Some of the more common types of temperature sensors used in our trade are thermocouples, resistance temperature detectors (RTD), negative temperature coefficient (NTC) thermistors, and transistors. Some may look similar when encased in a particular housing, but they all are quite different and cannot be interchanged.
Thermocouples are sensors that generate a very low dc voltage. They are made of two dissimilar metals in contact with each other. A voltage is created across this junction which is proportional to the junction’s temperature. The “type” of thermocouple describes the metals used to make the junction, e.g., a J-type thermocouple uses iron in one wire and a copper/nickel alloy in the other. The higher the temperature, the higher the voltage produced by the thermocouple.
There are two steps that can be used to check a thermocouple:
1. Check for a short between the terminals, and
2. Make sure that voltage tracks with the temperature.
The first test can be performed with an ohmmeter. A good thermocouple should have a low resistance reading. If you see more than a few ohms, you probably have a faulty thermocouple.
The second test requires a meter that can measure down to tenths of millivolts. Connect the meter to the terminals of the thermocouple. Grabbing the end of the thermocouple should cause the voltage to increase slightly. As you release the junction, the voltage should drop.
An RTD is a sensor that will exhibit a change in resistance with a change in temperature. Most RTDs are simply wire-wound or thin-film resistors made of wire with a known resistance-versus-temperature relationship. Platinum is one of the most widely used materials for RTDs. The relationship between an RTD’s resistance and the surrounding temperature is highly predictable, allowing for accurate and consistent temperature measurement. By supplying an RTD with a constant current and measuring the resulting voltage drop across the resistor, the RTD’s resistance can be calculated, and the temperature can be determined.
Thermally sensitive resistors (thermistors) are similar to RTD’s; they are electrical resistors whose resistance also changes with temperature. Thermistors are made of semiconductor material. The resistance of the NTC thermistor decreases with an increase in temperature. The temperature-versus-resistance curve tends to be extremely nonlinear when compared to the RTD’s temperature-versus-resistance curve, which is more linear.
Both RTDs and thermistors can be tested by measuring the resistance value of the sensor at a known temperature. Compare the measured temperature/resistance to the temperature/resistance supply for the manufacturer. If there is a deviation between the measure resistance and the charted resistance, the sensor is likely defective.
Certain transistors can also be used to measure temperature. They will generally produce a fairly linear millivolt output with a change in the surrounding temperature. Some transistors will provide a 10 millivolt dc change in their output per 1˚F change in temperature. They can be tested by measuring the output voltage at a known temperature and comparing the measured values to the values supplied by the manufacturer. If there is a deviation, the sensor is defective.
When it comes to testing a temperature sensor, it is best to know the sensor type. And when the sensor needs to be replaced, it is generally best to use the OEM sensor. This reduces the probability of selecting the wrong sensor type.
Publication date: 10/7/2013