A thermocouple is a commonly used type of sensor that is used to measure temperature. Thermocouples will be well-known in industrial control thermocouple sensor applications because of their relatively low priced and wide measurement ranges. Specifically, thermocouples excel at measuring high temperatures where various other common sensor types cannot functionality. Try operating an integrated circuit (LM35, AD 590, etc.) at 800C.

Thermocouples are usually fabricated from two electrical conductors manufactured from two different metal alloys. The conductors are typically built into a cable having a heat-resistant sheath, usually with an essential shield conductor. At one stop of the cable, both conductors are electrically shorted collectively by crimping, welding, etc. This end of the thermocouple–the hot junction–is thermally attached to the object to be measured. Another end–the cold junction, 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 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 make an open-circuit voltage, called 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 is necessary to know both voltage and reference junction heat so that you can determine the temp at the hot junction. As a result, a thermocouple measurement method must either measure the reference junction temperature or handle it to keep it at a fixed, known temperature.

There is a misconception of how thermocouples run. The misconception is certainly that the hot junction is the way to obtain the output voltage. That is incorrect. The voltage is generated over the length of the wire. Hence, if the complete wire length is at the same temperature no voltage would be generated. If this weren’t true we hook up a resistive load to a uniformly heated thermocouple inside an oven and use additional high temperature from the resistor to make a perpetual motion machine of the first kind.

The erroneous model also claims that junction voltages are generated at the frosty end between the special thermocouple cable and the copper circuit, hence, a cold junction heat range measurement is required. This idea is wrong. The cold -end temperature is the reference level for measuring the temperature distinction across the amount 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 is almost always less expensive 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 measure the thermocouple reference junction temperature by means of a separate analog input channel. Dedicating a special channel to the function serves two functions: no application channels are ingested by the reference junction sensor, and the dedicated channel is certainly automatically pre-configured for this function without requiring host processor support. This special channel is made for direct connection to the reference junction sensor that is standard on numerous Sensoray termination boards.

Linearization Within the “useable” temperatures range of any thermocouple, there is a proportional romance between thermocouple voltage and heat range. This relationship, however, is by no means a linear relationship. Actually, most thermocouples are extremely non-linear over their functioning ranges. So that you can obtain temperature data from a thermocouple, it’s important to transfer the non-linear thermocouple voltage to temperatures units. This process is called “linearization.”

Several methods are commonly employed 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 resolution. At the contrary end of the spectrum, specific thermocouple interface components (incorporated circuits or modules) are available to perform both linearization and reference junction compensation in the analog domain. Generally, neither of the methods is well-appropriate for cost-effective, multipoint data acquisition devices.


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