A thermocouple is a commonly used type of sensor that’s used to measure temperature. Thermocouples happen to be well-known in industrial control applications because of their relatively low cost and wide measurement ranges. In particular, thermocouples master measuring high temperatures where additional common sensor types cannot performance. Try operating an integrated circuit (LM35, AD 590, etc.) at 800C.
Thermocouples will be fabricated from two electrical conductors manufactured from two different metallic alloys. The conductors are typically built into a cable having a heat-resistant sheath, frequently with an integral shield conductor. At one conclusion of the cable, both conductors are electrically shorted jointly by crimping, welding, etc. This end of the thermocouple–the sizzling 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 close to the hot junction.
It should be mentioned that the “hot” junction, which is somewhat 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 generate 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 really a function of the temperature variation between junctions, it’s important to learn both voltage and reference junction temperature to be able to determine the heat at the hot junction. Therefore, a thermocouple measurement system 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 will be that the hot junction is the source of the output voltage. This is inappropriate. The voltage is generated over 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 in a oven and use additional temperature from the resistor to create a perpetual motion machine of the initial kind.
The erroneous model likewise claims that junction voltages are usually generated at the frosty 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 -ending temperature is the reference stage for measuring the temperature difference across the amount of the thermocouple circuit.
Most industrial thermocouple measurement systems opt to measure, instead of control, the reference junction temperatures. This is due to the fact that it is almost always less expensive to simply add a reference junction sensor to an existing measurement system than to include on a full-blown temperature controller.
Sensoray Smart A/D’s measure the thermocouple reference junction temperature through a dedicated analog input channel. Dedicating a special channel to the function serves two reasons: no application stations are consumed by the reference junction sensor, and the dedicated channel can be 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 numerous Sensoray termination boards.
Linearization Within the “useable” temperatures range of any thermocouple, you will find a proportional connection between thermocouple voltage and temperatures. This relationship, however, is by no means a linear relationship. In fact, most thermocouples are really non-linear over their working ranges. As a way to obtain temperature data from the thermocouple, it is necessary to change the non-linear thermocouple voltage to temperatures 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 is nearly linear to within the measurement resolution. At the opposite end of the spectrum, particular thermocouple interface components (incorporated circuits or modules) can be found to perform both linearization and reference junction settlement in the analog domain. In general, neither of these methods is well-suited for cost-effective, multipoint data acquisition devices.