Guide to the types of temperature sensors and their applications
Introduction
In many cases, we need to measure the exact temperature, and in some cases, we need precise control. If we study the types of temperature sensors from the perspective of their characteristics, we realize that in order to choose a suitable sensor, we must consider certain points. In this article, we will introduce different types of temperature sensors and explain how they work, and point out some of their uses.
Types of temperature sensors
Did you know that there is a temperature sensor in almost every electronic device? For example, a smartphone. Or a personal computer. Even kitchen appliances.
There are many types of dual sensors on the market, many of which we will discuss in this article. But the two most commonly used in process applications are resistance temperature sensors (RTDs) and thermocouples. You’ve probably come in contact with both temperature sensors at least once in your life.
Other sensors on our market are infrared sensors and biometric sensors. They have fewer uses in automation systems, but you need to know a little bit about them as well.
1) Resistant Temperature Sensors (RTD)
These sensors, as one of the most accurate sensors available, are well known and offer good accuracy in a variety of applications. They also have excellent stability and repeatability.
How do RTDs work?
These temperature sensors control the temperature by measuring the resistance in the circuit. As the temperature changes, the resistance of the sensor changes with a certain and measurable value. Therefore, the sensor can convert these resistance changes to measurable values for controllers.
When evaluating RTDs, we usually determine the sensor according to its resistance at zero degrees Celsius. One of the most widely used sensors on the market is the 100 ohm, abbreviated PT100. This means that at zero degrees Celsius, the sensor reads 100 ohms resistance.
When we evaluate the market for these sensors, we come across different types. What is the reason for this difference and diversity? Well, let’s start with the constituent elements, such as platinum, nickel and copper (three common ones). Most platinum industries consider it the best element for RTD because it has stable resistance over a wide range of temperatures. Nickel has a more limited range because it does not provide a linear response after 150. C. The next item is copper. This material provides very linear resistance changes throughout the measurement range, but we cannot use copper more than 150. C because it oxidizes.
We can also mention the different manufacturing categories of RTDs, such as Think-film, wire-wound and Coil-element, which are the most common in the industry. For some applications, we need special sensors, such as carbon resistance elements, to measure very low temperatures.
RTD sensors with two, three and four wires
When we talk about RTDs, we know that the change in resistance indicates a proportional change in temperature. Now we have a small point here. A platinum temperature sensor is not made entirely of platinum. Usually in a platinum sensor, the measuring element is connected to the device using a cable made of different (cheaper) materials such as copper.
Yes, in fact, this cable has a certain amount of resistance that can change the value of the sensor element. So here we have to consider the importance of cables. Special cables compensate for the amount of resistance and reduce deviation.
Two-wire RTDs do not have this type of compensation, so we use two wires when we only need an approximate value for the temperature. Most devices use three-wire RTDs.
This type of sensor uses the Wheatstone bridge circuit to compensate for the change in resistance in the transmitter. Four-wire RTDs have the most error correction and are more accurate.
Advantages of RTD
- Good linear coefficient
- Stable response (typically 0.05% per year)
Disadvantages
- Self-heating
- Expensive
2) Thermocouple
Now, let’s take a look inside the world of thermocouples! Industries around the world use these common sensors to measure temperature, but do you know how it works?
A thermocouple uses two different metals to produce a phenomenon called “thermal effect.” The device then converts this voltage to the numbers we can read.
For this type of sensor, we need a reference table to convert the voltage to temperature. The reference table announces the temperature depending on the voltage measured by the sensor, and each type of thermocouple on the market uses a specific table. Therefore, we must use the appropriate thermocouple table.
We have a wide range of thermocouples available. They are different in durability, temperature range, chemical resistance, vibration resistance. They also use acronyms as symbols, such as K or R. Let’s take a look at the details of the most common thermocouples on the market.
Types of thermocouples
Temperature thermocouples have a wider range than RTDs and are up to three times less expensive. However, if you need high accuracy and stability, you should use RTDs. Otherwise, one of these may be your request.
Type K thermocouple
Type K, made with nickel chromium and nickel aluminum, is used for a wide range of industrial applications due to its accuracy, reliability and flexibility.
Its range is between 270 and 1260 degrees Celsius and its communication wire covers from 0 to 200 degrees Celsius. It also has an error rate of 0.75% and a specific error (SLE) of about 0.4%.
Type J thermocouple
Type J uses iron and concentrate and has a lower temperature, and its lifespan is shorter at high temperatures than type K. This temperature sensor has a range between -210 to 760 degrees Celsius and communication wires from 0 to 200 degrees Celsius. The standard accuracy is about 0.75% and the SLE is about 0.4% like the K-type.
