TEMPERATURE MEASUREMENT TECHNOLOGIES
Temperature switches: mechanical and electronic, for alarms and shutdown
A temperature switch does one job and does it reliably: it opens or closes an electrical contact when the temperature passes a set point. Mechanical switches do it with a sensing bulb or element and a snap-action contact; electronic switches with a sensor, usually an RTD, and an electronic output. Both protect equipment and people — alarms, interlocks and emergency shutdown — independently of the control system. Our catalogue includes the temperature switches of United Electric Controls (100, 117, 12, 120, 400, 54, 55 and 800 Series, and the electronic Excela) and the Delta Mobrey 700.
How a temperature switch works
In a mechanical switch the temperature acts directly on the mechanism. Usually a bulb filled with a liquid or gas sits in the process, joined to the switch by a fine capillary tube or a rigid stem: as the temperature rises, the fill expands and pushes on a diaphragm or bellows, and at the set point that movement flips a snap-action contact. Other designs use a bimetal element. No power is needed to sense the temperature.
The contact changes at the set point when the temperature rises, but it does not return as soon as the temperature drops back below it: it waits until the temperature has fallen by a few degrees, to the reset point. The gap between the two is the deadband, and it stops the contact from chattering on and off when the temperature hovers around the set point.
An electronic switch measures the temperature with a sensor, usually an RTD, and its electronics compare the reading with set points you program, switching a relay or solid-state output. It can show the temperature on a display and allows adjustable deadbands and delays. The Excela from United Electric is an electronic switch designed to install with your existing wiring.
Because a switch works on its own, it keeps protecting the plant even if the control system or its network fails. For safety duty, look for hermetically sealed contacts — the 117 and 12 Series use them, the 12 Series to ANSI/ISA 12.27.01 — and for certification such as SIL 2, which the 12 Series carries. Contacts are usually SPDT or DPDT, as on the 117 and 120 Series.
- The temperature, rising and then falling over time
- The set point (solid line), the reset point (dashed) and the deadband between them
- The contact: it changes at the set point on the way up and returns only below the reset point
Where a temperature switch fits — and where it does not
A strong choice for
- Overtemperature alarms and trips on motors, bearings, pumps, compressors, heaters and process lines
- Emergency shutdown and safety instrumented systems — the 12 Series carries SIL 2 certification
- Protection that must not depend on the control system: a switch keeps working if the PLC or the network fails
- Hazardous areas, with explosion-proof versions such as the 120 Series
- Harsh and corrosive environments: the 12 Series is built in 316 stainless steel, with NACE MR0175 options
- Simple local control, such as switching a heater or a fan on and off, with no programming
Think twice when
- You need to see, record or trend the temperature: a switch only says above or below the set point — use a sensor with a transmitter
- You need continuous control (for example PID control of a heater): that needs a measured value
- You need a very narrow deadband or several set points: an electronic switch, or a transmitter with alarms in the control system, does it better
- There is no room for a bulb fully immersed in the medium, or the capillary cannot be routed safely
How it compares with the other technologies
- RTDs (Pt100)−200…+600 °C
- Thermocouples−200…+1700 °C
- Temperature transmitters−200…+1700 °C *
- Temperature switches (the technology of this article)−50…+400 °C
Typical ranges, not a model commitment. RTD accuracy to IEC 60751 at 0 °C; thermocouple accuracy to IEC 60584, by type and class; for transmitters (*), the range is that of the sensor connected to them. We confirm the figures of each instrument for your application.
Installing a temperature switch
The sensing part — bulb, stem or sensor — must sit fully in the medium whose temperature it watches, usually in a thermowell so that it can be removed without opening the line. A mechanical switch reacts to the temperature of the whole bulb, so a bulb that is only partly immersed responds late and at the wrong temperature.
Then:
- Choose the set-point range so that the set point sits in the middle part of it, not at the very end.
- Set the deadband wide enough to avoid chattering, but narrow enough that the alarm clears when the cause is gone.
- Test the switch at commissioning and at regular intervals, especially in safety duty.
- Use the contact rating, enclosure and certification that the circuit and the area call for.
Temperature switches by industry
Temperature switches in our catalogue
The temperature switches in our catalogue, each with its own page and technical data.
Temperature switches: frequently asked questions
Ask about a temperature switch
Tell us the medium, the set point and the temperature range, the duty (alarm, trip or control), the contact you need, the process connection and the area classification — and our engineers will suggest the right switch, with price and delivery time.