The Complete Guide to Temperature Sensors

Introduction

Temperature sensors are a specific type of sensor that measures the temperature or temperature gradient. They typically consist of a measurement element that detects the change in temperature. Temperature sensors work based on different principles and react to temperature changes differently. A few common temperature sensors include RTDs(Resistance Temperature Detector), thermistors, thermocouples, infrared measurements, and thermostats - each of these has a range of temperature that is more accurate, with advantages and disadvantages. 

This tutorial will cover some of the most commonly used thermometers and some ways of interfacing them with PLCs. To follow along, it is important to understand the concepts of temperature and heat; PLCs, current, and voltage connections.

Types of Temperature Sensors 

Temperature sensors can be classified as contact and non-contact sensors. 

Contact temperature sensors need the sensing element to be physically in contact with the surface whose temperature is being measured. For example, when thermocouples are used to measure the temperature of a furnace, they need to be in physical contact with the hot surface whose temperature is being measured. This is because heat flows from the hot surface being measured to the thermometer that causes a change in electrical signals. A change in these signals can be measured and converted into a change in temperature. 

Non-contact temperature sensors do not need the sensing element to be in contact with the surface being measured. As an example, an IR radiation thermometer can be used to measure the temperature of a boiler from a safe distance and without needed physical contact. This is a key difference between the contact and non-contact type of measurements.

Contact Temperature Sensors 

This section will cover some common industrial contact type temperature sensors.

Resistance Temperature Detector (RTD) Sensors

The sensing element of RTDs is a fine metal wire whose resistance changes with temperature. Therefore, measuring the change in resistance shows us the change in temperature. RTDs can be made from various metals such as platinum, nickel, and copper. Their range of accurate temperature measurement varies from -200 C to 1000 C. For example, PT100 is a platinum RTD with a resistance of 100 Ohms at 0 C; 138.5 Ohms at 100 C. By measuring the resistance and applying a basic formula, we can calculate the temperature. 
High accuracy, ease of installation, wide operating range, precise measurement, and stability over time are some of the advantages of using RTDs for temperature measurement in industrial applications.

Higher costs, power supply requirements, slower response, and low sensitivity to temperature changes are some of the disadvantages of using RTDs. Even though RTDs can measure up to 1000 C, the accuracy is low for temperatures above 850 C. PLCs have RTD expansion modules for reading RTD signals. RTDs can also be connected to PLC analog modules after converting the RTD signals to a current or a voltage signal using a signal converter. RTDs are used in applications such as in automobiles and industrial measurements below 600 C.

Thermocouples

A thermocouple is two different metallic wires that are joined at one end (hot junction). As the temperature at the hot junction changes, a voltage is induced at the other end (cold junction). This principle is called the Seebeck effect. By measuring the voltage across the open ends of the cold junction, we can calculate the temperature. Typically the hot junction is in contact with the process being measured. The cold junction is maintained at a fixed temperature - traditionally 0 C.

Thermocouples have a very wide range of measurements from -330 C to 3100 C. The relation between temperature and generated voltage is not linear. Unlike RTDs, thermocouples are passive sensing elements and do not need a power supply.

Thermocouples are cheaper than RTDs and are very rugged and responsive. But they are not as stable or accurate as RTDs. Since the voltage doesn’t change linearly with temperature additional signal manipulation is necessary to linearize the outputs. When thermocouples are used to measure temperatures of corrosive fluids it is necessary to insulate the measuring end to prevent corrosion, this slows the response of the temperature. There are various types of thermocouples - E, J, B, K - they are made of different metal combinations and ratios. Different thermocouples are suitable for use in different environments such as for inert materials, oxidizing or reducing environments.

PLC manufacturers have input expansion modules designed to read thermocouple voltages. Due to the non-linear temperature-to-mV relation, it is necessary to have specific PLC modules in order to get reusable results. The below PLC code shows analog input reading from a thermometer converted into a human-readable temperature used to control a heater power.             

The Siemens sample code above shows how to convert the raw value to a scaled and human readable temperature value. The temperature is then used for checking a condition that controls a PLC digital output to turn off a heater.  

Thermostats

A key difference between thermostats and the other temperature measurements is that there are only two states, ON & OFF, around a certain temperature as opposed to a range of temperatures. A thermostat is made of a bimetallic strip that changes shape at a certain temperature. These are purely mechanical and do not have a voltage or a current output. The construction of bimetallic strips is very simple - two metals with different temperature coefficients are stuck together. As temperature changes, the strip bends one way or another causing the on/off action. These are inexpensive and are used where accuracy is not critical such as in household room heater control. Due to their relatively simple construction, they are inexpensive and easy to maintain. 

