A thermocouple is a popular type of sensor that’s used to measure temperature. Thermocouples will be favorite in industrial control applications because of the relatively low priced and wide measurement ranges. In particular, thermocouples excel at measuring high temperatures where thermocouple sensor some other common sensor types cannot function. Try operating a built-in circuit (LM35, AD 590, etc.) at 800C.
Thermocouples happen to be fabricated from two electric conductors made of two different metallic alloys. The conductors are typically built into a cable connection having a heat-resistant sheath, typically with an essential shield conductor. At one stop of the cable, both conductors are electrically shorted mutually by crimping, welding, etc. This end of the thermocouple–the very hot junction–is thermally attached to the thing to be measured. Another end–the cold junction, occasionally called reference junction–is connected to a measurement system. The objective, of course, is to determine the temperature near the hot junction.
It should be observed that the “hot” junction, that 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, named the Seebeck voltage, that is proportional to the temperature difference between your hot and reference junctions :
Vs = V(Thot-Tref)
Since thermocouple voltage is a function of the temperature difference between junctions, it’s important to know both voltage and reference junction heat range in order to determine the temperature at the hot junction. Consequently, a thermocouple measurement method must either gauge the reference junction temperature or command it to keep it at a set, known temperature.
There exists a misconception of how thermocouples run. The misconception can be that the hot junction is the way to obtain the output voltage. That is inappropriate. The voltage is generated over the amount of the wire. Hence, if the complete wire length is at the same temperature no voltage will be generated. If this weren’t true we hook up a resistive load to a uniformly heated thermocouple in a oven and use additional warmth from the resistor to produce a perpetual motion machine of the first kind.
The erroneous model also claims that junction voltages will be generated at the frosty end between the special thermocouple cable and the copper circuit, hence, a cold junction temp measurement is required. This concept is wrong. The cold -ending temperature is the reference level for measuring the temperature difference across the length of the thermocouple circuit.
Most industrial thermocouple measurement methods opt to measure, rather than 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 an existing measurement system than to add 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 reasons: no application stations are consumed by the reference junction sensor, and the dedicated channel can be automatically pre-configured for this reason without requiring host processor support. This special channel is designed for direct connection to the reference junction sensor that is standard on many Sensoray termination boards.
Linearization Within the “useable” heat range of any thermocouple, you will find a proportional partnership between thermocouple voltage and temperatures. This relationship, however, is in no way a linear relationship. In fact, most thermocouples are extremely non-linear over their functioning ranges. To be able to obtain temperature data from a thermocouple, it is necessary to transform the non-linear thermocouple voltage to temperature units. This process is called “linearization.”
Several methods are commonly utilized 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, special thermocouple interface components (included circuits or modules) are available to perform both linearization and reference junction settlement in the analog domain. In general, neither of the methods is well-suited for cost-effective, multipoint data acquisition devices.