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Friday 30th July 2021 02:40 AM
Temperature Sensors – Thermistors versus Thermocouples
We Weigh the Pros and Cons of Each Type of Temperature Sensor
Thermistors vs Thermocouples. They’re both good choices for temperature measurement and temperature control applications depending on your application requirements. For electrical engineers faced with the task of managing one or more temperature detection circuits, choosing the right temperature sensing device for a Printed Circuit Board (PCB) can be confusing.
When you go through the temperature sensor selection process, you should consider the requirements of the circuitry as well as any attached machinery. For example, will the PCB be monitoring a heat sink? And is current-time delay an issue?
There are several factors to consider that can impact the overall performance of the PCB such as:
- Stability: How long must the sensor last? Is drifting a concern?
- Accuracy: Is precise temperature measurement a requirement?
- Packaging: What are you measuring? What are the environmental conditions?
- Response Time: Where will the sensor be placed? What are the Housing Requirements?
- Noise Immunity: Is immunity to noise a critical factor for the application?
- Temperature Range: Is the temperature sensor rated to perform at the desired temperature?
Whether you’re designing a new temperature detection circuit or maintaining an existing system, it’s prudent to not focus on cost alone. When you are considering an NTC thermistor vs thermocouple, be sure to review the “Five Selection Considerations” listed below. They will help determine the right choice for your application.
Thermistor vs Thermocouple. Which One is Right for Me?
There are many types of temperature sensing devices available today, and they come in different sizes, shapes, and styles to accommodate any application. So, selecting the right temperature sensor for an application can mean the difference between your system performing optimally, or failing due to equipment overheating.
Both NTC Thermistors and Thermocouples are temperature sensing devices. What are some of the advantages, or disadvantages of choosing one or the other? Read on as we break down some fundamental differences between NTC Thermistors vs Thermocouples.
We’ll cover their composition, as well as some of the applications that each are capable of handling. In the end, you’ll see why NTC Thermistors vs Thermocouples weigh in to be an efficient, cost-effective solution for designing or maintaining a temperature detection circuit system.
NTC (negative temperature coefficient) Thermistors
An NTC thermistor is a temperature sensing device made of sintered semiconductor material that contains a mix of several metal oxides. These materials possess charge carriers that allow current to flow through the thermistor displaying large changes in resistance proportional to small changes in temperature.
Inherently, NTC Thermistors produce high resistance at low temperature. As temperature increases, the resistance of the thermistor decreases. Since Thermistors experience such a large change in resistance per degrees Celsius, the smallest change in temperature is accurate with a quick response time.
Locating the right thermistor for an application requires a calculation of the resistance versus temperature using the thermistor beta (β) formula. This method uses a two-point calibration to calculate the resistance versus the temperature curve and it calibrates the resistance at both temperature points.
Achieving the right curve is important because it represents the relationship between the selected resistance and temperature. Additionally, because NTC Thermistors are non-linear, their output requires linearization. This makes their effective operating range between -50°C to 250 °C for standard thermistors.
NTC Thermistor Applications
NTC Thermistors are available in a variety of sizes and styles such as customizable probe assemblies; glass encapsulated, surface mount, and disc and chip styles. These attributes make them adaptable to perform well in many industries, such as green energy, automotive, aerospace, medical, and HVAC, to name a few.
Though many applications that use NTC Thermistors focus on resistance versus temperature characteristics, Thermistors also fill a need in other electrical applications such as Current-Time and Voltage-Current uses.
Current-Time uses can include:
- Time Delay
- Surge Suppression
- Sequential Switching
- Voltage-Current uses can include:
- Fluid Velocity
- Liquid Level Control
- Voltage Regulation
- Temperature Control Circuits
If a higher rate of accuracy is necessary, you can use NTC Thermistors in conjunction with a Wheatstone Bridge. A Wheatstone Bridge is often referred to as a (null comparator) because it makes a measurement by comparing two quantities. While the value of one quantity is known, the other quantity value is unknown and must be adjusted until it equals the known quantity value. This process enables the detector between the two quantities to give a zero, or null reading.
To learn more about NTC Thermistors and their uses, features, and benefits, visit our page “What is an NTC Thermistor”
A thermocouple is an electrical device constructed of two dissimilar conducting metals connected at one point. Together, they form two electrical junctions; the measuring (hot) junction, and the reference (cold) junction. When those junctions maintain different temperatures, they produce a low temperature-dependent DC voltage aka (thermo-electric voltage). Thermo-electric voltage can be translated into temperature so the resistance can be measured.
Referenced in this article is the “K” type. This thermocouple is commonly used as an all-purpose sensor because it can operate over a broad range of temperatures ranging from -200°C up to 1250 °C. Furthermore, because of the metals used, it is one of the least expensive types of thermocouples. Nevertheless, two limitations of thermocouples, in general, are reduced accuracy and their susceptibility to calibration drift with use over time.
Figure 1: Example K-Type Thermocouple Standard Configuration
Thermocouples are used in many applications but perform best in extreme temperatures, so they’re used extensively in the steel and iron industries. Engineers rely on thermocouples to measure and control temperature in furnaces, kilns, and boilers. Additionally, they are used in diesel engines and gas powered turbines.
Unfortunately, the lifespan of a thermocouple is hard to predict even if all application details are known. One method to predict their stability is to install the thermocouple, then evaluate its performance to determine its estimated lifespan. As you can see, this method would not be considered a cost-effective approach.
Although thermocouples function well in various atmospheres where oxidization can occur, beware of the “Green Rot” phenomenon, which gets its name from the color of the affected alloy. Green Rot can occur when the chromium in the chromel alloy is exposed to hydrogen; a reducing gas, via contact through metal wires. Consequently, should this happen, the thermocouple will produce a low or (error) reading caused from the reduced output of the EMF (Electromotive Force).
Thermistors vs Thermocouples. Five Areas to Consider When Selecting Your Temperature Sensor
When choosing a temperature sensor, the first consideration should be the temperature range of the application. Since NTC Thermistors perform well in an operating range between -50 to 250 °C, they are well-suited for a wide range of applications in different industries. Although thermocouples work in many of the same applications as NTC Thermistors, they lack accuracy in low temperature applications. However, they excel in operating environments that utilize extreme temperatures.
Stability is important in applications where long-term operation is the goal. Temperature sensors can drift over time depending on their materials, construction, and packaging. An epoxy-coated NTC thermistor can change by 0.2 °C per year while a hermetically sealed one changes by only 0.02 °C per year. While thermocouples have much lower stability of approximately 1-2 °C per year.
Of the basic temperature sensor types, an NTC Thermistors ability to achieve the highest accuracy is within the -50 to 150°C & up to 250 °C range for glass encapsulated. Accuracy ranges from 0.05 to 0.20 Degree Celsius with high long-term stability. If a thermocouple is used, the accuracy of the measurement could be off by up to 5 °. Furthermore, the responsiveness of such a thermocouple is on the order of 20 s.
NTC Thermistors offer excellent immunity to electrical noise and lead resistance because they possess high resistance during initial switch-on. Though not affected by lead resistance, thermocouples are susceptible to electrical noise because they output a small signal which can be affected by electrical noise.
Packaging requirements are dictated by the environment the temperature sensor will be used in. NTC Thermistors can be customized and potted into various housings dependent on application requirements. They can also be epoxy coated or glass encapsulated for further protection.
Thermistors vs Thermocouples
Thermistors vs Thermocouples? You be the judge. For their overall performance and cost effectiveness, NTC Thermistors weigh in to be an excellent choice for your temperature sensing solutions.
- Fast Response
- Greater sensitivity
- Stability and accuracy within their temperature range