A thermocouple is a commonly used type of sensor that’s used to measure temperature. Thermocouples are usually preferred in industrial control applications because of the relatively low priced and wide measurement ranges. Specifically, thermocouples master measuring high thermocouple types temperatures where other common sensor types cannot performance. Try operating a built-in circuit (LM35, AD 590, etc.) at 800C.
Thermocouples are fabricated from two electrical conductors manufactured from two different steel alloys. The conductors are usually built into a cable connection having a heat-resistant sheath, frequently with an essential shield conductor. At one conclusion of the cable, the two conductors are electrically shorted together by crimping, welding, etc. This end of the thermocouple–the warm junction–is thermally attached to the thing to be measured. Another end–the cold junction, sometimes called reference junction–is linked 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 relatively of a misnomer, may in fact be at a temperature less than that of the reference junction if reduced temperatures are being measured.
Reference Junction Compensation Thermocouples create an open-circuit voltage, referred to as the Seebeck voltage, that is proportional to the temperature difference 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 know both voltage and reference junction heat range to be able to determine the heat at the hot junction. Therefore, a thermocouple measurement technique must either gauge the reference junction temperature or control it to keep it at a set, known temperature.
There exists a misconception of how thermocouples operate. The misconception is that the hot junction is the way to obtain 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 will be generated. If this were not true we connect a resistive load to a uniformly heated thermocouple inside an oven and use additional warmth from the resistor to produce a perpetual motion machine of the first kind.
The erroneous model furthermore claims that junction voltages are generated at the frosty end between the special thermocouple cable and the copper circuit, hence, a cold junction temperature measurement is required. This idea is wrong. The cold -conclusion temperature is the reference level for measuring the temperature variation across the length of the thermocouple circuit.
Most industrial thermocouple measurement techniques 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 put in a reference junction sensor to a preexisting measurement system than to include on a full-blown temperature controller.
Sensoray Smart A/D’s gauge the thermocouple reference junction temperature by means of a separate analog input channel. Dedicating a special channel to this function serves two purposes: no application channels are consumed by the reference junction sensor, and the dedicated channel can be automatically pre-configured for this reason without requiring host processor assistance. This special channel is made for direct link with the reference junction sensor that’s standard on various Sensoray termination boards.
Linearization Within the “useable” heat range range of any thermocouple, there exists a proportional relationship between thermocouple voltage and temperatures. This relationship, however, is in no way a linear relationship. Actually, most thermocouples are extremely non-linear over their working ranges. As a way to obtain temperature data from a thermocouple, it is necessary to change the non-linear thermocouple voltage to heat range units. This technique is called “linearization.”
Several methods are commonly used to linearize thermocouples. At the low-cost end of the answer spectrum, you can restrict thermocouple operating range in a way that the thermocouple ‘s almost linear to within the measurement resolution. At the contrary end of the spectrum, unique thermocouple interface components (built-in circuits or modules) are available to execute both linearization and reference junction reimbursement in the analog domain. In general, neither of these methods is well-appropriate for cost-effective, multipoint data acquisition techniques.