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THE BASICS OF THERMOCOUPLES

by Bob Perrin

Start ı A Tail of Two Metals ı Breaking the Loop ı Measuring Temperature ı Practical Matters ı Circuits ı Closing Time ı Sources and PDF

MEASURING TEMPERATURE

Thermocouples produce a voltage (or loop current) that is proportional to the difference in temperature between the hot junction and the reference junction. If you want to know the absolute temperature at the hot junction, you must know the absolute temperature of the reference junction.

There are three ways to find out the temperature of the reference junction. The simplest method is to measure the temperature at the reference junction with a thermistor or semiconductor temperature sensor such as Analog Devicesı TMP03/04. Then, in software, add the measured thermocouple temperature (the difference between the hot junction and the reference junction) to the measured temperature of the reference junction. This calculation will yield the absolute temperature of the hot junction.

The second method involves holding the reference junction at a fixed and known temperature. An ice bath, or an ice slushy, is one of the most common methods used in laboratory settings. Figure 2 shows how this is accomplished.

Figure 2ıBy inserting a short pigtail of Metal A onto the terminal block where Metal B would normally connect, we move the cold junction.

Alternately, we could have omitted the pigtail of Metal A and just immersed the terminal block in the ice. This would work fine, but it would be much messier than the method shown in Figure 2.

Sometimes, the temperature of the cold junction (terminal block) in Figure 1c is allowed to float to ambient. Then ambient is assumed to be "about 25ıC," or some other "close enough" temperature. This method is usually found in systems where knowing the temperature of the hot junction is not overly critical.

The third method used to nail down the cold junction temperature is to use a cold junction compensation IC such as the Analog Devices AD594 or Linear Technology LT1025. This method sort of combines the first two methods.

These ICs have a temperature sensor in them that detects the temperature of the cold junction. This is presumably the same temperature as the circuit board on which the IC is mounted. The IC then produces a voltage that is proportional to the voltage produced by a thermocouple with its hot junction at ambient and its cold junction at 0ıC. This voltage is added to the EMF produced by the thermocouple. The net effect is the same as if the cold junction were physically held at 0ıC.

The act of knowing (or approximating) the cold junction temperature and taking this information in to account in the overall measurement is referred to as cold junction compensation. The three techniques I discussed are each methods of cold junction compensation.

The ice bath is probably the most accurate method. An ice slushy can maintain a uniformity of about 0.1ıC without much difficulty. Iıve read that an ice bath can maintain a uniformity of 0.01ıC, but Iıve never been able to achieve that level of uniformity. Ice baths are physically awkward and therefore usually impractical for industrial measurements.

The off-the-shelf cold junction compensation ICs can be expensive and generally are only accurate to a few degrees Celsius, but many systems use these devices.

Using a thermistor, or even the PN junction on a diode or BJT, to measure the cold junction temperature can be fairly inexpensive and quite accurate. The most common difficulty encountered with this system is calibration. Prudent positioning of the sensor near, or on the terminal block is important.

If the terminal block is to be used as the cold junction (see Figure 1b), the terminal block must be kept isothermal. In practice, keeping the terminal block truly isothermal is almost impossible. So, compromises must be made. This is the stock and trade of engineers. Knowing what is isothermal "enough" for your application is the trick.

Lots of money can be wasted on precision electronics if the terminal blockıs screw terminals are allowed to develop a significant thermal gradient. This condition generally happens when power components are placed near the terminal blocks. You must pay careful attention to keeping the temperature stable around the terminal blocks.

There are two broad classes of temperature-measurement applications. The first class involves measuring absolute temperature. For example, you may want to know the temperature of the inside of an oven relative to a standard temperature scale (like the Celsius scale). This type of application requires that you know precisely the absolute temperature of the reference junction.

The second type of measurement involves measuring differences in temperature. For example, in a microcalorimeter, you may want to measure the temperature of the system, then start some chemical reaction and measure the temperature as the reaction proceeds. The information of value is the difference between first measurement and the subsequent ones.

Systems that measure temperature differences are generally easier to construct because control or precise measurement of the reference junction isnıt required. What is required is that the reference junction remain at a constant temperature while the two measurements occur. Whether the reference junction is at 25.0ıC or 30.0ıC isnıt relevant because the subtraction of consecutive measurements will remove the reference junction temperature from the computed answer.

You can use thermocouples to make precise differential temperature measurements, but you must ensure the terminal block forming the cold junction is "close enough" to isothermal. You must also ensure that the cold junction has enough thermal mass so it will not change temperature over the time you have between measurements.

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