In most cases, we need to monitor exact temperatures, and in some, we need precise control. When we learn about temperature sensor types from the maintenance point of view, we’ll find that we only need to consider certain points to choose the right sensor for a process. This article will guide you through the multitude of temperature sensor types, explain how their work, and give some pointers about their applications.
By now we’ve probably heard about external temperature sensors. Vendors know that simple solutions sell, so they create complex devices to make temperature monitoring simple.
Anyway, these new devices pull data from our process, like the material and thickness of the pipe, ambient temperature, and more. Then they use special algorithms to calculate the right temperature for the pipes.
Unfortunately, these devices only monitor or control the temperature in pipes. Right now we lack similar solutions for other temperature applications.
To know more about temperature transmitters, you can read our article on the types of temperature transmitter
Temperature sensor types
Did we know that nearly every electronic device has a temperature sensor? Take a smartphone, for example. It probably uses a semiconductor-based sensor on its integrated circuits to monitor the temperatures our phone encounters.
We have a ton of different temperature sensor types on the market, way too many to talk about in this article. But two in particular really stand out in most process applications, the resistance temperature detector (RTD) and the thermocouple. We’ve probably had contact with both of these temperature sensors at least once in your life.
From the others in the market, we’ll discuss a couple more, the infrared sensor and the bimetallic sensor. They have fewer applications in process automation, but you should know a little about them too.
Let’s start with the RTD.
Resistance temperature detector (RTD)
This sensor has a well-earned reputation as one of the most accurate sensors available, providing good accuracy in a variety of applications. Beyond that, it’ll also give us excellent stability and repeatability. How does it do all that?
This temperature sensor monitors temperature by detecting resistance in an electrical current. When the temperature changes, the resistance will change inconsistent and measurable ways. Therefore, the sensor can translate these shifts into numbers we can read.
When we scale out an RTD, usually the vendor specifies the sensor according its resistance at zero degrees Celsius. In the market, a lot of sensors spec out at 100 ohms. That means that at zero degrees Celsius, the sensor will read a resistance of 100 ohms.
To know more RTD senosr, you can read our article on RTD sensor
Types of RTDs
When we scan the market, we see so many different types. How do these differences factor in? Well, let’s start with sensing elements, like platinum, nickel, and copper, the three most commonly used.
Most industries consider platinum the best element for RTDs because it offers stable resistance over a wide range of temperature. Nickel has a more limited range because it doesn’t offer a linear answer after 150 degrees Celsius.
Last but not least, we have copper. This material provides very linear resistance changes throughout the measurement range. However, we can’t use copper over 150 degrees Celsius because the sensor will oxidize.
We can also find different build categories of RTDs, like thin-film, wire-wound, and coil-element., the most common in the industries. For certain applications, we need particular sensors, like carbon resistor elements for ultra-low levels of temperature measurement.
RTD sensors with two, three, and four wires
When we talk about RTDs, we know the change in resistance indicates a proportional change in temperature value. So far, so good. Now here we have a small secret. A platinum temperature sensor is not completely built with platinum. Usually in a platinum sensor, the sensing element connects to the transmitter using a cable made of a different (cheaper) material, like copper.
Yes, indeed. The cable has a resistance value that can alter the value coming from the sensor element. And here we have the importance of the number of cables. These cables will compensate for the value of the resistance, reducing interference.
Two-wire RTDs won’t have this kind of compensation, so we use a two-wire when we only need an approximate value for the application. Most field applications use three-wire RTDs.
This kind of sensor uses the Wheatstone bridge circuit to compensate for the resistance shift in the transmitter. And of course, the four-wire RTD will eliminate the most voltage drop in your measurements, reducing its contribution to the error margin.
To know more about this device, you can check out our product review
- Good linearity
- Great accuracy
- Stable response (typically 0.05 percent per year with respect to span).
- Low output resistance
Now, let’s dive into the thermocouple universe! Industries around the world use this common solution to temperature measurement, but do we know how it works?
A thermocouple uses two different metals to produce the phenomenon called “thermoelectric effect.” That means the sensor generates a voltage when the temperature differs from one end of the thermocouple to the other. The device then translates that voltage into numbers we can read.
Now, for this kind of sensor, we’ll need a reference table to interpret those numbers. The reference table will tell us the temperature depending on the voltage measured by your sensor, and each type of thermocouple in the market uses a different table. So we need to make sure to use the right table for the thermocouple.
We have a wide range of thermocouples available. They differ in durability, temperature range, chemical resistance, vibration resistance, and compatibility. They also use letters as designations, like type K or R. Let’s check out the details of the most common thermocouples in the market.
Types of thermocouples
Thermocouples have more range of temperature measurement than RTDS and can cost up to three times less. However, if we need high accuracy and stability, then we need to stick with RTDs. If we don’t, then one of these may suit our application.
Type K thermocouple
Built with nickel-chromium and nickel-aluminum, type K rules the roost because of its accuracy, reliability, and flexibility to cover a wide range of applications.
It has a range from -270 to 1260 degrees Celsius, and the extension wire covers 0 to 200 degrees Celsius. It also has an accuracy of +-0.75 percent and special limits of error (SLE) of +-0.4 percent.
