Using Precision Digital Multimeters for Quick Checks on Temperature TransducersNovember 1, 2003 By: David Pereles, Fluke Corp.
Does this thing work? How a digital multimeter can tell you if your temperature transducer is working, broken, or confused.
If your processes use temperature transducers, then the ability to quickly see if they're good or bad is important. This article describes how to use a digital multimeter (DMM) to perform some quick and simple tests for the most common ones—thermocouples, resistance temperature detectors (RTDs), and thermistors.
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. When a production line is down, speed is of the essence. The failed component must be isolated and replaced ASAP. 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, they will tell you if a transducer has failed. And sometimes that's exactly what you need.
Does This Thermocouple Work?
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. If the reading at room temperature is close to 110 Ω, then you have an RTD on your hands—read on.
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.
Multimeters with min./max. recording and the ability to graph electrical signals (similar to an oscilloscope) are also convenient for this application. Min./max. recording lets you connect the meter, walk over to the tip of the thermocouple, warm it for a few seconds, and walk back to the meter to check the results. Typical values for a good thermocouple are shown in Figure 1.
Figure 1. Using the digital multimeter's min./max. record function allows you to monitor thermocouple voltage changes over time and make sure voltage increases with increasing temperature.
The figure shows that it took 37 s to warm the tip. Of course, if you had to walk to the end of the transducer, this time would be longer.
Does This RTD Work?
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.
To measure an RTD or any resistance, the measurement system drives a current through the device and measures the voltage drop.
While most low-cost DMMs with a millivolt and resistance function can be used to check thermocouples or thermistors, they may not have sufficient resolution and accuracy for testing RTDs. For checking RTDs you'll need a meter that is capable of indicating changes of tenths of an ohm, and you'll want a meter that measures to hundredths—the absolute value of the resistance isn't important, but the ability to track small changes is. Look for multimeters with resolutions down to 0.01 mV or 0.01 Ω and added features such as min./max. recording or a graphical display. Since small changes in resistance reflect large changes in temperature, their additional resolution and improved accuracy give you a clearer picture of how well the RTD being tested is functioning, giving you more confidence in your results.
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.
Does This Thermistor Work?
Thermistors are made of semiconductor material and they work in a way opposite to RTDs. While RTDs experience an increase in resistance with increasing temperature, thermistors tend to exhibit a lower resistance with higher temperatures. This is because semiconductor materials tend to conduct more electrons as the temperature goes up.
Although many types of thermistors are available, two-wire thermistors are the most common for general-purpose temperature measurement. Checking a thermistor involves performing a resistance measurement. Using the DMM's resistance function, you should be able to watch the resistance of the transducer stabilize at room temperature and drop as the tip of the transducer is heated.
Thermistors generally have a large change in resistance per degree of temperature, so just about any meter can be used to quickly test a thermistor's response. Graphical multimeters can take advantage of this property by graphically displaying the changing resistance. Figure 2 shows a plot of resistance over time for a thermistor that was heated briefly.
Figure 2. Using a multimeter with a graphing function lets you see how the thermistor behaves when the temperature changes—this thermistor was heated briefly, causing its resistance to drop.
Words to the Wise
Temperature transducers usually fail in a big way. Rather than drifting, they usually just stop working. While there can be no substitute for regular calibration and certification, in a pinch a precision DMM can work for you as a solid troubleshooting tool.
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