Types of Thermocouples

One of the problems in designing a temperature sensor is trying to match the sensor to the range of temperatures that it's expected to measure. This means that no one sensor is able to accurately measure temperatures below 32°F and above 1000°F. For this reason a wide variety of thermocouples is needed to provide accurate readings over a wide range of temperatures. ill. 1 provides a graph that shows the temperature range for five common types of thermocouples. Each type of thermocouple is identified by a single letter of the alphabet. The most common type is a J-type thermocouple made of iron and constantan. From the horizontal axis of the graph notice that its temperature range is 0°C - 750°C. This converts to 32°F - 1382°F. The vertical axis of the graph shows that the J-type thermocouple produces approximately 40 mV at 750°C . A complete table for the J-type thermocouple is available here.

Above: ill. 1 Graph of the temperature range and amount of voltage various thermocouples produce.

This graph also shows that the S-type thermocouple has a range of 0°C - 1450°C, which converts to 32°F - 2642°F. Notice that the S type only produces approximately 17 mV at 1450°C.

EXERCISE

Suppose you are asked to find the type of thermocouple that produces the most voltage for each degree of temperature. This thermocouple would provide the most resolution (mV/°C).

SOLUTION

From the graph in ill. 1 notice from the vertical axis that the E-type thermocouple produces the most voltage at over 70 mV. Its resolution would be determined by dividing 70 mV by 900°C.

70 mV / 900°C = 0.077 mV/°C