A thermocouple is a popular type of sensor that’s used to measure temperature. Thermocouples are usually common in industrial control applications because of their relatively low cost and wide measurement ranges. In particular, thermocouples excel at measuring high temperatures where different common sensor types thermocouple manufacturers cannot work. Try operating an integrated circuit (LM35, AD 590, etc.) at 800C.
Thermocouples are usually fabricated from two electrical conductors made of two different metal alloys. The conductors are usually built into a cable connection having a heat-resistant sheath, frequently with an essential shield conductor. At one end of the cable, the two conductors are electrically shorted collectively by crimping, welding, etc. This end of the thermocouple–the scorching junction–is thermally attached to the thing to be measured. The other end–the cold junction, quite often called reference junction–is linked to a measurement system. The objective, of course, is to determine the temperature near the hot junction.
It should be noted that the “hot” junction, which is fairly of a misnomer, may in fact be at a temperature less than that of the reference junction if low temperatures are being measured.
Reference Junction Compensation Thermocouples crank out an open-circuit voltage, referred to as the Seebeck voltage, that’s proportional to the temperature variation between the hot and reference junctions :
Vs = V(Thot-Tref)
Since thermocouple voltage is a function of the temperature distinction between junctions, it is necessary to learn both voltage and reference junction heat as a way to determine the temperature at the hot junction. Consequently, a thermocouple measurement system must either measure the reference junction temperature or handle it to keep up it at a fixed, known temperature.
There exists a misconception of how thermocouples run. The misconception is definitely 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 entire wire length is at the same temperature no voltage would be generated. If this weren’t true we link a resistive load to a uniformly heated thermocouple inside an oven and use additional heat from the resistor to produce a perpetual motion machine of the initial kind.
The erroneous model as well claims that junction voltages are generated at the chilly end between the special thermocouple wire and the copper circuit, consequently, a cold junction temp measurement is required. This idea is wrong. The cold -finish temperature is the reference point for measuring the temperature difference across the length of the thermocouple circuit.
Most industrial thermocouple measurement methods opt to measure, rather than control, the reference junction temperature. This is due to the fact that it is 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 gauge the thermocouple reference junction temperature by means of a dedicated analog input channel. Dedicating a particular channel to this function serves two functions: no application channels are taken by the reference junction sensor, and the dedicated channel is automatically pre-configured for this reason without requiring host processor help. 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, there exists a proportional relationship between thermocouple voltage and temperature. This relationship, however, is in no way a linear relationship. Actually, most thermocouples are extremely non-linear over their functioning ranges. To be able to obtain temperature data from a thermocouple, it’s important to switch the non-linear thermocouple voltage to heat units. This process is called “linearization.”
Several methods are commonly utilized to linearize thermocouples. At the low-cost end of the answer spectrum, one can restrict thermocouple operating range in a way that the thermocouple ‘s almost linear to within the measurement quality. At the contrary end of the spectrum, specific thermocouple interface components (integrated circuits or modules) are available to execute both linearization and reference junction payment in the analog domain. Generally, neither of the methods is well-appropriate for cost-effective, multipoint data acquisition systems.