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Application Guide

The Art of TemperatureSensing

Voltage Signals Sensors generating varying voltagesignals are thermocouples. Thermo-couples combine dissimilar metallicelements or alloys to produce a voltage.Using specific combinations of metalsand alloys in the thermocouple’s legs produces a predictable change in volt- age based on a change in temperature.

Contact Temperature Sensor Typesand Comparisons

Thermocouples, RTDs andThermistors Resistance Values Sensors generating varying resistancevalues are resistance temperature detectors (RTDs) and thermistors.Resistive devices use metals or metal oxides that provide repeatable changes of resistance with temperature.A variation of the thermistor notcovered in this Application Guide isthe integrated circuit (IC). It’s a ther- mistor that has a computer chip tocondition and amplify its signal. Thecomputer chip limits the IC’s use to anarrow temperature range. Temperature sensors, aside fromcapillary/bulb thermometers and bi- metal sensors, use varying voltage signals or resistance values. Ref. 22
ThermocoupleRTDThermistor VT RT RT Voltage Resistance Resistance Temperature Temperature Temperature • No resistance lead wire • Most stable, accurate• High output, fastproblems• Contamination resistant• Two-wire ohms• Fastest response to • More linear thanmeasurementtemperature changesthermocouple• Point temperature sensing• Simple, rugged• Area temperature sensing• High resistance• High temperture• Most repeatable • High sensitivity to smalloperation 1704°C (3100°F) temperature measurementtemperature changes• Point temperature sensing• Non-linear• Current source required• Non-linear• Low voltage• Low absolute resistance• Limited temperature range• Least stable, repeatable• Self-heating• Fragile• Least sensitive to small• Slow response time• Current source requiredtemperature changes• Low sensitivity to small • Self-heatingtemperature changes DisadvantagesAdvantages Conclusion Thermocouples are best suited tohigh temperatures, environmental extremes, or applications requiringmicroscopic size sensors.RTDs are best for most industrialmeasurements over a wide temper-ature range, especially when sensor stability is essential for proper control.Thermistors are best for lowtemperature applications with limitedtemperature ranges. 36