A thermocouple is a commonly used type of sensor that is used to measure temperature. Thermocouples are usually famous in industrial control applications because of the relatively low priced and wide measurement ranges. Specifically, thermocouples excel at measuring thermocouple sensor high temperatures where some other common sensor types cannot function. Try operating an integrated circuit (LM35, AD 590, etc.) at 800C.
Thermocouples are usually fabricated from two electric conductors made of two different steel alloys. The conductors are usually built into a cable connection having a heat-resistant sheath, often with an essential shield conductor. At one ending of the cable, the two conductors are electrically shorted mutually by crimping, welding, etc. This end of the thermocouple–the scorching junction–is thermally attached to the object to be measured. Another end–the cold junction, often called reference junction–is linked 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 relatively of a misnomer, may actually be at a temperature less than that of the reference junction if low temperatures are being measured.
Reference Junction Compensation Thermocouples generate an open-circuit voltage, referred to as the Seebeck voltage, that is proportional to the temperature distinction between the hot and reference junctions :
Vs = V(Thot-Tref)
Since thermocouple voltage is really a function of the temperature variation between junctions, it is necessary to know both voltage and reference junction temperature so as to determine the temp at the hot junction. Consequently, a thermocouple measurement method must either measure the reference junction temperature or control it to keep it at a set, known temperature.
You will find a misconception of how thermocouples function. The misconception is certainly 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 exactly the same temperature no voltage would be generated. If this weren’t true we connect a resistive load to a uniformly heated thermocouple in a oven and use additional warmth from the resistor to make a perpetual motion machine of the initial kind.
The erroneous model in addition claims that junction voltages happen to be generated at the cold end between your special thermocouple cable and the copper circuit, consequently, a cold junction heat range measurement is required. This concept is wrong. The cold -finish temperature is the reference level for measuring the temperature difference across the amount of the thermocouple circuit.
Most industrial thermocouple measurement systems opt to measure, rather than control, the reference junction temperature. This is due to the fact that it’s almost always less expensive to simply add a reference junction sensor to a preexisting measurement system than to add on a full-blown temperature controller.
Sensoray Smart A/D’s measure the thermocouple reference junction temperature through a separate analog input channel. Dedicating a special channel to this function serves two needs: no application stations are consumed by the reference junction sensor, and the dedicated channel will be automatically pre-configured for this reason without requiring host processor assistance. This special channel is made for direct connection to the reference junction sensor that is standard on several Sensoray termination boards.
Linearization Within the “useable” temperatures range of any thermocouple, there exists a proportional marriage between thermocouple voltage and heat. This relationship, however, is in no way a linear relationship. In fact, most thermocouples are extremely non-linear over their functioning ranges. So that you can obtain temperature data from the thermocouple, it’s important to change the non-linear thermocouple voltage to heat range units. This process 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 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 (incorporated circuits or modules) can be found to perform both linearization and reference junction payment in the analog domain. In general, neither of these methods is well-suited for cost-effective, multipoint data acquisition techniques.