Numerous technologies are available to measure temperature. And, with many overlapping characteristics, choosing the ideal methodology can be challenging. Often used to replace thermocouples or electromechanical thermostats, thermistors are gaining popularity. The ability to simplify circuits, eliminate system calibration requirements, and provide low power designs are among the benefits offered by thermistors. But, are they right for your application? This article will review the basic thermistor technology and provide some pointers that will help you decide if your application can benefit from using them.
Thermistors derive their name from the device’s major characteristics -- they are thermally sensitive resistors. Thermistor-based temperature sensors exhibit a change in electrical resistance in response to a change in ambient temperature conditions. They are highly sensitive and have reproducible resistance vs. temperature properties.
Thermistors can be further classified as either “positive temperature coefficient” (PTC) or “negative temperature coefficient” (NTC) devices. PTC thermistors demonstrate a rapid increase in resistance near a fixed temperature point, called the Curie temperature, which is typically in the range of 212 to 302°F (100 to 150°C). PTC thermistors are useful for self-regulating heaters and simple thermal switch applications.
NTC thermistors exhibit a steep drop in resistance as temperature increases. The resistance changes approximately three orders of magnitude in a 180°F (100°C) range. This high sensitivity to temperature change allows NTC thermistors to accurately measure very small temperature variations. NTC thermistors commonly are used for precise temperature measurement in environments where the cost of failure is high.
This accuracy makes NTC thermistors unique in their functionality in many applications compared to temperature sensors or switches such as thermocouples or electromechanical thermostats. Additional factors that come into play when evaluating temperature-sensing methodologies are interchangeability, the installability and accuracy of the two-wire connection, ruggedness, hermetic seal capability and flexibility. Because the remainder of this article will focus on NTC thermistors, I will consider each feature separately.
Interchangeability. Through process control and precise manufacturing practices, thermistor sensors can be produced with accurate tracking of a specified resistance vs. temperature curve. These precision thermistors can be “dropped into” a circuit to measure temperature accurately over a wide range.
Referred to in the industry as interchangeable thermistors, they are available with tolerances as tight as +/-0.09°F (0.05°C), not just at a single point, but over a temperature range from 32 to 158°F (0 to 70°C). This interchangeability eliminates the need for individual sensor calibrations, saving time and cost. Thermocouples, while producing a repeatable voltage output based on temperature, have a much wider tolerance of this output, and thus are less accurate in indicating temperature for critical thermal applications. Typical thermocouple accuracy is +/-3.9°F (2.2°C) for a Type K; while the best thermocouples can attain +/-0.9°F (0.5°C) over a short span, they are 10 times less sensitive than thermistors.1
Thermocouples measure relative temperature, which means they can only sense temperature differences. To be useful, thermocouple-measuring circuits require an absolute temperature sensor (such as a thermistor) to determine the cold junction temperature. And, by definition, electromechanical switches “switch” at a designated temperature level, making the devices less useful in applications where temperature indication over a range is required.
Two-Wire Connections. Thermistors operate with two wire connections; unlike thermocouples, reference junction compensation is not necessary for thermistors. The inherently higher resistance and higher temperature coefficient for thermistors allows longer lead length without introducing significant errors compared to platinum RTDs, which must operate in a three- or four-wire mode. For instance, 10' of 26 AWG two-conductor wire has a resistance of 0.82 Ω. This resistance represents an error of 3.8°F (2.1°C) for a 100 Ω platinum RTD, but for a 10 kΩ thermistor, it is 0.0036°F (0.002°C). Thermocouples with two-wire connections are limited by the necessity to carefully match metal types in connectors and extension cables to avoid temperature error from spurious thermocouple effects.
Ruggedness. The more severe the application environment for the temperature sensor, the more rugged the device must be. Monitoring of the manufacturing plant or even office ambient temperature is far less demanding than monitoring of temperature on space station batteries -- the latter being an NTC thermistor application.
NASA and ESA (European Space Agency) qualifications programs include numerous tests of ruggedness that NTC thermistors are able to pass. Durability also is a deciding factor, which is one reason that NASA has specified thermistors qualified for extended space flight. To be fair, from a ruggedness standpoint, thermocouples could be used, but their relative inaccuracy compared to thermistors makes them less attractive. Switches are only useful when a temperature setpoint is involved and, depending upon their construction, may not be suitable for use in space because of mechanical constraints.
Hermetic Seal. Glass-encapsulated thermistors achieve a hermetic seal between the environment and the thermistor element. This permits measurement in severe moisture environments without concern for silver migration and provides stability over a range of operating temperatures.
Size. Size parameters are another concern. Thermistors can be miniaturized and constructed as a device as small as 0.012" (0.3 mm) OD, allowing them to be using in very tight quarters. This is critical in applications where a small footprint and light weight are vital. While thermocouples are available in very small sizes (to 0.008" [0.2 mm]), if high accuracy is required, thermistors are a better choice. Electromechanical thermostats, because of their mechanical nature, are more difficult to miniaturize.
Flexibility. Thermistors are offered in a range of resistance, slope characteristics, lead configurations and encapsulation materials, allowing them to be tailored to many temperature-sensing applications within industrial, medical, commercial, military and aerospace applications.
Determining whether a specific application will benefit from the use of a thermistor depends upon the unique parameters of the project. A better understanding of the attributes of this temperature-sensing technology is a good starting point to help you determine which sensor type is best suited for your particular requirements.