Both are suitable technologies for a myriad of industrial cooling applications, but in some applications one may work better than the other.

Process cooling encompasses hundreds of applications that require careful attention when selecting the best temperature control components to complement the equipment design and process. For instance, in industrial refrigeration, temperature sensors are used to monitor the performance of the refrigeration circuit, heating modules and fluid temperature safeties. This article compares two popular temperature sensor styles - the RTD and the thermistor - and describe their application benefits, common uses, control methodology and relative costs.

RTDs

An RTD - or resistance temperature detector - utilizes a specific volume of metal that, based on its inherent temperature co-efficient, has the ability to change resistance as the temperature surrounding the sensor changes. An RTD sensor is well suited for precision industrial and laboratory equipment where accuracy is a priority and a specific temperature range is available.

RTD sensors are offered in two accuracy groups: Class A or Class B. Class A sensors offer accuracy of ±0.5°F (±0.3°C) over a temperature range of -325 to 1,200°F (-200 to 650°C). The accuracy of a Class B sensor degrades to ±1°F (±0.6°C) but offers a wider temperature range of -325 to 1,560°F (-200 to 850°C).

RTDs can be produced in a range of metals to provide accuracy and temperature scales to fit most applications. The most common type of RTD is constructed of platinum due to that material’s almost linear temperature coefficient. Platinum also offers resilience to corrosion, stable repeatability and a widely used temperature range of 32 to 212°F (0 to 100°C).

An RTD functions by pushing a 0.5 to 4.5 V control signal (red) through a volume of metal (the RTD element) and reading the resulting amperage signal (white) created from the resistance of the RTD element. An example of the control wiring is shown in figure 1.

The most common RTD sensors are designed with a 100 Ω resistance at 32°F (0°C) and 138.4 Ω at 212°F (100°C). This results in a very small change in resistance - approximately 0.384 Ω - per 1.8°F (1°C) of temperature change. An RTD with a 1,000 Ω baseline is also available that increases the resistance interval per degree of temperature, resulting in better repeatability and accuracy.

These small changes in resistance require the RTD sensor and control wiring to be isolated from sources of high voltage that may emit noise into the signal and cause temperature inaccuracies. With any temperature sensor, the length of control wiring should be kept to a minimum. However, with an RTD, in general, a 100 Ω RTD should be kept to a maximum lead of 15'. Wiring for a 1,000 Ω sensor can be extended to a maximum of 125'.

Accuracy is improved in a three- and four-wire RTD by referencing the change in resistance of the additional wires, due to temperature, and comparing them back to that of the signal resistance. This allows for the sensor to have an accurate baseline when calculating the actual change in resistance from the nominal 100 Ω.

Thermistors

A thermistor incorporates a specific volume of polymer or ceramic with the function to display a wide, repeatable and precise change in resistance based on the temperature of the surrounding medium. The greatest benefit of a thermistor stems from the high temperature coefficient of resistance, which enables the sensor to maintain precision within the total range of the temperature scale. Due to this precision, thermistors commonly are designed with a temperature range of -100 to 600°F (-77 to 315°C) in order to encompass a large number of applications. Accuracy of a thermistor varies with the class of material but commonly is found at 0.36°F (0.2°C). Higher-grade versions can be produced to exhibit an accuracy of 0.18°F (0.1°C).

Thermistors can be produced for less than an RTD, making them a less expensive option, while still maintaining similar design specifications. Although the temperature range of a thermistor is narrower than an RTD’s, they are still an acceptable fit for applications with a specific temperature range and lower cost as the highest priority.

A thermistor is available with two different methods of functionality. A positive temperature coefficient (PTC) sensor will exhibit an increase in electrical resistance as the temperature of the medium increases. A negative temperature coefficient (NTC) will display a decrease in electrical resistance as the temperature of the medium increases. An example of each is shown in figure 2.

The control method of a thermistor is similar to that of an RTD. A control signal, usually 0.5 to 4.5 V DC, is pushed through the volume of material exhibiting the change in resistance, and the resulting voltage drop is inputted. Thermistors are generally produced with a 10K resistance at room temperature.

Due to the high resistance range and large temperature scale, thermistors are less susceptible to the noise generated by nearby high voltage wiring. However, great care should still be taken to avoid contact and close proximity with any high voltage wiring or high frequency emitting devices. In addition, a 10K thermistor can run much longer control wiring, above 1,000', than that of an RTD while still maintaining the specified accuracy and precision. PC

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