How Do You Tell a Thermocouple From an RTD?
Some thermocouples look very similar to an RTD. There are, however, usually some small differences. You usually can’t just replace a thermocouple with an RTD without making some changes, such as, reprogramming the controller for a different type of temperature sensor.
The Clues to Tell the Difference
There can be several clues as to whether you have a thermocouple or an RTD
(Resistance Temperature Detector). For example, if the temperature sensor has three leads, the sensor is an RTD. Thermocouples only have two leads. On the RTD with three leads, almost always two of the leads will have one color and the third lead will have another color.
For a two wire RTD, usually both leads will be of the same color. For a thermocouple, the two leads will always be of different colors.
The “Gold Standard” Difference
The “gold standard” for determining whether a sensor is a thermocouple or is an RTD is to measure the resistance using a Volt Ohm Meter. If the sensor has two lead wires, measure the resistance between the two leads. If the sensor is a thermocouple, you are just measuring the resistance of that length of wire. You would probably measure a few ohms. If you are measuring an RTD, you are measuring the resistance of the wires plus the resistance of the RTD
element. For the plastics industries, this RTD element is almost always 100 ohm at 0 0C. At room temperature, this element will read more like 105 to 110 ohms. Thus, if we are measuring an RTD, the ohm meter will read a few ohm more than 105 to 110 ohms, certainly much greater than just a few ohms like the thermocouple would.
If the sensor has three leads, it will be an RTD. There will be three combinations where the resistance can be measured. Two of the leads go to one side of the RTD and the third lead goes to the other side of the RTD
element. Thus, two of the reading will read 105 to 110 ohms and the third will read just a few ohms.
If there is a plug on the sensor, a white plug will be an RTD, a black plug will be Type J thermocouple and a yellow plug will be Type K thermocouple.
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