A thermocouple is a popular 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. In particular, thermocouples master measuring high temperatures where additional common sensor types cannot function. Try operating an integrated circuit (LM35, AD 590, etc.) at 800C.
Thermocouples are fabricated from two electrical conductors manufactured from two different steel alloys. The conductors are typically built into a cable having thermocouple temperature range a heat-resistant sheath, usually with an integral shield conductor. At one end of the cable, the two conductors are electrically shorted along by crimping, welding, etc. This end of the thermocouple–the very hot junction–is thermally attached to the object to be measured. Another 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 noted 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 produce an open-circuit voltage, named the Seebeck voltage, that’s proportional to the temperature difference between the hot and reference junctions :
Vs = V(Thot-Tref)
Since thermocouple voltage is a function of the temperature distinction between junctions, it’s important to learn both voltage and reference junction heat so that you can determine the temperatures at the hot junction. As a result, a thermocouple measurement program must either gauge the reference junction temperature or command it to maintain it at a fixed, known temperature.
There exists a misconception of how thermocouples work. The misconception can be that the hot junction is the source of the output voltage. That is incorrect. The voltage is generated across the length of the wire. Hence, if the complete wire length is at exactly the same temperature no voltage will be generated. If this weren’t true we connect a resistive load to a uniformly heated thermocouple inside an oven and use additional warmth from the resistor to make a perpetual motion machine of the first kind.
The erroneous model also claims that junction voltages are usually generated at the wintry end between the special thermocouple wire and the copper circuit, hence, a cold junction temp measurement is required. This idea is wrong. The cold -conclusion temperature is the reference point for measuring the temperature distinction across the amount of the thermocouple circuit.
Most industrial thermocouple measurement devices opt to measure, rather than control, the reference junction temperatures. That is due to the fact that it is almost always less costly to simply put in a reference junction sensor to an existing 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 particular channel to the function serves two uses: no application stations are ingested by the reference junction sensor, and the dedicated channel is automatically pre-configured for this reason without requiring host processor support. This special channel is made for direct link with the reference junction sensor that is standard on several Sensoray termination boards.
Linearization Within the “useable” heat range range of any thermocouple, you will find a proportional romantic relationship between thermocouple voltage and heat range. This relationship, however, is in no way a linear relationship. In fact, most thermocouples are extremely non-linear over their operating ranges. So as to obtain temperature data from the thermocouple, it is necessary to turn the non-linear thermocouple voltage to heat range units. This process is called “linearization.”
Several methods are commonly utilized to linearize thermocouples. At the low-cost end of the solution spectrum, you can restrict thermocouple operating range in a way that the thermocouple is nearly linear to within the measurement image resolution. At the contrary end of the spectrum, unique thermocouple interface components (integrated circuits or modules) can be found to execute both linearization and reference junction compensation in the analog domain. Generally, neither of the methods is well-suited for cost-effective, multipoint data acquisition methods.