Today, let's take a closer look at each temperature sensor type, including RTDs, thermocouples, and thermistors. While these names might seem a bit technical, we’ll explain them in an approachable way!
RTDs: Precise platinum Temperature Sensors
First on the list are RTDs (Resistance Temperature Detectors). These sensors measure temperature by detecting changes in electrical resistance within a wire. The process behind this is relatively simple. Imagine a platinum wire: as the temperature rises, the atoms in the metal vibrate more, which increases resistance to the flow of electricity, allowing for precise temperature readings.
RTDs take advantage of this principle by measuring the wire’s resistance to determine the temperature. It’s similar to how you can gauge heat by how long you can touch something hot. The more resistance, the hotter it is, which is a key principle in temperature measurement.
RTDs are known for their precision, with tolerance classes (for example Class A and Class B according to IEC 60751) that allow accuracy of a few tenths of a degree in typical industrial ranges. It’s like hitting a target from far away and hitting it almost every time.
RTDs measuring sensors in Industrial Applications
One common industrial use for RTDs is in food processing plants for reliable temperature readings. For instance, an RTD sensor might be installed in a thermometer placed inside a cooking vat to ensure accurate temperature readings. As the temperature of the food changes, the RTD detects this by measuring the resistance, sending the information to the thermometer, which then displays the temperature readings for workers to monitor.
This ensures that the food is cooked safely and at the right temperature, adhering to strict temperature measurement guidelines. RTDs may not sound like the most exciting piece of technology, but they’re essential for accuracy and safety in industries like these!
Thermocouples: Temperature Detection with Wires
Next up are thermocouples, which operate using the Seebeck effect. This occurs when two different metals are joined to create a circuit. If you take a copper wire and an iron wire and heat one end, a small voltage appears due to the difference in temperature between the two ends. This thermoelectric voltage (EMF) is used to measure the temperature difference between the hot junction and the cold junction.
Thermocouples come in various types depending on the metals used. For example, Type K thermocouples use nickel-chromium and nickel-aluminum alloys, while Type J thermocouples combine iron and constantan, an alloy of copper and nickel. These devices can measure a broad temperature range, from about -200°C up to around 1800°C, depending on the thermocouple type.
Thermocouples in Industry: From Factories to Laboratories
Thermocouples are incredibly useful in industries that require extreme temperature control, such as factories producing ceramics, glass, or bricks, where precise temperature measurement is critical. These products need to be heated to very high temperatures, and thermocouples are essential in making sure the furnace temperature is monitored accurately for effective temperature measurement.
In addition, thermocouples are employed in boilers to prevent overheating and in scientific labs for measuring the temperature of materials and chemicals during experiments.
So, while they might seem complex at first, thermocouples are quite simple - they’re just wires generating electricity when heated, which can then be used to measure temperature.
Thermistors: The Resistance-Based Temperature Game
Finally, we have thermistors. These sensors detect temperature changes by monitoring how their resistance varies with temperature. There are two main types: NTC (Negative Temperature Coefficient) and PTC (Positive Temperature Coefficient). NTC thermistors decrease in resistance as the temperature rises, while PTC thermistors increase their resistance as the temperature goes up.
Industrial Uses of Thermistors
Thermistors are widely used in HVAC systems to ensure proper climate control. For example, in an air conditioning system, a thermistor monitors the air temperature coming from the vents. If it detects that the air isn’t cool enough, it signals the system to adjust accordingly.
Thermistors are also used in medical devices, such as thermometers, and in automotive engines to monitor temperature and prevent overheating.
What Is a Temperature Sensor and How Is the Signal Used?
A temperature sensor is a device that converts the temperature of an object, liquid or gas into an electrical signal: a resistance change, a small voltage or a digital value. In automation, that signal goes to a PLC, a controller or a temperature meter, where it is scaled to degrees and used to control heaters, fans, valves or alarms.
Temperature sensors are often divided into two groups:
- Contact sensors - the sensing element touches the measured medium. Contact sensors include RTDs, thermocouples, thermistors and semiconductor-based temperature sensors. They are usually mounted in temperature probes and protective thermowells.
- Non-contact sensors - infrared sensors and pyrometers with thermopile detectors measure the heat radiated by the object. They are used for moving, very hot or hard-to-reach objects.
