Calibration and troubleshooting are two very different requirements. Calibration maintains product quality; troubleshooting affects product quantity. Calibration happens on a schedule; troubleshooting happens in emergencies. Calibration must be precise; troubleshooting must be fast. With a precision multimeter, you can perform quick, go/no-go checks on most temperature transducers, and while these tests tell nothing about transducer accuracy, but if a transducer has failed. And sometimes that s exactly what you need.
Thermocouples
Thermocouples are unpowered transducers that generate a very low voltage. When two dissimilar metals are in contact with each other, a potential is created across the junction—the Seebeck effect. This voltage across the junction of the two metals 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 junction of the metals may be sheathed in various configurations or may be exposed. The higher the temperature, the higher the voltage produced by the thermocouple. (It is somewhat misleading to use the terms “high” and “voltage” in this context, since the voltage across a common, J Type thermocouple is about 1.0 mV at a room temperature of 68°F and about 1.9 mV at body temperature, 99°F). There are two steps to checking thermocouples. The first is to check for a short on the terminals and the second, to make sure that voltage tracks with the temperature. The first test can be performed with any quality multimeter. Put the meter in ohms or continuity mode; on a good thermocouple, you should see 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 (0.0001 V). A meter that can measure hundredths of millivolts (0.00001 V) makes it even easier to do this check, because the added resolution shows very small temperature changes. Connect the meter to the terminals of the thermocouple. Grabbing the end of the thermocouple should cause the voltage to increase slightly, since you re warming it up. As you release the junction, the temperature (and voltage) should drop.
RTD
RTDs operate on the principle that the resistance of any conductor changes with temperature. As the temperature of a conductor rises, the increased molecular vibration impedes electron flow. Thus, the higher the temperature, the higher the material s resistance. Most RTDs are of the PT-100 variety. They consist of a platinum wire coil with a nominal resistance of 100 Ω at the freezing point (or, for purists, the triple point) of water. Resistances other than 100 Ω at 32°F are less common, but do occur. It helps to know what the resistance of your RTD should be. Sometimes copper or another metal is substituted for the platinum. For example, in some electric motors and transformers an extra set of copper windings functions as an RTD, indicating overtemperature conditions within the motor. In these special applications, and with metals other than platinum, you will probably find freezing-point resistances other than 100 Ω. RTDs can have two, three, or four leads. In a two-wire configuration, simply connect the meter across the leads and measure the resistance. For a PT-100 RTD at room temperature, this should be about 110 Ω (±20%). If you grab the tip of the RTD, you should see the resistance increase. Let go, and you should see the resistance gradually settle back after you release the tip. Three-wire RTDs are commonly used when a measurement system is made up of resistance bridges. The wires that connect the tip to a measuring device have a temperature-dependent resistance of their own (as do all metals). The extra wire helps the bridge balance out the effects of lead resistance. When checking a three-wire RTD with an ohmmeter, all you need to know is that two of the three wires should be shorted. Usually, the shorted wires are the same color. Between any of the shorted wires and the third wire, the transducer should act just like its two-wire counterpart. That is, at room temperature, the meter should read about 110 Ω for a PT-100 RTD, and resistance should increase slightly as the temperature at the tip increases Four-wire RTDs are less common than the other types. If you do come across one, it should have two shorted pairs of wire. Again, the shorted wires are generally the same color. The resistance between different colored wires should have a reasonable value at room temperature and should increase if you heat the tip.
Ref: www.sensorsmag.com
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