Type J thermocouple
Type J uses iron and constantan, and it has a smaller temperature range and shorter lifespan in high temperatures than type K. This temperature sensor grade has a range from -210 to 760 degrees Celsius and extension wires from 0 to 200 degrees Celsius. Standard accuracy hovers around 0.75 percent and SLE around 0.4 percent, like type K.
Type T thermocouple
Type T mostly appears in low-temperature measurement. It uses copper and constantan and has a range from -270 to 370 degrees Celsius, with extension wires from 0 to 200 degrees Celsius. The accuracy and SLE fall in the same ballpark as the first to, +-0.75 percent and +-0.4 percent, respectively.
Type E thermocouple
A Type E thermocouple offers better accuracy and signal quality compared to type K, and a good range of temperature measurement as well. Using nickel-chromium and constantan as its materials, this sensor ranges from -270 to 870 degrees Celsius, and the extension cable from 0 to 200 degrees Celsius. Although it has a similar SLE to the other three, it sports an accuracy of +-0.5 percent.
Type N thermocouple
The type N thermocouple has similar accuracy and temperature range to the K, although it has nicrosil and nisil for its materials, making it more expensive than a K. This grade supports a range from -270 to 1300 degrees Celsius, with the same extension cable as the others, 0 to 200 degrees Celsius. The accuracy is +-0.75 percent and SLE +-0.4 percent.
Type S thermocouple
Type S thermocouples have a high temperature range with high accuracy and stability. Built from platinum and 10 percent rhodium, this grade can cover -50 to 1480 degrees Celsius and the extension wire 0 to 200 degrees Celsius. At an accuracy of 0.25 percent and SLE of 0.1 percent, this represents one of the most accurate sensors in our lineup.
Type R thermocouple
The type R thermocouple also measures high temperatures in different applications. It only differs from the type S in ratio of metals, at 13 percent rhodium instead of 10. This grade goes from -50 to 1480 degrees Celsius, with an accuracy of +-0.25 percent and SLE of 0.1 percent, just like the type S.
You can find plenty of types of thermocouples on the market if you want to check out some of the less common varieties.
To know more about the difference between RTD and thermocouple, read our article on RTD vs Thermocouple vs Thermistor.
We also have an article about reading a thermocouple reference table.
Construction of the junctions on a thermocouple can also change its functions and features.
Grounded: This common junction style has the sheath and the thermocouple welded together to create one junction at the probe tip. It responds faster to temperature changes than ungrounded but can pick up transient noise on the circuit.
Ungrounded: This junction has mineral insulation, which protects it from transient noise but slows its response time.
Exposed: Welding the thermocouple wires together can allow you to insert the sensor directly into the process, increasing response time. However, this sensor can degrade or corrode quickly.
Ungrounded uncommon: This one has dual sensors insulated from each other by a sheath. It also insulates its elements from each other.
- Wide temperature range (0 to 1800 °C)
- Less stable than RTDs
- Less accurate than RTDs
Infrared temperature sensors
We’ve seen one of these devices in our daily life. Supermarkets usually have pyrometers to monitor the temperature of their freezers. An infrared temperature sensor detects thermal radiation emitted by equipment or material. This device has the useful feature of non-contact temperature measurement, which means we can check temperatures from a distance.
How does it work? Basically, a lens inside the transmitter focuses thermal radiation onto a detector. The detector converts the radiant power to an electrical signal, and the transmitter will show on its display the temperature in the proper units.
Of course, we need to know the emissivity, or how much infrared energy your equipment or material can emit, to figure out the temperature. Therefore, the device has a database of materials and their emissivity values. It also compensates for ambient temperature in its reading.
- Good accuracy
- No interference
- Easy and precise measurement
- Minimal cable
- Ineffective in fluids
- Fragile and easy to contaminate
Metals expand and contract with a change in temperature. Bimetallic thermometers rely on this property to measure temperature by converting the mechanical displacement into numbers we can read.
The temperature sensor consists of a strip with two different metals that expand and contract at different rates when exposed to temperature changes, most commonly steel and copper. Usually built in the form of a spiral tube, the mechanical expansion of the materials results in rotation. One point of the bimetallic system remains stationary, while the other side rotates a pointer to indicate the temperature.
- Limited range (-80 to 400 ˚C)s
- Regular use can result in warping
A thermistor is a “thermally sensitive resistor,” also known as a semiconductor sensor. It monitors heat by measuring changes in resistance. We classify them by negative or positive temperature coefficient (NTC or PTC), depending upon the resistance change.
Medical equipment, cars, toasters, and many more use thermistors.
- Fast output response
- Good sensitivity
- Minimal lead resistance error
- Limited range (-40 to 150 degrees Celsius)
- Non-linear measurement
In addition, to know more about how a thermocouple work, you can read our article on thermocouple types
Everything else about temperature measurement
You can find so many more temperature sensor on the market, like silicon diodes, thermistors, and others. But for the daily activities of the instrumentation engineer, the most important temperature measurement devices are the RTD and thermocouple.
If you need help choosing the right temperature sensor for your application, take a look at our new smart assistant.
If you’d like to know more about temperature sensor types and their applications or temperature measurement in general, feel free to get in touch with our engineers!