Thermocouple Cold Junction Compensation
Cold Junction techniques for Thermocouple Thermometry
What is Cold Junction Compensation?
There are calibration tables (emf tables) for each thermocouple type, ralting output voltage to the temperature of the measuring junction. Throughout thermocouple thermometry it is clearly necessary to refer sensor voltage output to these in some way to ascertain true temperature
Most importantly, different net voltage outputs are produced for a given temperature difference between the measuring and reference junctions if the reference junction temperature itself is allowed to vary. So, the calibration tables mentioned above always expressly assume that the reference junction is held at 0°C.
Simple Copper Junctions in Ice
This can be achieved by inserting the copper junction(s) into melting ice, via insulating glass tubes, or into a temperature controlled chamber, like an isothermal block with suitable temperature sensors. For industrial measurement today, this kind of function is normally performed by temperature correcting electronics - while linearising electronics (usually digital), harnessing curve fitting techniques, look after the inherent non-linearities as per the calibration tables.
Practical Industrial Compensation
Essentially, reference temperature variations are sensed by a device such as a thermistor as close as possible to the reference junction. An EMF is then induced which varies with temperature so as to compensate for temperature movements at the reference terminals.
Cold Junction Compensation Methods
As explained earlier, thermocouples provide an output which is related to the temperatures of the two junctions. For them to function as absolute temperature measuring devices, rather than differential, the reference junctions must be maintained at a known temperature (see Figure 5.1).
Compensation in Laboratory Use
An established, simple method of maintaining reference temperature stability, still used in laboratories today, is to immerse the reference junctions in a slush of melting ice. Given that you have pure water ice, the temperature plateau during the melting process is established at a constant 0°C within ±0.001°C. In practice, all that is required is an ice-filled Dewar flask and the set-up is then potentially quite accurate. However, it does require regular attention and replenishment for anything other than short term use and is clearly inappropriate for industrial requirements. Sources of error include the 4°C reference offset which will occur if enough ice melts so that the reference junctions are actually immersed only in water - with the ice floating above! Also if the ice used has been stored in a freezer, it will be a lot colder than 0°C.
Not surprisingly, there are more practical cold junction alternatives for industrial use, also designed to provide a reference temperature of 0°C. One involves an automatic temperature-controlled enclosure, into which the reference junctions are inserted. This holds the junctions continuously at the ice point, using semiconductor thermoelectric cooling (Peltier) devices. Here temperature errors are typically less than 0.1°C. The use of an ice point reference, or its equivalent (however generated), is still preferable to the alternatives, not only on the grounds of accuracy and stability, but also because the reference tables for thermocouples are based on a 0°C reference temperature.
Integral Electronic Cold Junction Compensation (CJC)
Another very common device today is based on a temperature sensitive electrical network (there are several options) which tracks the reference junction temperature and develops an equivalent voltage. Such so-called cold junction compensation is incorporated into each thermocouple circuit, or the measuring instrument itself at the point of connection (see below). These devices are available as discrete modules, mains or battery powered and provide for accuracy within a few °C.
Many of the instruments designed to operate with thermocouples provide terminals for direct connection to the thermocouple or extension cable conductors without any need for a separate reference junction as such. Such devices as electronic thermometers, temperature controllers, data loggers, etc, frequently incorporate their own equivalent ice point reference voltage generators (as described above).
The temperature at the connection point might be determined by an integral resistance thermometer, thermistor or transistor, and thus a suitable reference voltage developed. Incidentally, it is worth taking care over the physical siting of the reference generator, since accuracy and stability of the thermocouple reading are dependent on the network involved actually being in the same temperature environment as the connections themselves.
In any event, the reference voltage can be added to the thermocouple output either by inclusion in the electrical circuit, or, particularly in the case of controllers, data loggers and other digital systems, by data manipulation in the temperature calculations. In fact, many modern controllers, loggers, etc can accommodate the latter approach for all cold junction compensation needs.
For large schemes involving many thermocouples, racking systems and cabinets are also available having, say, 100 equivalent reference (cold) junctions already fitted into a uniform temperature enclosure. The enclosure might be an ice point unit as already described, or it might equally be a thermally stable metal block which maintains a reasonably steady temperature close to that of its surroundings. In the latter case, the temperature of the block is continuously monitored by an electrical compensator, and again, the equivalent ice point voltage is then available to be added to each thermocouple signal output - electrically, or numerically.
Beyond these, there are also reference units designed for enclosures operating at elevated temperatures. These can be useful in areas with particularly high ambient temperatures, but the thermocouple outputs will have to be adjusted to the equivalent 0°C values. In essence, as long as the reference temperature is known, the temperature of the measuring junction can be derived by adding in a correction factor from standard tables covering the thermocouple concerned.