Designing with temperature sensors, part one: sensor types

Most people have heard the phrase “Birds of a feather flock together,” which describes people who have similar characteristics or interests and choose to spend time together. Is it possible that some temperature sensors tend to flock together, too?

Bonnie Baker, Texas Instruments -- EDN, September 22, 2011

Of the sensing technologies, temperature sensing is the most common due to the multitude of applications in which it is critical to know and process the actual or the relative temperature. For instance, pressure, force, flow, level, and position sensors often require temperature monitoring to ensure accuracy. Most sensors use resistive-bridge configurations to measure pressure and force. The temperature errors of the resistive elements in these bridges can exceed the sensor’s actual measurement range, making the pressure sensor’s output useless—unless you know the temperature of the bridge. Flow- and level-sensor accuracies depend on the density of the liquid or the gas. The temperature of that material is one variable that affects accuracy.


Keep in mind that no one temperature sensor is right for all applications. The thermocouple has an unrivaled temperature range, and the RTD sensor has excellent linearity. Table 1 summarizes the main characteristics of thermocouples, RTDs, thermistors, and silicon-based temperature sensors. This table can be useful during your first pass in the sensor-selection process.

A thermocouple comprises two wires of dissimilar metals that are bonded together at one end. This configuration produces an EMF (electromotive-force) voltage between the two wires at the unbounded, or measurement, end. The EMF level is a function of the two dissimilar metals and the temperature gradient along the length of the thermocouple wires. The thermocouple is not particularly accurate; however, it can quickly sense over a wide temperature range.

RTDs provide excellent accuracy in a temperature-sensing environment. Their temperature range is narrower than that of thermocouples but wider than those of thermistors and silicon-based sensors. Select an RTD sensor if your application requires a high-quality, accurate temperature measurement.

Thermistors often provide the lowest-cost approach for your temperature-sensing system. You can overcome the devices’ high nonlinearity with a simple resistive network. Although this type of network reduces thermistors’ temperature range, this trade-off is acceptable in many temperature-sensing applications.

IC-temperature or silicon-based sensors offer another alternative to solving temperature-measurement problems. Their advantages include user-friendly output formats and easy installation during PCB (printed-circuit-board) assembly. Although silicon-based temperature sensors respond slowly due to their package mass, their plug-and-play features make them attractive. Table 2 complements the specifications in Table 1 with a list of typical applications for these four temperature sensors. Examples of appropriate applications include biophysics and metal-cutting research for thermocouples, cold-junction compensation and calibration for RTDs, pyrometer calibration for thermistors, and battery management for silicon-based sensors.

Of the temperature sensors now on the market, thermocouples, RTDs, thermistors, and silicon-based sensors continue to dominate. The thermocouple is most appropriate for higher-temperature sensing, whereas the RTD is best suited for lower-temperature applications requiring good linearity. The thermistor is a low-cost alternative for applications having smaller temperature ranges, and silicon-based sensors sometimes win out because of their ease of use. The next four Baker’s Best columns will dig into the temperature-sensor details of these four families of sensors.

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References

Product Book, Thermometrics, 1997. Schraff, Fred, “The Principles and Methods of Using Thermocouples,” Measurement & Control, June 1996, pg 126. Sulciner, James, “Understanding and Using PRTD Technology, Part 1: History, Principles and Designs," Sensors, August 1996. Technical Reference, Omega. Texas Instruments.