A thermocouple is a popular kind of sensor that is used to measure temperature. Thermocouples are popular in industrial control applications because of the relatively affordable and wide measurement ranges. In particular, thermocouples do well at measuring high temperatures where other common sensor types cannot function. Try operating an integrated circuit (LM35, AD 590, etc.) at 800C.
Thermocouples are fabricated from two electrical conductors made from two different metal alloys. The conductors are typically built into a cable using a heat-resistant sheath, often with an integral shield conductor. At one end of the cable, the 2 conductors are electrically shorted together by crimping, welding, etc. This end in the thermocouple–the recent junction–is thermally connected to the object to get measured. One other end–the cold junction, sometimes called reference junction–is connected to a measurement system. The objective, naturally, is to determine the temperature close to the hot junction.
It ought to be noted that this “hot” junction, which is somewhat of any misnomer, may the truth is be at a temperature lower compared to the reference junction if low temperatures are being measured.
Since thermocouple voltage is really a function of the temperature distinction between junctions, it really is needed to know both voltage and reference junction temperature in order to determine the temperature with the hot junction. Consequently, a thermocouple measurement system must either measure the reference junction temperature or control it to maintain it at a fixed, known temperature.
There exists a misconception of how thermocouples operate. The misconception is the fact that hot junction is the source of the output voltage. This is certainly wrong. The voltage is generated across the size of the wire. Hence, in case the entire wire length is at a similar temperature no voltage could be generated. If the were not true we connect a resistive load to some uniformly heated temperature controller inside an oven and use additional heat from the resistor to create a perpetual motion machine of your first kind.
The erroneous model also claims that junction voltages are generated with the cold end in between the special thermocouple wire as well as the copper circuit, hence, a cold junction temperature measurement is required. This idea is wrong. The cold -end temperature will be the reference point for measuring the temperature difference across the size of the thermocouple circuit.
Most industrial thermocouple measurement systems choose to measure, as opposed to control, the reference junction temperature. This is certainly due to the fact that it must be almost always more affordable to simply put in a reference junction sensor to a existing measurement system than to add-on an entire-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 the function serves two purposes: no application channels are consumed from the reference junction sensor, and the dedicated channel is automatically pre-configured for this function without requiring host processor support. This special channel is made for direct connection to the reference junction sensor which is standard on many Sensoray termination boards.
Linearization Inside the “useable” temperature array of any thermocouple, there is a proportional relationship between thermocouple voltage and temperature. This relationship, however, is in no way a linear relationship. The truth is, most thermocouples are exceedingly non-linear over their operating ranges. To be able to obtain temperature data from your thermocouple, it is necessary to convert the non-linear thermocouple voltage to temperature units. This thermocoup1er is named “linearization.”
Several methods are commonly accustomed to linearize thermocouples. With the low-cost end from the solution spectrum, one can restrict thermocouple operating range in a way that the thermocouple is nearly linear to inside the measurement resolution. In the opposite end of the spectrum, special thermocouple interface components (integrated circuits or modules) are available to perform both linearization and reference junction compensation from the analog domain. On the whole, neither of these methods is well-designed for inexpensive, multipoint data acquisition systems.
Along with linearizing thermocouples from the analog domain, it can be easy to perform such linearizations in the digital domain. This is certainly accomplished by using 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 process that was created to hold costs into a practical level without sacrificing performance.
First, both thermocouple and reference junction sensor signals are digitized to obtain thermocouple voltage Vt and reference junction temperature Tref. The thermocouple signal is digitized in a higher rate compared to the reference junction since it is assumed that this reference junction is pretty stable in comparison to the hot junction. Reference junction measurements are transparently interleaved between thermocouple measurements without host processor intervention.