A thermocouple is a widely used sort of sensor that is utilized to measure temperature. Thermocouples are popular in industrial control applications because of their relatively inexpensive and wide measurement ranges. In particular, thermocouples do well at measuring high temperatures where other common sensor types cannot function. Try operating a built-in circuit (LM35, AD 590, etc.) at 800C.
Thermocouples are fabricated from two electrical conductors made from two different metal alloys. The conductors are usually that are part of a cable developing a heat-resistant sheath, often with an integral shield conductor. At one end in the cable, both conductors are electrically shorted together by crimping, welding, etc. This end in the thermocouple–the junction–is thermally linked to the object to become measured. Another end–the cold junction, sometimes called reference junction–is linked to a measurement system. The goal, obviously, is to ascertain the temperature near to the hot junction.
It should be noted how the “hot” junction, which is somewhat of your misnomer, may actually be at a temperature lower than that of the reference junction if low temperatures are now being measured.
Since thermocouple voltage is a function of the temperature distinction between junctions, it really is necessary to know both voltage and reference junction temperature in order to determine the temperature on the hot junction. Consequently, a thermocouple measurement system must either look at the reference junction temperature or control it to keep up it with a fixed, known temperature.
You will find a misconception of how thermocouples operate. The misconception would be that the hot junction is definitely the way to obtain the output voltage. This is certainly wrong. The voltage is generated across the size of the wire. Hence, in case the entire wire length are at exactly the same temperature no voltage would be generated. If this were not true we connect a resistive load to some uniformly heated thermocouple controller inside an oven and make use of additional heat in the resistor to create a perpetual motion machine from the first kind.
The erroneous model also claims that junction voltages are generated at the cold end in between the special thermocouple wire as well as the copper circuit, hence, a cold junction temperature measurement is needed. This idea is wrong. The cold -end temperature may be the reference point for measuring the temperature difference across the length of the thermocouple circuit.
Most industrial thermocouple measurement systems prefer to measure, as an alternative to control, the reference junction temperature. This really is simply because that it is usually less expensive just to add a reference junction sensor for an existing measurement system than to add-on a complete-blown temperature controller.
Sensoray Smart A/D’s study the thermocouple reference junction temperature through a dedicated analog input channel. Dedicating a particular channel to this particular function serves two purposes: no application channels are consumed with the reference junction sensor, along with the dedicated channel is automatically pre-configured just for this function without requiring host processor support. This special channel is made for direct link with the reference junction sensor that is standard on many Sensoray termination boards.
Linearization In the “useable” temperature range of any thermocouple, you will find a proportional relationship between thermocouple voltage and temperature. This relationship, however, is by no means a linear relationship. In reality, most thermocouples are incredibly non-linear over their operating ranges. As a way to obtain temperature data from your thermocouple, it really is needed to convert the non-linear thermocouple voltage to temperature units. This thermocoup1er is called “linearization.”
Several methods are typically used to linearize thermocouples. At the low-cost end of the solution spectrum, one could restrict thermocouple operating range such that the thermocouple is nearly linear to within the measurement resolution. With the opposite end of your spectrum, special thermocouple interface components (integrated circuits or modules) are available to perform both linearization and reference junction compensation within the analog domain. On the whole, neither of such methods is well-designed for inexpensive, multipoint data acquisition systems.
Along with linearizing thermocouples inside the analog domain, it is actually easy to perform such linearizations within the digital domain. This can be accomplished by means of either piecewise linear approximations (using look-up tables) or arithmetic approximations, or in some instances a hybrid of those two methods.
The Linearization Process Sensoray’s Smart A/D’s hire a thermocouple measurement and linearization process that is designed to hold costs into a practical level without sacrificing performance.
First, both thermocouple and reference junction sensor signals are digitized to get thermocouple voltage Vt and reference junction temperature Tref. The thermocouple signal is digitized at a higher rate compared to the reference junction as it is assumed that the reference junction is comparatively stable compared to the hot junction. Reference junction measurements are transparently interleaved between thermocouple measurements without host processor intervention.