With the display panel, care should be taken to ensure that the display is easily recognisable in terms of brightness and contrast as well as character size. Furthermore, in the case of the DIN-rail measuring device with different measuring functions, not only the numerical values should be displayed, but also additional information on the measured variable or its unit.

Power supply, operating conditions, IP protection rating and impact protection
The DIN-rail measuring device is usually used for continuous measurements and in most cases it is supplied with the necessary voltage via cable. When selecting a device model, it is important to consider which supply voltage is available at the installation location. Often, the versions for DC and AC voltage are available, each for different voltage values.

As with other devices, care must also be taken with the DIN rail measuring device that the design is adapted to the environmental conditions at the installation location. This applies to temperature and humidity as well as to exposure to moisture, dust and foreign bodies. If there is a risk of external mechanical stress against which the unit housing cannot be protected by other measures, a model with a classified IK shock resistance rating should be used.

Parameter settings, interfaces and memory in the DIN rail measuring unit
After connection, the models with operating keys and display can usually be configured directly for the specific application conditions. With other types of devices, an adapter is used for this purpose or the DIN-rail measuring device is connected to a computer via a data interface, e.g. RS232, and set to the desired values using configuration software.

In the case of the DIN-rail measuring device, attention must be paid not only to the type of the measuring inputs and signal or data outputs, but also, if there is little space available at the installation site, also to their exact position and the maximum cross-sections that can be connected. Often, the device specifications also indicate the grid spacing of the individual connections. For applications in which a sensor is to be supplied with voltage via the device, an additional connection with suitable voltage must be available. If the storage of the measurement data is intended, you can choose from device versions that are equipped with internal memory or an SD card compartment.