Comparison of Common Temperature Sensor Types
The table below summarizes the different types of temperature sensors used in industry. The values are typical ranges; always check the datasheet of the specific sensor.
| Sensor type | Principle | Typical range | Strengths | Limitations |
|---|---|---|---|---|
| RTD (Pt100, Pt1000) | Resistance of platinum increases with temperature | About -200 to 600°C | High accuracy, stability, near-linear output | Higher cost, needs excitation current |
| Thermocouple (J, K, T, N and others) | Voltage from two dissimilar metals (Seebeck effect) | About -200 to 1800°C, depending on type | Very wide range, rugged, fast response | Millivolt signal, needs cold junction compensation |
| NTC thermistor | Resistance decreases strongly with temperature | Typically about -50 to 150°C | Very sensitive, low cost | Strongly nonlinear, needs a lookup table or equation |
| Semiconductor IC sensor | Temperature-dependent voltage of a transistor junction | About -55 to 150°C | Analog or digital output, easy to use | Limited range, mainly for electronics |
| Infrared (non-contact) | Thermal radiation measured by a thermopile | Wide, depending on model | No contact, moving or hot objects | Depends on surface emissivity |
Semiconductor-based temperature sensors and digital temperature sensors (for example IC sensors with I2C or 1-Wire interfaces) are popular in electronics, but in industrial installations the classic RTD, thermocouple and thermistor dominate, usually combined with a transmitter.
How to Choose a Temperature Sensor for Your Application
Before you choose a sensor, answer these questions:
- Temperature range - for high temperature processes such as furnaces and kilns, a thermocouple is the only practical option. For a wide temperature range up to several hundred degrees with high accuracy, choose a platinum RTD (Pt100 or Pt1000).
- Accuracy and stability - RTDs offer the best long-term stability. Thermistors are very sensitive to small temperature changes, but only across a narrow temperature range.
- Response time - a thin probe or an exposed junction reacts faster to a change in temperature than a sensor inside a massive thermowell.
- Signal and distance - a thermocouple produces only millivolts, and an RTD signal depends on the wire resistance. For long cables, use a head-mounted transmitter that converts the temperature signal to 4-20 mA.
- Environment - vibration, moisture and chemical media decide the probe material, the protection rating and the type of thermowell.
Connecting Temperature Sensors to a PLC
Modern PLCs accept temperature sensors in two ways. Dedicated RTD and thermocouple input modules measure the sensor directly and return the value in degrees, with the linearization tables built into the module. Alternatively, a transmitter converts the signal to a standard analog 4-20 mA or 0-10 V signal, which you connect to a universal analog input module and scale in the program.
We describe the first method in detail in our article on RTD sensors and the Siemens S7-1200, and the second in analog vs digital signals in a Siemens PLC. To practice scaling analog values and programming temperature control in TIA Portal, take a look at our Siemens TIA Portal LAD/FBD course.
Temperature Sensors - FAQ
What are the three main types of temperature sensors?
In industrial automation, the three most common contact sensors are RTDs (resistance temperature detectors), thermocouples and thermistors. Infrared sensors are the most common non-contact option.
Are temperature sensors analog or digital?
RTDs, thermocouples and thermistors are analog sensors: their resistance or voltage changes continuously with temperature. Digital temperature sensors contain a converter and send the measured value over a digital interface. In a PLC, the analog signal is converted to a digital value by the input module.
Which temperature sensor is best?
There is no single best sensor. A Pt100 RTD is the best choice for accurate measurement in most process applications, a thermocouple for very high temperatures and fast response, and an NTC thermistor for low-cost measurement in a small temperature range, for example in HVAC devices.
Summary
In conclusion, temperature sensor technology might seem complex, but it plays a vital role in many industries. Today, we explored RTDs, thermocouples, and thermistors.
- RTDs, which use platinum wires to measure temperature, are commonly used in food processing for accurate temperature control.
- Thermocouples generate electricity when two metals are heated and can measure temperatures from extremely cold to incredibly hot, making them ideal for industrial applications like kilns and boilers.
- Thermistors, which change their resistance based on temperature, find use in various systems such as HVAC, medical equipment, and vehicle engines.
Thank you for reading! If you're interested in learning more, explore our courses for in-depth training: Control Byte Courses.
