Temperature Sensors
A Temperature sensor is a device that provides temperature measurement in a readable form, which can be digital or analog. Temperature sensors can come in many different forms depending on what you are trying to measure and have a variety of domestic uses as well as many industrial and manufacturing applications where process control is needed. They are used in various applications around the house such as HVAC systems, refrigerators, water heaters, microwaves, ovens etc. They are used in the automotive industry to measure engine temperatures and climate control. Temperature sensors serve a variety of uses in other industries such as food and beverage, medical, chemical/oil, renewable energy, consumer electronics, aerospace, agriculture, among others. The following will be an overview and comparison of thermocouples, thermistors, and resistance temperature detectors (RTD).
A Thermocouple is a type of sensor has 2 dissimilar metal wires that are twisted or welded together at one end, which is referred to as the measuring point. At the other end of these wires is what is called the reference point, or also called the cold junction, which is where the wires connect to a voltage reader. When there is a temperature difference at the measuring point, this generates an electromotive force (EMF) at the connection point, this phenomenon is called the Seebeck effect.
The EMF that is generated is measured at the connection point by the voltage reader and used to determine the difference in temperature. Note that the thermocouple doesn’t measure the absolute temperature but instead measures the temperature difference between both junctions. Due to the fact it only measures the temperature difference, the temperature of the cold junction needs to be known for accurate temperature readings. This is referred to as cold junction compensation and can be done in different ways. Historically thermocouples have used a known temperature, such as an ice bath (0°C) to get a temperature reading, but modern thermocouples will utilize electronic compensation by using a thermocouple or resistance temperature detector (RTD). Another method to compensate for this would be using software that incorporates an algorithm into the temperature calculation.
The type of thermocouple will be determined by the type of metal that is used and is denoted by a lettering system. Each letter refers to a specific combination of 2 different metals that are used in the thermocouple. Type J, K, T, & E are base metals and are the most common type of thermocouples. Type R, S, and B are noble metals that are used to withstand extreme temperatures.
Click here for an overview from TE Connectivity on the different types of thermocouples.
Thermocouples are ideal for a wide range of applications due to their rapid reaction rate and ability to withstand certain extreme conditions. These sensors are versatile and widely used because they offer a good balance of cost, durability and temperature range make them suitable for various applications.
The main disadvantage of thermocouples compared to other temperature sensors is they are not quite as accurate and sensitive to temperature change. They are also normally less stable and tend to drift over time, especially at higher temperatures. Another added complexity for thermocouples is the need for cold junction compensation for an accurate temperature calculation.
The term thermistor comes from a combination of 3 words, thermally sensitive resistor, and this sensor exhibits a continuous, incremental change in resistance correlated to variations in temperature. A thermistor is a type of temperature sensor that is made of a semiconductor material that reacts like a resistor sensitive to temperature. This means that they have a greater resistance than conducting materials but a lower resistance than insulating materials. A thermistor actually doesn’t measure the temperature but instead measures the resistance of the thermistor as temperature changes.
There are two different types of thermistors, Negative Temperature Coefficient (NTC) and Positive Temperature Coefficient (PTC). The main difference between the two is their temp-resistance relationship which gives them versatility for different applications.
Negative Temperature Coefficient (NTC) thermistors will see a decrease in resistance as the temperature increases and conversely, the resistance will increase as the temperature decreases. NTC sensors are more commonly used because they tend to be more accurate and are more sensitive to temperature changes, making them ideal for precise temperature measurement and control. Contrary to NTC thermistors, the resistance of Positive Temperature Coefficient (PTC) thermistors will increase as the temperature increases. Although PTC thermistors are not as commonly used as the NTC type, they are still commonly used in self-regulated heating elements and are ideal for overcurrent protection because of their ability to limit power surges by absorbing excess energy. While NTC thermistors offer more efficient active monitoring techniques such as soft-start or pulse width modulation (PWM), PTC thermistors offer great self-heating or resetting properties that help protect against overloads or short circuits without the need for manual intervention.
Click HERE to view epoxy-coated NTC thermistors from Molex
The main advantages of thermistors are that they are highly sensitive and offer a quick response time. They normally have a compact design which makes them easy to integrate and ideal for applications with space constraints. Thermistors are also inexpensive compared to other common types of temperature sensors making them cost-effective for a variety of applications.
The disadvantage of thermistors is that they do not have a linear response when it comes to the resistance-temperature relationship which can make it more complicated to convert resistance measurements to temperature readings. The operational range of a thermistor is dependent on probe type and is typically between -100°C and 300°C, which is more of a limited range compared to other temperature sensors.
A Resistance Temperature Detector (RTD) is a device that acts as a variable resistor that changes resistance in direct proportion to the change in temperature. RTD’s will measure the temperature based on the principle that the electrical resistance of a metal will change as the temperature changes. If the temperature increases and the metal conductor heats up, the resistance of the current flow increases. On the other hand, if the temperature decreases then the resistance will decrease as well. RTD’s use this variation in electrical resistance to measure the change in the temperature.
What makes RTD sensors unique is that they use a sensing element that is exposed to the medium being measured and responds to the temperature changes by altering its electrical resistance. The most common type of sensing element used is normally Platinum because it has a stable and repeatable resistance-temperature relationship. This sensing element is either composed of a metal wire that is an etched grid on a substrate or wire wound on a coil.
Click here for TE Connectivity’s thin-film Platinum RTD’s
The main advantage of RTD’s over thermocouples and thermistors is that they are much more precise, offering an accuracy tolerance within .01%. Their true temperature-resistance relationship is not technically linear but is approximated as such under specific temperature range. Some RTD’s also feature a robust design which is ideal for industrial applications where durability is needed.
The main disadvantage of RTD’s compared to other temperature sensors is cost, especially if platinum is being used as the sensing element. They also tend to have a slower response time and lower operating range compared to thermocouples but are also considered not as versatile or durable.
Conclusion
Thermocouples, thermistors, and resistance temperature detectors (RTD) are used in a wide range of applications in both domestic and industrial settings. Each sensor has unique characteristics that make them suitable for specific uses: thermocouples are versatile and robust, thermistors offer high sensitivity and precision, while RTDs provide exceptional accuracy and stability. Understanding the differences between these sensors allows for the selection of the most appropriate technology for accurate temperature measurement and control in various environments.
When choosing a temperature sensor, it’s crucial to consider the specific requirements of your application, such as the desired temperature range, sensitivity, response time, and environmental conditions. By selecting the right sensor, you can ensure reliable and precise temperature monitoring, which is essential for maintaining optimal performance, safety, and efficiency in both everyday household devices and complex industrial systems.
For a full list of Thermocouples, Thermistors and Resistance Temperature Detectors, click here