A thermocouple is a commonly used type of sensor that’s used to measure temperature. Thermocouples are usually famous in industrial control applications because of their relatively low priced and wide measurement ranges. Specifically, thermocouples excel at thermocouple manufacturers measuring high temperatures where some other common sensor types cannot functionality. Try operating a built-in circuit (LM35, AD 590, etc.) at 800C.
Thermocouples are usually fabricated from two electric conductors manufactured from two different metallic alloys. The conductors are typically built into a cable connection having a heat-resistant sheath, generally with an integral shield conductor. At one conclusion of the cable, both conductors are electrically shorted together by crimping, welding, etc. This end of the thermocouple–the scorching junction–is thermally attached to the object to be measured. The other end–the cold junction, oftentimes 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 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 distinction 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 learn both voltage and reference junction temperatures as a way to determine the temperatures at the hot junction. Consequently, a thermocouple measurement technique must either gauge the reference junction temperature or management it to maintain it at a fixed, known temperature.
There is a misconception of how thermocouples run. The misconception is that the hot junction may be the source of the output voltage. This is incorrect. The voltage is generated across the amount of the wire. Hence, if the complete wire length is at the same temperature no voltage would be generated. If this weren’t true we hook up a resistive load to a uniformly heated thermocouple in a oven and use additional temperature from the resistor to produce a perpetual motion machine of the initial kind.
The erroneous model in addition claims that junction voltages are usually generated at the chilly end between the special thermocouple cable and the copper circuit, consequently, a cold junction temperature measurement is required. This idea is wrong. The cold -stop temperature is the reference level for measuring the temperature distinction across the length of the thermocouple circuit.
Most industrial thermocouple measurement systems opt to measure, instead of control, the reference junction heat range. That is due to the fact that it is almost always less expensive 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 gauge the thermocouple reference junction temperature by means of a separate analog input channel. Dedicating a special channel to this function serves two uses: no application stations are ingested by the reference junction sensor, and the dedicated channel is definitely automatically pre-configured for this reason without requiring host processor help. This special channel is designed for direct connection to the reference junction sensor that’s standard on countless Sensoray termination boards.
Linearization Within the “useable” temperatures range of any thermocouple, there exists a proportional connection between thermocouple voltage and temperature. This relationship, however, is in no way a linear relationship. Actually, most thermocouples are really non-linear over their working ranges. As a way to obtain temperature data from the thermocouple, it’s important to switch the non-linear thermocouple voltage to temp units. This technique is called “linearization.”
Several methods are commonly used to linearize thermocouples. At the low-cost end of the perfect solution is spectrum, you can restrict thermocouple operating range such that the thermocouple is nearly linear to within the measurement image resolution. At the contrary end of the spectrum, exclusive thermocouple interface components (integrated circuits or modules) are available to perform both linearization and reference junction reimbursement in the analog domain. In general, neither of the methods is well-appropriate for cost-effective, multipoint data acquisition methods.