In most cases, we need to monitor exact temperatures, and in some, we need precise control. When we learn about temperature sensor from the maintenance point of view, we realize that we need to consider certain points to choose the right sensor for your application.

The article guides us through the multitude of temperature sensor available in the market and discusses the applications where it can be used.

Temperature sensor types

Today, we have a ton of different temperature sensor types in the market. But two in particular really stand out in most process applications, the resistance temperature detector (RTD) and the thermocouple.

From the others on the market, we’ll discuss a couple more, the infrared sensor and the bimetallic sensor. Though they have fewer applications in process automation, we will touch base with them too.

To know more about the difference between RTD and thermocouples, you can read the Visaya Article here

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.

Endress+Hauser
iTHERM TM411 Innovative, modular resistance thermometer for the food industry
in the shop from 425 €
TM411

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. On the market, a lot of sensors spec out at 100 ohms (P t100). That means that at zero degrees celsius, the sensor will read a resistance of 100 ohms (Pt 100)

To know more about how temperature sensors is connected to a control system, you can read the Visaya Article here

Types of RTD

When we scan the market, we  see so many different types. So, how do these differences factor? Well, let’s start with sensing elements, like platinum, nickel, and copper, the three most commonly used.

Most industries consider platinum as 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, you 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. 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.

Endress+Hauser
Easytemp TMR35 Compact hygienic thermometer
in the shop from 116 €
TMR35

Two-wire RTDs won’t have this kind of compensation, so use a two-wire when we only need an approximate value for your application. Most field applications use three-wire RTDs.

This kind of sensor uses the Wheatstone bridge circuit to compensate for the resistance shift in your transmitter. And of course, the four-wire RTD will eliminate the most voltage drop in your measurements, reducing its contribution to your error margin.

 

To know more about TMR35, you can check out the Visaya Product Review

Pros

  1. Good linearity
  2. Great accuracy
  3. Stable response (typically 0.05 percent per year with respect to span).

Cons

  1. Self-heating
  2. Low output resistance

Thermocouple

Industries around the world use this common solution to temperature measurement, but do you know how it works?

So 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 will need a reference table to interpret those numbers. The reference table will tell us the temperature depending on the voltage measured by the sensor, and each type of thermocouple on the market uses a different table. So we have to make sure to use the right table for the thermocouple we have.

thermocouple types
Courtesy of Temperature Sensors

As we mentioned before,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.

B+B Thermo-Technik GmbH
FF-GLT Type AF Humidity/ temperature sensor for outdoors with voltage output 
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FF-GLT-10V-AFT

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 RTDs is the right choice for us. For other reasons, we can check out the types of thermocouples.

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.

B+B Thermo-Technik GmbH
Type K Thermocouple Ø1.0 mm with miniature plug Mineral insulated thermocouples sensor
in the shop from 11 €
K-M625-10

Type J thermocouple

Type J uses iron and constantan, and it has a smaller temperature range and shorter lifespan in high temperatures than typeK. 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.

B+B Thermo-Technik GmbH
Surface thermocouple type J Thermocouple sensor with glass fibre insulated connecting cable
in the shop from 41 €
J98AA08 083.0-7

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 two, +-0.75 percent and +-0.4 percent, respectively.

Type E thermocouple

This 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 N has a 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 supports 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 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.

Thermocouple junctions

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.

Pros

  1. Wide temperature range (0 to 1800 °C)
  2. Rugged
  3. Cost-efficient

Cons

  1. Less stable than RTDs
  2. Less accurate than RTDs

Infrared sensors

We have seen one of these devices in our daily life, even if we didn’t know it. Supermarkets usually have pyrometers to monitor the temperature of their freezers. An infrared sensor detects thermal radiation emitted by equipment or material. This device has the useful characteristic of non-contact, which means we can check temperatures from a distance.

B+B Thermo-Technik GmbH
Flashbeam 8869 Infrared thermometer for contactless temperature measurements
in the shop from 41 €
0560 8869

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.

Pros

  1. Good accuracy
  2. No interference
  3. Easy and precise measurement
  4. Minimal cable

Cons

  1. Ineffective in fluids
  2. Expensive
  3. Fragile and easy to contaminate

Bimetallic sensors

Metals expand and contract with a change in temperature. Bimetallic devices 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 result in rotation. One point of the bimetallic system remains stationary, while the other side rotates a pointer to indicate the temperature.

Pros

  1. Simple
  2. Robust
  3. Inexpensive

Cons

  1. Limited range (-80 to 400 ˚C)s
  2. Regular use can result in warping

    Thermistor

    A “thermally sensitive resistor,” also known as a semiconductor sensor, 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.

    Pros

    1. Fast output response
    2. Good sensitivity
    3. Minimal lead resistance error

    Cons

    1. Limited range (-40 to 150 degrees Celsius)
    2. Non-linear measurement
    3. Self-heating

Key takeaways

Temperature sensors are useful instruments for determining process temperature. The type of temperature sensor we use primarily depends on the application concerned.

In addition, parameters such as temperature range, interchangeability, accuracy, tolerance and individual characteristics play a decisive role in selecting the right sensors needed for our applications.

To know more on selecting the right temperature sensor, you can get in touch with our engineers!

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