Thermistors, Monolithic Linear Sensors Aid Temperature Detection

AT A GLANCE

Low mass = quick response

Using Steinhart-Hart equation

Handling joule heating

Online Extra to June 2003 Control Engineering article, “Thermistors, Monolithic Linear Sensors Aid Temperature” Click here to read the online extra

Temperature is one of the most frequently measured physical phenomena. However, if a traditional thermocouple or resistive temperature detector (RTD) isn’t appropriate for a specific application, thermistors and monolithic linear temperature sensors are suitable for temperatures from -50 to 150 °C.

Both sensor types are relatively inexpensive and easy to configure because no reference junction is required. Both provide good performance with instruments of moderate sensitivity. The thermistor’s large change in resistance per degree Celsius facilitates high-resolution measurements. Monolithic linear temperature sensors provide a very linear output easily scaled to a temperature reading, and they are frequently used as cold-junction sensors for thermocouple isothermal terminal blocks.

Thermistors

Thermistors ( therm ally sensitive resistor ) include a variety of resistance-based temperature sensors commonly used in biological applications, environmental control systems, and consumer-grade temperature measurement devices. Although thermistors are resistive devices, their operation and use are far different from RTDs.

Calculation of a two-wire thermistor’s resistance is straightforward use of Ohm’s Law. When the series resistance of the lead configuration is significant, the four-wire configuration can be used. Current applied to the thermistor should always be limited to the minimun needed to prduce a readable voltage (typically 100ua or less).

Both negative temperature coefficient (NTC) and positive temperature coefficient (PTC) thermistors are available. Resistance of an NTC thermistor decreases as temperature increases, while resistance of a PTC thermistor increases as temperature increases; NTC types are used more often for temperature measurement applications.

Thermistors can be manufactured in very small sizes, resulting in low mass that allows quick response to slight temperature changes. However, they can be affected by self-heating errors that result from excitation current being dissipated in the thermistor. Thermistors are also relatively fragile, so they must be handled and mounted carefully to avoid damage.

Another important difference between thermistors and RTDs is that thermistors offer a significantly broader range of base resistance values, extending from kilo-ohms to mega-ohms. The temperature coefficient of a typical thermistor is much larger—several percent or more per degree Celsius. Typical thermistor coefficients of temperature range from -2% to -8% per °C, and are generally larger at the lower end of the temperature range.

Using solid-state, monolithic linear temperature sensors is fairly straightforward. The current sensor illustrated here is inserted in series with a resistor that provides a voltage drop readable by a digital voltmeter.

High resistance values and high temperature coefficients of thermistors produce a resistance change of up to several thousand ohms per degree Celsius. Therefore, resistance of the wires connecting the instrumentation to the thermistor is usually insignificant, and special techniques—such as high-gain instrument inputs and three- or four-wire measurement configurations—are unnecessary to achieve high accuracy. Resolution of 0.01 to 0.02 °C is possible.

While thermistors have relatively few drawbacks, it’s important to be aware of their limitations to achieve accurate, reliable measurements.

Thermistors have a typical measurement range of -80 to 150 °C. Some thermistors can be used at temperatures up to 300 °C, but even this range is significantly lower than for thermocouples and RTDs. Exposure to higher temperatures can decalibrate a thermistor permanently, producing measurement inaccuracies.

A final consideration with thermistors is that their non-linear response requires use of the Steinhart-Hart equation to calculate temperature from resistance. The equation uses three coefficients that must either be supplied by the thermistor manufacturer or derived by the user. These coefficients can be taken from a T vs. R chart, if supplied, or by making three resistance measurements of the thermistor at known temperatures and then calculating the coefficients. [See the online version of this article for the equation; method for calculating it; and the logic for this process.]

Instruments that support four-wire ohms measurement, or are directly compatible with thermistors, can simplify resistance concerns. Techniques such as four-wire configurations and sensitive measurement capability are required for thermistors only in more critical applications, because any resistance in the test leads is relatively insignificant when compared to the resistance of the thermistor itself.

Monolithic linear sensors

Monolithic linear temperature sensors are another type of transducer that offer a measurement range and ease of operation comparable to thermistors. Typically, these sensors are two- or three-pin active electronic devices that operate from a nominal 3-30 V dc supply (actual supply voltage range depends on device type), and output a current or voltage proportional to temperature.

The temperature range of monolithic linear temperature sensors is approximately -50 °C to 150 °C, making them suitable for a relatively narrow measurement range compared to thermocouples and RTDs. Within this range, output is extremely linear with temperature, and no reference junctions or complex calculations are required to use them. Although these sensors are often used as CJC reference junctions for thermocouple circuits, manufacturers maintain their suitability for general temperature measurements in the supported temperature range.

One of the first monolithic linear temperature sensors, Analog Devices’ AD590, was designed and calibrated so that its current output increased linearly by 1

Joule heating; thermal shunting

One potential source of error in using any type of resistive or powered temperature sensor is resistive (“joule”) heating that results from excitation current dissipated by the sensor. Although the amount of heat energy may be slight, it can still affect measurement accuracy.

In the case of thermistors, a DMM/ohmmeter applying a test current of 100

Actual power dissipation depends on the specific test current of the meter and the resistance of the thermistor at the temperature of interest. For monolithic devices, self-heating is also a minor concern. Supply currents are in the microamp range, so power consumption is generally very low. Typical maximum self-heating in still air is just 0.1-0.2 °C.

In addition, all temperature detectors possess some mass in the form of a sensor element, protective sheath or encapsulation, leads, and other physical components. When the sensor is placed in contact with a medium to measure its temperature, the sensor will absorb some heat energy, thereby altering the heat content and temperature of the medium. This process is called “thermal shunting.”

Thermal shunting can be minimized by using temperature sensors of the smallest possible mass. However, the choice sometimes imposes tradeoffs. For example, thermocouples generally have lower mass than RTDs, but they are less accurate. Powered resistive sensors of lower mass are more prone to joule heating than more massive sensors. A temperature sensor with lower mass may lack a protective coating or sheath, which can make it more susceptible to damage or other problems.

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