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At times one will need to compare the operation of a thermocouple, an RTD, a thermistor, and an IC temperature sensor. ill. 1 shows a comparison of the thermocouple, RTD, thermistor, and IC sensor. From this chart note the graph of the output of each device as it's compared with a change in temperature. e.g., the thermocouple provides a millivolt signal, the RTD provides a change of resistance that increases when temperature increases, the thermistor provides a change of resistance that decreases when temperature increases, and the IC sensor provides a voltage or current signal when temperature increases.
Above: ill. 1: A comparison of thermocouples, RTDs, thermistors, and IC temperature sensors.
At this point we will review the advantages and disadvantages for each sensor. For instance, some of the advantages of the thermocouple include that it's self-powered because it produces a millivolt signal which means it does not need a power supply, its two wire operation is simple to use, it's rugged and can be used in applications where the temperature is extreme and where it may get bumped frequently, and they have a wide range of devices available for different temperature ranges. The disadvantages of the thermocouple include that it's nonlinear in parts of its response curve, it requires a temperature compensation circuit (reference), it's the least stable, and it's the least sensitive.
The RTD is the most stable and most accurate of the temperature devices discussed in this section. Some disadvantages of the RTD are that it's more expensive than the other devices, it requires a current source such as a wheatstone bridge circuit with a power supply to provide a useful signal, it has a low absolute resistance, a small ΔR, and it's self-heating which means that it will add heat to itself as current flows through it which will cause errors in some readings.
The advantages of the thermistor are that it has a high output change of resistance for a change of temperature, it's faster reacting to a temperature change, and it can provide a signal from a two-wire configuration. The disadvantages of the thermistor are that it's not linear which means that only a small portion or its range can be used at a time, or it must be used in applications that don't require complete linearity such as a high/low alarm, it has a very limited temperature range, it's fragile and can be broken easily, a current source like the wheatstone bridge is required, and it's self-heating like the RTD.
The IC sensor is the most linear of these sensors. It has the highest output for the amount of temperature change, and it's by far the most inexpensive. Its disadvantages include its temperature range is less than 200°C, it requires a power supply like a traditional solid state circuit, it's slow to react to temperature changes, it's self-heating, and it has a limited number of configurations.
