A thermocouple is a commonly used type of sensor that is used to measure temperature. Thermocouples happen to be preferred in industrial control applications because of their relatively low cost and wide measurement ranges. Specifically, thermocouples excel at measuring high temperatures where some other common sensor types cannot function. Try operating a circuit (LM35, AD 590, etc.) at 800C.
Thermocouples are usually fabricated from two electric thermocouple types conductors made of two different steel alloys. The conductors are usually built into a wire having a heat-resistant sheath, usually with an essential shield conductor. At one stop of the cable, both conductors are electrically shorted together by crimping, welding, etc. This end of the thermocouple–the warm junction–is thermally attached to the object to be measured. Another end–the cold junction, in some cases called reference junction–is connected to a measurement system. The target, of course, is to determine the temperature near the hot junction.
It should be mentioned that the “hot” junction, which is considerably of a misnomer, may in fact be at a temperature lower than that of the reference junction if reduced temperatures are being measured.
Reference Junction Compensation Thermocouples create an open-circuit voltage, known as the Seebeck voltage, that’s proportional to the temperature variation between the hot and reference junctions :
Vs = V(Thot-Tref)
Since thermocouple voltage is a function of the temperature variation between junctions, it’s important to learn both voltage and reference junction temp so as to determine the temperature at the hot junction. Consequently, a thermocouple measurement system must either gauge the reference junction temperature or handle it to keep up it at a fixed, known temperature.
There is a misconception of how thermocouples function. The misconception will be that the hot junction is the source of the output voltage. This is wrong. The voltage is generated over the amount of the wire. Hence, if the entire wire length is at exactly the same temperature no voltage would be generated. If this weren’t true we connect a resistive load to a uniformly heated thermocouple in a oven and use additional high temperature from the resistor to produce a perpetual motion machine of the first kind.
The erroneous model furthermore claims that junction voltages will be generated at the wintry end between your special thermocouple wire and the copper circuit, consequently, a cold junction heat measurement is required. This idea is wrong. The cold -end temperature is the reference level for measuring the temperature difference across the length of the thermocouple circuit.
Most industrial thermocouple measurement devices opt to measure, instead of control, the reference junction heat range. That is due to the fact that it’s almost always less costly to simply add a reference junction sensor to a preexisting measurement system than to include on a full-blown temperature controller.
Sensoray Smart A/D’s measure the thermocouple reference junction temperature by means of a separate analog input channel. Dedicating a particular channel to this function serves two purposes: no application stations are taken by the reference junction sensor, and the dedicated channel is usually automatically pre-configured for this function without requiring host processor assistance. This special channel is made for direct link with the reference junction sensor that’s standard on several Sensoray termination boards.
Linearization Within the “useable” temperature range of any thermocouple, there is a proportional marriage between thermocouple voltage and heat range. This relationship, however, is by no means a linear relationship. Actually, most thermocouples are extremely non-linear over their working ranges. In order to obtain temperature data from the thermocouple, it is necessary to switch the non-linear thermocouple voltage to heat units. This technique is called “linearization.”
Several methods are commonly employed to linearize thermocouples. At the low-cost end of the perfect solution is spectrum, one can restrict thermocouple operating range such that the thermocouple ‘s almost linear to within the measurement resolution. At the contrary end of the spectrum, exceptional thermocouple interface components (built-in circuits or modules) can be found to execute both linearization and reference junction compensation in the analog domain. Generally, neither of the methods is well-appropriate for cost-effective, multipoint data acquisition devices.