For many industries, being able to measure precise temperature variations is an absolute must. Temperature ranges can vary greatly and environmental temperatures typically change slowly, so the temperature devices chosen need to be able to span a large temperature range and read subtle changes in temperature.
Electronic temperature measurement is commonly used for those applications due to its higher measurement resolution and accuracy, as compared with mechanical devices such as liquid-in-glass or dial thermometers.
There are three main electronic temperature sensors that are currently used in various industries. They are Thermocouples, Resistance Temperature Detectors (RTDs), and Thermistors.
Each device has its own advantages and disadvantages and so it is important to weigh the differences between them so that the correct sensor is being used for the measurement being made.
Thermocouples consist of two wires of dissimilar metals connected together to form a junction. The change in temperature of the junction can be determined by the voltage change between the wires. The other end of the signal wires is connected to a reference junction, which is usually a part of the measuring instrumentation. The reference junction is electronically compensated for the ambient temperature to provide accurate measurement results
Thermocouples are very popular in that they are simple devices and are used for various applications in furnaces, ovens, water heaters, and aircraft engines. They are easily interchangeable, inexpensive, have standardized connectors and color-coding, and can measure a wide range of temperatures.
Thermocouples are made in many different configurations depending on the different metals used to create the thermocouple junction. Thermocouple types K, J, T, E, and S are the most popular metal combinations due to their cost, availability, chemical properties, and temperature ranges.
RTDs, sometimes referred to as Platinum Resistance Thermometers (PRTs), are temperature sensors that change resistance to variations in temperatures. RTDs are most often made using platinum due to its linear resistance-temperature relationship as well as its chemical inertness. RTDs are often used for laboratory temperature measurements since the element’s resistance increases with temperature in a linear and repeatable manner, which leads to higher accuracies and stability than thermocouples.
RTDs are usually offered in either 3-wire or 4-wire configurations to minimize any self-heating in the wires due to the excitation current that is required in order to determine the resistance. The additional wires carry the current while the other wires only carry the voltage into the measurement system to minimize any lead resistance in the measurement.
Thermistors are also a type of resistive temperature sensor that typically possesses a large negative temperature coefficient so that the indicated resistance drops with an increase in temperature. The main difference from RTDs is that thermistors are semiconductor components and are made out of ceramic or polymers where RTDs are made from pure metals. For that reason, thermistors typically cannot be used at higher temperatures or they will be permanently damaged.
Thermistors produce a non-linear resistance curve and because of this are most accurate in a narrow temperature range depending on the thermistor type. The sensitivity in that range is much better than that of RTDs but the useful temperature range is limited.
Measurement Characteristics Comparison
RTDs work better in below-freezing temperatures and in environments up to approximately 1200°F (650°C) while thermocouples are commonly used for measuring higher temperatures and larger temperature ranges. Thermistors should only be used in a range of about 100°F around a target temperature due to diminishing accuracies outside of this range.
Accuracy is one of the major factors to consider in the selection of temperature sensors. RTDs, thermistors, and thermocouples perform with different accuracies in different temperature ranges. Over their entire range, RTDs are the most accurate of the sensors and precision RTDs can reach accuracies better than 0.1°F. The accuracy of standard thermocouples is roughly no better than 4°F, although special-limit thermocouples can achieve accuracies down to 2°F. Thermistors can achieve accuracies similar to RTDs in a narrow temperature range and are best used if you don’t expect a lot of temperature variation.
If you have any questions on the use of Thermocouples, RTDs, or Thermistors, contact e2b Calibration and we will help you choose the proper sensor for your application.