A thermocouple is a popular form of sensor that is used to measure temperature. Thermocouples are popular in industrial control applications because of the relatively inexpensive and wide measurement ranges. Specifically, thermocouples excel at measuring high temperatures where other common sensor types cannot function. Try operating an internal circuit (LM35, AD 590, etc.) at 800C.
Thermocouples are fabricated from two electrical conductors created from two different metal alloys. The conductors are typically built in a cable possessing a heat-resistant sheath, often with an integral shield conductor. At one end of your cable, both the conductors are electrically shorted together by crimping, welding, etc. This end from the thermocouple–the hot junction–is thermally attached to the object to become measured. Another end–the cold junction, sometimes called reference junction–is linked to a measurement system. The objective, needless to say, is to ascertain the temperature near to the hot junction.
It should be noted that this “hot” junction, which happens to be somewhat of the misnomer, may in reality attend a temperature lower than that of the reference junction if low temperatures are now being measured.
Since thermocouple voltage is really a purpose of the temperature distinction between junctions, it can be necessary to know both voltage and reference junction temperature so that you can determine the temperature with the hot junction. Consequently, a thermocouple measurement system must either measure the reference junction temperature or control it to keep up it in a fixed, known temperature.
There exists a misconception of methods thermocouples operate. The misconception would be that the hot junction will be the way to obtain the output voltage. This can be wrong. The voltage is generated across the length of the wire. Hence, if the entire wire length is at exactly the same temperature no voltage could be generated. If this type of were not true we connect a resistive load to a uniformly heated thermocouple controller inside an oven and employ additional heat from your resistor to make a perpetual motion machine of the first kind.
The erroneous model also claims that junction voltages are generated at the cold end between your special thermocouple wire and also the copper circuit, hence, a cold junction temperature measurement is necessary. This concept is wrong. The cold -end temperature is definitely the reference point for measuring the temperature difference across the length of the thermocouple circuit.
Most industrial thermocouple measurement systems opt to measure, as opposed to control, the reference junction temperature. This is certainly due to the fact that it must be more often than not more affordable to simply include a reference junction sensor to a existing measurement system instead of 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 an exclusive channel to this function serves two purposes: no application channels are consumed from the reference junction sensor, as well as the dedicated channel is automatically pre-configured with this function without requiring host processor support. This special channel is made for direct link to the reference junction sensor which is standard on many Sensoray termination boards.
Linearization In the “useable” temperature variety of any thermocouple, there exists a proportional relationship between thermocouple voltage and temperature. This relationship, however, is by no means a linear relationship. The truth is, most thermocouples are really non-linear over their operating ranges. So that you can obtain temperature data from a thermocouple, it can be necessary to convert the non-linear thermocouple voltage to temperature units. This thermocoup1er is named “linearization.”
Several methods are typically accustomed to linearize thermocouples. At the low-cost end from the solution spectrum, one can restrict thermocouple operating range such that the thermocouple is nearly linear to throughout the measurement resolution. With the opposite end in the spectrum, special thermocouple interface components (integrated circuits or modules) are for sale to perform both linearization and reference junction compensation inside the analog domain. Generally, neither of the methods is well-suitable for inexpensive, multipoint data acquisition systems.
Along with linearizing thermocouples in the analog domain, it really is easy to perform such linearizations from the digital domain. This is certainly accomplished by means of either piecewise linear approximations (using look-up tables) or arithmetic approximations, or occasionally a hybrid of the two methods.
The Linearization Process Sensoray’s Smart A/D’s use a thermocouple measurement and linearization procedure that is designed to hold costs to your practical level without having to sacrifice performance.
First, the two thermocouple and reference junction sensor signals are digitized to acquire thermocouple voltage Vt and reference junction temperature Tref. The thermocouple signal is digitized with a higher rate than the reference junction as it is assumed that this reference junction is comparatively stable compared to the hot junction. Reference junction measurements are transparently interleaved between thermocouple measurements without host processor intervention.