Temperature Switches & Thermal Switches for Liquids
Temperature Switches & Temperature Sensors for Reliable Liquid Monitoring
Temperatures can have a decisive influence on the safety, quality and functionality of industrial processes. If permitted limit values are exceeded or not reached, liquids may change their properties, components may be damaged or system processes may be impaired. A temperature switch therefore monitors whether a previously defined temperature is reached and triggers an electrical switching operation if required.
Temperature monitoring is particularly efficient when it is combined directly with liquid level detection. Depending on the design, our float switches can be combined with a temperature switch or temperature sensor. This allows the liquid level and temperature to be monitored using a compact, individually configured sensor unit.
What Is a Temperature Switch?
A temperature switch, also referred to as a thermal switch, is a component that responds when a specific temperature is reached. As soon as the defined switching point is reached, the temperature switch opens or closes an electrical contact.
The resulting switching signal can then, for example:
- switch off a machine or pump,
- activate heating or cooling,
- switch on a fan,
- trigger a warning message,
- transmit a signal to a higher-level control system.
Unlike continuous temperature measurement, a simple thermal switch does not provide a continuous measured value. Instead, it monitors whether a specific temperature limit has been reached.
Temperature switches are therefore particularly suitable for applications in which a clearly defined response to a specific temperature is required.
How a Thermal Switch Works: Temperature Monitoring Explained
The function of a thermal switch is based on a temperature-dependent change within the switching element. In mechanical versions, for example, a bimetal can be used. This consists of bonded metals that expand to different degrees as the temperature rises.
The resulting deformation actuates an electrical contact when the defined switching temperature is reached. Depending on the contact function, the electrical circuit is opened or closed.
A temperature switch can therefore trigger a clearly defined response without continuously transmitting measured values. However, the exact operating principle depends on the design and version of the switching element used.
Switching Point, Reset Temperature and Hysteresis
The switching point is the temperature at which the contact state of the temperature switch changes. For example, if a switching temperature of 80 °C is defined, the thermal switch triggers the intended switching operation when this value is reached.
The switch often resets only at a slightly lower or higher temperature. The difference between the switching temperature and the reset temperature is referred to as hysteresis.
This hysteresis prevents the contact from switching back and forth immediately and repeatedly in response to small temperature fluctuations. Without sufficient separation between the switching point and the reset point, frequent switching operations could occur if the temperature remains close to the limit value.
When designing a temperature switch, both the desired switching point and the required reset behaviour should therefore be taken into account.
Normally Closed or Normally Open: Which Switching Function Is Suitable?
Depending on the application, the temperature switch can be designed as a normally closed or normally open contact.
A normally closed contact is closed under normal operating conditions. When the switching temperature is reached, the electrical circuit is opened. This function is suitable, for example, for switching off a machine, heater or pump in the event of excessive temperature.
A normally open contact is open under normal operating conditions. When the defined temperature value is reached, the contact closes. In this way, for example, a fan, cooling system or warning signal can be activated.
Which contact function is suitable depends in particular on the response that is to be triggered when the temperature is reached and on the desired switching state in the event of a cable break or another fault.
Typical Applications of Temperature Switches
Temperature switches are used wherever maintaining a specific temperature range is necessary for safe or reliable operation.
Typical applications include:
- Tanks and storage vessels
- Hydraulic and lubricating oil systems
- Cooling circuits
- Pumps and compressors
- Machines and systems
- Water treatment
- Heating and cooling systems
- Industrial process vessels
- Cleaning and dosing systems
- Equipment and apparatus engineering
In a hydraulic tank, for example, a thermal switch can indicate when the oil heats up beyond a permitted limit value. In a coolant reservoir, it can activate a cooling system or warning message. It can also be used to monitor whether a liquid has reached the required minimum temperature.
When selecting a temperature switch, the properties of the medium and the specific operating conditions must be taken into account. These include temperature, pressure, chemical resistance, viscosity and the installation situation within the vessel.
Level and Temperature Switches for Combined Monitoring
In many applications, it is not sufficient to monitor only the temperature or the liquid level. Both parameters must be taken into account to ensure safe system operation.
A low liquid level can, for example, cause pumps to run dry, heating elements to no longer be sufficiently covered or cooling circuits to lose their function. At the same time, an excessively high liquid temperature may indicate an overload, insufficient cooling or a process malfunction.
A combined level and temperature switch integrates both monitoring functions into a single sensor unit. This allows different operating conditions within the tank to be detected at an early stage and targeted measures to be initiated.
Monitoring Liquid Level and Temperature with a Single Sensor Unit
Our individually configurable float switches can additionally be equipped with a temperature switch and, optionally, with a PT100 or PT1000 temperature sensor.
The float switch monitors one or more defined liquid levels. A magnet integrated into the float actuates a reed contact located inside the guide tube when the respective switching position is reached. Depending on the configuration, this can be used to detect minimum and maximum liquid levels, pre-warning levels or additional switching points.