Type T thermocouple
Type T is mostly used to measure low temperatures. It uses copper and canvas and has a range of 270 to 370 degrees Celsius and its connection wires range from 0 to 200 degrees Celsius. Accuracy and SLE are in the first place, respectively, 5.75% and 0.4%, respectively.
Type E thermocouple
The E-type thermocouple has better signal accuracy and quality than the K-type, as well as better temperature measurement. This sensor uses Nickel Chrome and your counter as its materials from -270 to 870 degrees Celsius and the communication cable from 0 to 200 degrees Celsius. Although it has a similar SLE to the other three groups, its accuracy is 0.5%.
Type N thermocouple
The N-type thermocouple has the same accuracy and temperature range as the K, although it uses microsil and nissil in its materials, but is more expensive than the K. This sensor is from 270 to 1300 degrees Celsius with the same cable conditions. Like others, 0 to 200 degrees Celsius. Its accuracy is 0.75% and SLE 0.4%.
Type S thermocouple
Type S thermocouples have a high temperature range with high accuracy and stability. The sensor, which is made of platinum and 10% rhodium, can cover from 50 to 1480 degrees Celsius and the communication wire from 0 to 200 degrees Celsius. With an accuracy of 0.25% and SLE 0.1%. This is one of the most accurate sensors in the industry.
Type R thermocouple
The R-type thermocouple also measures temperature in various applications. In terms of metals alone, it differs from type S in that it is 13 percent radium instead of 10 percent. Temperature range is from -50 to 1480 degrees Celsius, with an accuracy of 0.25% and SLE 0.1%, just like the S type.
Thermocouple connections
Building connections in a thermocouple can also change its function and features.
Ground ground: This method of joint connection with sheath and thermocouple is welded together to create a connection at the tip of the probe. It responds faster to temperature changes and can make transient noise on the circuit.
Ungrounded: This junction has mineral insulation that protects against transient noise but slows down the reaction time.
Exposed: Welding thermocouple wires together can allow you to enter the sensor directly into the process and increase response time. However, this sensor can be quickly damaged or corroded.
Ungrounded uncommon: This case has two sensors that are insulated from each other by a sheath. Also, the size of the components of the two sensors are insulated from each other.
Advantages
- Extensive temperature range (0 to 1800) C)
- Strong and hard
- Save money
Disadvantages
- Less stable than RTD
- Less accuracy than RTD
3) Infrared temperature sensors
We have seen one of these devices in our daily lives. Supermarkets usually have a thermometer to monitor the temperature of their freezers. An infrared temperature sensor detects thermal radiation emitted by equipment or materials. This device has useful features for measuring contact temperature without contact, which means that the temperature can be checked from a distance.
how it works? Basically, a lens inside the transmitter focuses heat radiation on the detector. The tracker converts the radiant power into an electrical signal, and the transmitter displays the temperature on its display in the appropriate units.
Of course, to determine the temperature, we need to know how much emissions or infrared energy your equipment or materials can emit. Therefore, the device has a database of materials and their release values. It also compensates for the ambient temperature by reading it.
Advantages
- Good accuracy
- Without intervention
- Easy and accurate measurement
- Minimum cable
Disadvantages
- Ineffective in liquids
- Expensive
- Fragile and easy contamination
4) Biometric thermometer
Metals expand and contract as the temperature changes. Biometric thermometers rely on this feature to measure temperature by converting mechanical displacement to numbers we can read.
The temperature sensor consists of a strip of two different metals, usually steel and copper, which contract at different speeds when exposed to temperature changes. It is usually made in the form of a spiral tube, the mechanical expansion of the material leading to rotation. One point of the two-metal system remains constant, while the other side rotates a marker to indicate the temperature.
Advantages
- Simple
- powerful
- Inexpensive
Disadvantages
- Limited range (80 to 400 degrees Celsius)
- Regular use can lead to twists and turns
5) Thermistor
The thermistor is “heat-resistant”, also known as a semiconductor sensor. It measures heat by measuring changes in resistance. Depending on the resistance change, we classify them with a negative or positive temperature coefficient (NTC or PTC).
Medical equipment, cars, toasters and many other thermistors are used.
Advantages
- Quick output response
- Good sensitivity
- Minimal lead resistance error
Disadvantages
- Limited range (40 to 150 degrees Celsius)
- Nonlinear measurement
- Self-heating
