A thermocouple is a popular type of sensor that is used to measure temperature. Thermocouples are usually common in industrial control applications because thermocouple of their relatively low cost and wide measurement ranges. Specifically, thermocouples excel at measuring high temperatures where additional common sensor types cannot performance. Try operating an integrated circuit (LM35, AD 590, etc.) at 800C.
Thermocouples will be fabricated from two electric conductors manufactured from two different steel alloys. The conductors are typically built into a wire having a heat-resistant sheath, often 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 popular 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, would be to determine the temperature near the hot junction.
It should be mentioned that the “hot” junction, that is somewhat of a misnomer, may in fact be at a temperature lower than that of the reference junction if minimal temperatures are being measured.
Reference Junction Compensation Thermocouples make 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 difference between junctions, it’s important to know both voltage and reference junction heat to be able to determine the temp at the hot junction. Consequently, a thermocouple measurement method must either gauge the reference junction temperature or control it to keep up it at a fixed, known temperature.
You will find a misconception of how thermocouples operate. The misconception will be that the hot junction is the source of the output voltage. That is incorrect. 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 were not true we link a resistive load to a uniformly heated thermocouple inside an oven and use additional temperature from the resistor to generate a perpetual motion machine of the first kind.
The erroneous model also claims that junction voltages are generated at the frosty end between the special thermocouple cable and the copper circuit, therefore, a cold junction heat range measurement is required. This idea is wrong. The cold -stop temperature is the reference stage for measuring the temperature variation across the amount of the thermocouple circuit.
Most industrial thermocouple measurement systems opt to measure, instead of control, the reference junction temp. 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 dedicated analog input channel. Dedicating a particular channel to the function serves two needs: no application stations are ingested by the reference junction sensor, and the dedicated channel is definitely 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 is standard on countless Sensoray termination boards.
Linearization Within the “useable” heat range range of any thermocouple, you will find a proportional marriage between thermocouple voltage and heat range. This relationship, however, is in no way a linear relationship. Actually, most thermocouples are really non-linear over their functioning ranges. In order to obtain temperature data from the thermocouple, it’s important to convert the non-linear thermocouple voltage to temperature units. This process is called “linearization.”
Several methods are commonly employed to linearize thermocouples. At the low-cost end of the solution spectrum, you can restrict thermocouple operating range such that the thermocouple ‘s almost linear to within the measurement image resolution. At the contrary end of the spectrum, exclusive thermocouple interface components (incorporated circuits or modules) are available to perform both linearization and reference junction compensation in the analog domain. In general, neither of the methods is well-appropriate for cost-effective, multipoint data acquisition systems.