A thermocouple is a commonly used type of sensor that thermocouple wire is used to measure temperature. Thermocouples are well-known in industrial control applications because of the relatively low priced and wide measurement ranges. Specifically, thermocouples master measuring high temperatures where some other common sensor types cannot performance. Try operating a built-in circuit (LM35, AD 590, etc.) at 800C.
Thermocouples are usually fabricated from two electric conductors manufactured from two different metal alloys. The conductors are typically built into a cable connection having a heat-resistant sheath, normally with an essential shield conductor. At one end of the cable, both 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. The other end–the cold junction, occasionally called reference junction–is linked to a measurement system. The target, of course, is to determine the temperature near the hot junction.
It should be noted that the “hot” junction, which is relatively of a misnomer, may in fact be at a temperature lower than that of the reference junction if low temperatures are being measured.
Reference Junction Compensation Thermocouples crank out an open-circuit voltage, known as the Seebeck voltage, that’s proportional to the temperature distinction between your hot and reference junctions :
Vs = V(Thot-Tref)
Since thermocouple voltage is really a function of the temperature difference between junctions, it is necessary to know both voltage and reference junction temp to be able to determine the temperatures at the hot junction. Consequently, a thermocouple measurement method must either gauge the reference junction temperature or command it to maintain it at a set, known temperature.
There is a misconception of how thermocouples run. The misconception is usually that the hot junction is the source of the output voltage. That is wrong. The voltage is generated across the amount of the wire. Hence, if the entire wire length is at exactly the same temperature no voltage will be generated. If this were not true we link a resistive load to a uniformly heated thermocouple inside an oven and use additional warmth from the resistor to generate a perpetual motion machine of the initial kind.
The erroneous model also claims that junction voltages happen to be generated at the frosty end between the special thermocouple wire and the copper circuit, therefore, a cold junction temperatures measurement is required. This concept is wrong. The cold -conclusion temperature is the reference level for measuring the temperature difference across the length of the thermocouple circuit.
Most industrial thermocouple measurement methods opt to measure, instead of control, the reference junction temp. This is due to the fact that it’s almost always less costly 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 by means of a dedicated analog input channel. Dedicating a special channel to this function serves two reasons: no application stations are ingested 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 link with the reference junction sensor that is standard on numerous Sensoray termination boards.
Linearization Within the “useable” temperature range of any thermocouple, there is a proportional relationship between thermocouple voltage and heat. This relationship, however, is by no means 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 temperature units. This process is called “linearization.”
Several methods are commonly used to linearize thermocouples. At the low-cost end of the answer spectrum, you can restrict thermocouple operating range in a way that the thermocouple is nearly linear to within the measurement image resolution. At the opposite end of the spectrum, specific thermocouple interface components (integrated circuits or modules) are available to execute both linearization and reference junction reimbursement in the analog domain. Generally, neither of the methods is well-appropriate for cost-effective, multipoint data acquisition systems.