A thermocouple is a commonly used type of sensor that is used to measure temperature. Thermocouples are common in industrial control applications because of the relatively low cost and wide measurement ranges. In particular, thermocouples excel at measuring high temperatures where various other common sensor types cannot performance. Try operating a built-in circuit (LM35, AD 590, etc.) at 800C.
Thermocouples happen to be fabricated from thermocouple temperature range two electric conductors manufactured from two different steel alloys. The conductors are typically built into a cable connection having a heat-resistant sheath, normally with an integral shield conductor. At one end of the cable, both conductors are electrically shorted together with each other 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, often called reference junction–is connected to a measurement system. The target, of course, would be to determine the temperature close to the hot junction.
It should be observed that the “hot” junction, that is considerably 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 distinction 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 temperature so as to determine the temperature at the hot junction. Therefore, a thermocouple measurement system must either measure the reference junction temperature or control it to keep up it at a set, known temperature.
There is a misconception of how thermocouples operate. The misconception is certainly that the hot junction may be the source of the output voltage. This is wrong. The voltage is generated over the amount of the wire. Hence, if the complete wire length is at the same temperature no voltage would be generated. If this were not true we connect 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 furthermore claims that junction voltages happen to be generated at the frosty end between your special thermocouple wire and the copper circuit, hence, a cold junction heat range measurement is required. This concept is wrong. The cold -end temperature is the reference stage for measuring the temperature difference across the length of the thermocouple circuit.
Most industrial thermocouple measurement systems opt to measure, instead of control, the reference junction heat. That is due to the fact that it’s almost always less expensive 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 through a dedicated analog input channel. Dedicating a particular channel to the function serves two needs: no application channels are ingested by the reference junction sensor, and the dedicated channel is certainly automatically pre-configured for this function without requiring host processor support. This special channel is made for direct connection to the reference junction sensor that is standard on numerous Sensoray termination boards.
Linearization Within the “useable” heat range of any thermocouple, there exists a proportional romance between thermocouple voltage and temperature. This relationship, however, is by no means a linear relationship. In fact, most thermocouples are really non-linear over their functioning ranges. So that you can obtain temperature data from a thermocouple, it is necessary to change the non-linear thermocouple voltage to temperature units. This technique 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 opposite end of the spectrum, exceptional thermocouple interface components (integrated circuits or modules) can be found to execute both linearization and reference junction settlement in the analog domain. In general, neither of these methods is well-appropriate for cost-effective, multipoint data acquisition techniques.