Temperature monitoring complements this limit-level detection. When the defined temperature is reached, the integrated temperature switch triggers a separate switching signal.
This makes it possible to determine simultaneously, for example:
- whether sufficient liquid is present,
- whether a maximum liquid level has been exceeded,
- whether the medium is too warm or too cold,
- whether a pump must be switched off,
- whether cooling or heating should be activated,
- whether a warning or fault message is required.
Miniature float switches are also available for small tanks or restricted installation spaces and can likewise be combined with temperature monitoring.
Advantages of Combined Level and Temperature Switches
Combining several monitoring functions in a single sensor unit can significantly simplify the design of a system.
The main advantages include:
- combined monitoring of liquid level and temperature,
- fewer separate sensors and tank connections,
- compact design,
- simplified installation and wiring,
- coordinated switching functions,
- individual adaptation to the tank and application,
- reduced design and integration effort.
Especially where space is limited, it can be advantageous to use a combined level and temperature switch instead of several separate components. At the same time, the positions of the liquid-level switching points and the temperature monitoring function can be adapted to the specific application.
Further information on selecting and positioning suitable sensors can also be found on our application page about liquid-level measurement in tanks.
Temperature Switch or PT100 / PT1000 Temperature Sensor?
Whether a temperature switch or a PT100 or PT1000 sensor should be used depends on the required type of temperature monitoring.
A temperature switch is often the appropriate choice when it is only necessary to determine whether a defined temperature has been reached. It provides a direct switching signal and can therefore trigger, for example, a shutdown, warning or cooling function.
A PT100 or PT1000, by contrast, is suitable when the precise temperature profile needs to be recorded. The sensor changes its electrical resistance depending on the temperature. A connected evaluation unit or control system uses this change to calculate the current measured value.
A thermal switch, by contrast, provides simple and direct limit-value monitoring without requiring a continuous temperature value to be processed.
Depending on the application, both versions can be combined with the liquid-level detection function of a float switch.
Temperature Switch or NTC / PTC Temperature Sensor?
NTC and PTC sensors detect temperature changes through changes in their electrical resistance. The main difference lies in the direction in which the resistance changes.
With an NTC sensor, resistance decreases as the temperature rises. These sensors are frequently used for continuous temperature measurement, for example in devices, tanks or electronic assemblies.
With a PTC sensor, resistance increases as the temperature rises. This makes it particularly suitable for protection and monitoring functions, such as detecting excessive temperatures in motors, windings or electrical components.
The most suitable version depends on the evaluation system and the required temperature range. Depending on the application, both sensor types can also be combined with liquid-level monitoring using a float switch.
Important Criteria When Selecting a Temperature Switch
A temperature switch should always be matched to the actual operating conditions. In addition to the required switching temperature, further technical and design-related factors are decisive.
The most important selection criteria include:
- Switching Temperature and Hysteresis:
The switching point must be selected so that critical conditions are detected in time. At the same time, the hysteresis should match the dynamics of the process. - Contact Function:
Depending on the required response, a normally closed or normally open contact may be necessary. - Electrical Load Capacity:
Voltage, current and switching capacity must be compatible with the connected control system or load. An additional switching stage may be required for higher loads. - Medium and Material Resistance:
All components in contact with the medium must be resistant to the liquid used. Depending on the application, stainless steel or various plastics may be suitable, for example. - Temperature and Pressure Range:
Not only the switching point, but also the permanently permissible operating and ambient temperatures must be taken into account. The same applies to the tank pressure. - Installation Position and Available Space:
The sensor geometry, position of the switching points, length of the guide tube and type of process connection must be suitable for the tank. - Degree of Protection and Ambient Conditions:
Moisture, dust, vibrations, cleaning processes or outdoor use may require special protective measures. - Industry-Specific Requirements:
Special materials, approvals or designs may be required for food applications or potentially explosive environments.
Careful consideration of these factors is essential for reliable and long-term stable temperature monitoring.
Customised Float Switches with Temperature Monitoring from Reed
Standardised sensors are not suitable for every tank geometry and process. We therefore develop and manufacture float switches that can be specifically adapted to the respective application.
Depending on the design, the following properties can be configured, among others:
- number and position of the liquid-level switching points,
- overall length and installation length,
- mechanical connection,
- electrical connections,
- contact functions,
- materials of the components in contact with the medium,
- float geometry,
- integrated temperature switch,
- temperature measurement with PT100 or PT1000,
- temperature measurement with NTC or PTC.
This results in a sensor unit that meets both the liquid-level detection and temperature-monitoring requirements.
Our combined level and temperature solutions are particularly suitable when several process parameters need to be monitored compactly and reliably without unnecessary additional tank connections. Together with you, we clarify the technical requirements and develop a suitable solution for your application.
