Cold Chain

Series on Measuring Temperature: RTDs, Thermistors and Thermocouples (Part 1)

We hope you will find this short series helpful in simplifying the differences between the 3 main types of temperature monitoring devices and the individual advantages of each based on your temperature monitoring scenario.  This introduction is intended to provide the functional user of temperature monitors with an explanation of the differences between the three types of measuring devices.

In simplest terms, a Resistance Temperature Detector (RTD) measures changes in the resistance of a current across a material (usually a metal such as copper, nickel or platinum).  A simple transmitter sends current through the sensor and measures the resistance of the current.  As the temperature of the metal material increases, so does resistance.

Similarly, a thermocouple also relies on changes in resistance to measure temperature.  Unlike an RTD, however, the change is measured as a result of differences in resistance properties between two different types of metal fused together at a single point, known as a junction.  As temperature changes at the junction, so does the resistance of the current across each of the two metals.  The difference in the value of measured resistance between the two metals allows the thermocouple to calculate a temperature (higher differences in resistance means higher temperatures).

Similar to an RTD and thermocouple, a Thermally Sensitive Resistor (Thermistor) measures temperature by (you guessed it!) measuring resistance.  The biggest difference between Thermistors and their temperature measuring cousins, RTD’s and thermocouples, is that thermistors measure temperature in a “non-linear” fashion.  This means, the graph of temperature change versus resistance for a thermistor is not a straight line.  More importantly, a thermistor relies on what is known as Negative Temperature Coefficient (NTC) to measure the impact of changes in resistance on temperature.  Unlike the RTD and thermocouple, an NTC thermistor’s temperature increases as resistance decreases.

Stay tuned next week as we dive a bit deeper into the reasons you may want to choose one temperature measuring approach over another.

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