A thermocouple is a commonly used type of sensor that is used to measure temperature. Thermocouples happen to be favorite in industrial control applications because of the relatively low priced and wide measurement ranges. In particular, thermocouples excel at measuring high thermocouple manufacturers temperatures where other common sensor types cannot functionality. Try operating a built-in circuit (LM35, AD 590, etc.) at 800C.
Thermocouples are fabricated from two electrical conductors made of two different metal alloys. The conductors are typically built into a cable having a heat-resistant sheath, normally with an integral shield conductor. At one ending of the cable, both conductors are electrically shorted jointly 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, in some cases called reference junction–is connected to a measurement system. The target, of course, is to determine the temperature near the hot junction.
It should be observed that the “hot” junction, that is relatively 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 generate an open-circuit voltage, known as the Seebeck voltage, that’s proportional to the temperature difference between your 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 learn both voltage and reference junction heat range so that you can determine the temp at the hot junction. Therefore, a thermocouple measurement system must either measure the reference junction temperature or command it to keep up it at a set, known temperature.
There exists a misconception of how thermocouples function. The misconception is certainly that the hot junction is the way to obtain 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 connect a resistive load to a uniformly heated thermocouple in a oven and use additional heat from the resistor to generate a perpetual motion machine of the first kind.
The erroneous model as well claims that junction voltages are usually generated at the wintry end between the special thermocouple cable and the copper circuit, therefore, a cold junction temperatures measurement is required. This idea is wrong. The cold -finish temperature is the reference point for measuring the temperature variation across the amount of the thermocouple circuit.
Most industrial thermocouple measurement methods opt to measure, rather than control, the reference junction temperatures. This is due to the fact that it’s almost always less expensive to simply put in 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 through a dedicated analog input channel. Dedicating a particular channel to the function serves two reasons: no application stations are consumed by the reference junction sensor, and the dedicated channel is usually automatically pre-configured for this reason without requiring host processor help. This special channel is designed for direct connection to the reference junction sensor that’s standard on countless Sensoray termination boards.
Linearization Within the “useable” temperature range of any thermocouple, there exists a proportional relationship between thermocouple voltage and heat range. This relationship, however, is in no way a linear relationship. In fact, most thermocouples are really non-linear over their running ranges. As a way to obtain temperature data from a thermocouple, it’s important to transfer the non-linear thermocouple voltage to temperatures units. This technique is called “linearization.”
Several methods are commonly used 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 image resolution. At the contrary end of the spectrum, particular thermocouple interface components (integrated circuits or modules) are available to execute both linearization and reference junction payment in the analog domain. In general, neither of the methods is well-appropriate for cost-effective, multipoint data acquisition techniques.