Sometimes that’s exactly what you need.

There are three common transducers for measuring temperature: thermocouples, RTDs, and thermistors.

THERMOCOUPLES

Thermocouples are unpowered transducers that generate a very low voltage. The voltage is generated by the Seebeck effect, by which a potential is created when dissimilar metals are in contact with each other. The voltage across the junction of the two metals is proportional to the temperature of the junction.

The junction of the metals may be sheathed in various configurations, or it may be exposed. The type of thermocouple describes the metals used to make the junction. For example, 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 — although it is somewhat misleading to use the terms “high” and “voltage” in this context. The voltage across a common, J-type thermocouple is about 1.0 millivolt at a room temperature of 68?F. The voltage at body temperature (99?) is about 1.9 millivolts.

There are two steps to checking thermocouples:

1. Check for a short on the terminals.

2. Make sure voltage tracks temperature.

The first test is easy and can be performed with any quality multimeter, such as the Fluke 110. Simply 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 W, 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:

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 temperature should drop.

Graphical multimeters (GMMs) afford another level of convenience in this application. They provide a large display that can convey large amounts of information at a glance. Min-max recording allows you to 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. This shows that it took 37 sec to warm the tip. Of course, if you had to walk to the end of the transducer, this time would be longer.

RTDs

RTDs operate on the principle that the resistance of any conductor changes with temperature. As the temperature of a conductor rises, the molecular structure gets shaken up, thus impeding electron flow. So, the higher the temperature, the higher the resistance.

Most RTDs consist of a platinum wire coil with a nominal resistance of 100 W at the freezing point (or, for the purists, the triple point) of water. Resistances other than 100 W at 32? are less common, but do occur. It helps to know what the resistance 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 function as an RTD, indicating over-temperature conditions within the motor. In these special applications, and with metals other than platinum, you will probably find freezing-point resistances other than 100 W.

To measure an RTD or any resistance, the measurement system drives a current through the device and measures the voltage drop.

Checking RTDs is even easier than checking thermocouples. But once again, it takes a precision multimeter to perform the test. You will require a meter that is capable of indicating changes of tenths of an ohm, and will want a meter that measures to hundredths. The absolute value of the resistance is not important, but you need to be able to track small changes.

RTDs can have two, three, or four leads. In a two-wire configuration, simply connect the meter across the leads and measure the resistance. The resistance of a platinum wire RTD should be about 110 W (plus or minus 20%) at room temperature. If you grab the tip of the RTD, you should see the resistance increase. The resistance should 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 (all metals do). 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 W for a platinum wire RTD. 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.

THERMISTORS

Thermistors work in a way opposite to RTDs. This is because semiconductors act oppositely to metals. Semiconductors tend to conduct more electrons at higher temperatures. Thermistors, which are made of semiconductor material, tend to exhibit a lower resistance with higher temperatures.

There are a variety of thermistors, but two-wire thermistors are the most common for general-purpose temperature measurement. Checking a thermistor involves performing ohm measurements. With an ohmmeter, 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. Because of this, just about any meter can be used to quickly test a thermistor’s response. Graphical multimeters can take advantage of this property in a unique way. Figure 2 shows a plot of resistance, over time, for a thermistor that was heated briefly.

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 a digital multimeter (DMM) can work for you as a solid troubleshooting tool.

Pereles works for Fluke Corp., P.O. Box 9090, Everett, WA 98206; 800-443-5853; www.fluke.com (website).

Publication date: 02/11/2002