Connecting a thermostat to a PLC input is similar to a switch. Below is a sample code showing how a thermostat input can control a heater’s power. In this example “Heater1” represents a relay that controls power to the heater being controlled.

The Siemens sample code above shows how a thermostat output controls a PLC digital output to turn on or off a heater.

Thermistors 

The word thermistor is a combination of thermal and resistor - thermistors are made of materials whose resistances are very sensitive to temperature. They differ from RTDs in a few different ways. RTDs are made of pure metals and thermistors are made of ceramics or polymers with impurities that determine how they react to temperature. There are broadly two types of thermistors, PTC (Positive Temperature Coefficient) thermistors, and NTC (Negative Temperature Coefficient) thermistors.

The resistance of PTCs increases with temperature and it is the opposite for NTCs. PTCs are used in inrush current limiters as the resistance increases with the current and the current flow is limited above a certain current value. 

Thermistors have an operating range of -90 C to 150 C. Their range of measurement is much lower than for RTDs, but they are preferred when high accuracy is desired.

Non-Contact Temperature Sensors 

Two of the most commonly used industrial non-contact temperature sensors are IR/Radiation thermometers and Optical pyrometers.

IR/Radiation thermometers

Objects emit heat and radiation of different wavelengths. Infrared radiation emitted by an object can be measured and the intensity measured corresponds to temperature. Since this type of thermometer measures the radiation being emitted by objects, they do not need to be in contact with the objects. There are several reasons why this makes an excellent choice, some of them include: object surfaces being too hot for a contact type thermometer, requiring a fast and accurate response, or just simply for the fact that they do not need contact - such as for measuring a patient's temperature during COVID. 

Different radiation thermometers have different sensitivities and can measure temperature accurately from shorter or longer distances. These are also excellent options when mobility is needed such as for fire-fighters or volcanoes or moving objects. This will ensure the safety of personnel allowing them to work from a safe distance. Some radiation thermometers also have a laser to help aim at the object for ease of use.

The temperature range of this thermometer is from -70 C to 540 C. Handheld radiation thermometers usually have a display to show the temperature reading. Industrial radiation thermometers usually support one of the standard protocols such as RS485 or PROFIBUS compatible with most PLCs in the market. Radiation thermometers are also called infrared pyrometers.

Optical pyrometers 

Just like radiation thermometers, optical pyrometers measure the surface temperature of objects. However, they are preferred for applications with high temperatures such as furnaces and molten metals.

The disappearing filament type optical pyrometers have a filament that is powered by an adjustable current source. As current through this filament circuit is manually adjusted, the brightness of the filament varies. Light from the object being measured is directed onto the filament. The goal is to vary the current passing through the filament until the brightness of the filament matches that of the light from the object being measured. When the filament outline disappears it means they are at the same temperature. The temperature of the object’s surface can then be inferred from the amount of current flowing through the filament circuit. Optical pyrometers work well at very high temperatures but they are not accurate at temperatures lower than 700 C.  

Wiring and Connecting a Thermometer to a PLC

Instruments use different communication protocols, currents, and voltages to transfer signals.

4-20 mA current loops are some of the most common signals in industrial applications. 4 mA corresponds to the lowest value of measurement and 20 mA corresponds to the highest value. The reason 4 mA is used instead of a 0 mA signal is to differentiate between a reading of the lowest value and a broken wire connection. Most PLCs have a built-in 4-20 mA connection or have expansion modules that can measure 4-20 mA signals.

Below is an image of how thermocouples are connected to a Siemens SM 1231 module 

Below is one example of a Siemens module used for RTD measurements. The numbers 1-4 represent an RTD. These are some different ways an RTD can be connected.

1-5 V DC signals are also some of the oldest and widely adopted, just like 4-20 mA signals. For example, RTD resistance changes can be translated into 1-5 V signals by signal manipulation. This voltage signal can then be wired to a PLC analog input module and the PLC can be programmed to convert the voltage into temperature. 

In addition to analog modules, PLCs also come with expansion modules for protocols such as RS485, PROFIBUS, and IOLink that are commonly supported by various instruments.

Conclusion

We have discussed different temperature measurement principles, types of sensors, ranges of temperature, accuracies, costs, advantages, disadvantages, and popular ways of connecting them to PLCs. RTDs can be connected to current or voltage analog modules with some signal manipulation. RTDs can also be connected directly to PLC modules made specifically for RTD measurements. Thermocouples require PLC modules (or alternative ways) that can linearize the output.