A thermocouple can be a commonly used form of sensor that is used to measure temperature. Thermocouples are popular in industrial control applications due to their relatively inexpensive and wide measurement ranges. Especially, 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 possessing a heat-resistant sheath, often with an integral shield conductor. At one end of your cable, the 2 conductors are electrically shorted together by crimping, welding, etc. This end of the thermocouple–the new junction–is thermally linked to the object to get measured. One other end–the cold junction, sometimes called reference junction–is linked to a measurement system. The goal, of course, is to ascertain the temperature near the hot junction.
It must be noted that the “hot” junction, which can be somewhat of the misnomer, may the truth is attend a temperature lower than that of the reference junction if low temperatures are now being measured.
Since thermocouple voltage can be a purpose of the temperature distinction between junctions, it is actually required to know both voltage and reference junction temperature as a way to determine the temperature at the hot junction. Consequently, a thermocouple measurement system must either study the reference junction temperature or control it to preserve it at the fixed, known temperature.
There is a misconception of methods thermocouples operate. The misconception is that the hot junction is the supply of the output voltage. This is certainly wrong. The voltage is generated across the size of the wire. Hence, if the entire wire length are at the identical temperature no voltage would be generated. If it were not true we connect a resistive load to your uniformly heated thermocouple sensors inside an oven and employ additional heat from the resistor to create a perpetual motion machine from the first kind.
The erroneous model also claims that junction voltages are generated with the cold end between your special thermocouple wire and also the copper circuit, hence, a cold junction temperature measurement is necessary. This idea 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 decide to measure, rather than control, the reference junction temperature. This is certainly simply because that it must be usually more affordable just to put in a reference junction sensor with an existing measurement system than to add on a complete-blown temperature controller.
Sensoray Smart A/D’s study the thermocouple reference junction temperature by means of a dedicated analog input channel. Dedicating a particular channel for this function serves two purposes: no application channels are consumed from the reference junction sensor, along with the dedicated channel is automatically pre-configured for this function without requiring host processor support. This special channel is made for direct link with the reference junction sensor that is certainly standard on many Sensoray termination boards.
Linearization In the “useable” temperature array 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 fact, most thermocouples are incredibly non-linear over their operating ranges. As a way to obtain temperature data from a thermocouple, it can be needed to convert the non-linear thermocouple voltage to temperature units. This thermocoup1er is called “linearization.”
Several methods are normally accustomed to linearize thermocouples. With the low-cost end of your solution spectrum, one could restrict thermocouple operating range in a way that the thermocouple is nearly linear to within the measurement resolution. On the opposite end of your 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 such methods is well-suited for inexpensive, multipoint data acquisition systems.
Together with linearizing thermocouples within the analog domain, it is actually possible to perform such linearizations from the digital domain. This really is accomplished by using either piecewise linear approximations (using look-up tables) or arithmetic approximations, or in some instances a hybrid of such two methods.
The Linearization Process Sensoray’s Smart A/D’s employ a thermocouple measurement and linearization process that was designed to hold costs to a practical level without sacrificing performance.
First, the two thermocouple and reference junction sensor signals are digitized to have thermocouple voltage Vt and reference junction temperature Tref. The thermocouple signal is digitized at a higher rate in comparison to the reference junction as it is assumed that the reference junction is fairly stable compared to the hot junction. Reference junction measurements are transparently interleaved between thermocouple measurements without host processor intervention.