A thermocouple can be a frequently used kind of sensor that is utilized to measure temperature. Thermocouples are popular in industrial control applications because of their relatively inexpensive and wide measurement ranges. Especially, thermocouples excel at measuring high temperatures where other common sensor types cannot function. Try operating an incorporated circuit (LM35, AD 590, etc.) at 800C.
Thermocouples are fabricated from two electrical conductors made of two different metal alloys. The conductors are usually built into a cable possessing a heat-resistant sheath, often with an integral shield conductor. At one end in the cable, both the conductors are electrically shorted together by crimping, welding, etc. This end of your thermocouple–the recent junction–is thermally coupled to the object to get measured. Other end–the cold junction, sometimes called reference junction–is attached to a measurement system. The goal, of course, is to discover the temperature nearby the hot junction.
It ought to be noted how the “hot” junction, that is somewhat of your misnomer, may in fact 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 is needed to know both voltage and reference junction temperature so that you can determine the temperature at the hot junction. Consequently, a thermocouple measurement system must either measure the reference junction temperature or control it to keep up it at the fixed, known temperature.
There exists a misconception of how thermocouples operate. The misconception is the fact that hot junction is definitely the source of the output voltage. This can be wrong. The voltage is generated across the size of the wire. Hence, in case the entire wire length is in a similar temperature no voltage would be generated. If this type of were not true we connect a resistive load to some uniformly heated thermocouple controller inside an oven and make use of additional heat through the resistor to generate a perpetual motion machine from the first kind.
The erroneous model also claims that junction voltages are generated on the cold end between the special thermocouple wire and also the copper circuit, hence, a cold junction temperature measurement is required. This concept is wrong. The cold -end temperature is the reference point for measuring the temperature difference across the length of the thermocouple circuit.
Most industrial thermocouple measurement systems choose to measure, as an alternative to control, the reference junction temperature. This is certainly because of the fact that it is typically less expensive to merely put in a reference junction sensor to a existing measurement system rather than to add-on an entire-blown temperature controller.
Sensoray Smart A/D’s appraise the thermocouple reference junction temperature through a dedicated analog input channel. Dedicating an exclusive channel to this particular function serves two purposes: no application channels are consumed through the reference junction sensor, and the dedicated channel is automatically pre-configured for this particular function without requiring host processor support. This special channel is designed for direct link with the reference junction sensor that is certainly standard on many Sensoray termination boards.
Linearization Inside the “useable” temperature selection of any thermocouple, you will discover a proportional relationship between thermocouple voltage and temperature. This relationship, however, is by no means a linear relationship. In reality, most thermocouples are extremely non-linear over their operating ranges. In order to obtain temperature data from your thermocouple, it is actually necessary to convert the non-linear thermocouple voltage to temperature units. This thermocoup1er is referred to as “linearization.”
Several methods are typically used to linearize thermocouples. At the low-cost end of the solution spectrum, you can restrict thermocouple operating range to ensure that the thermocouple is nearly linear to within the measurement resolution. On the opposite end in the spectrum, special thermocouple interface components (integrated circuits or modules) are offered to perform both linearization and reference junction compensation within the analog domain. In general, neither of the methods is well-suitable for cost-effective, multipoint data acquisition systems.
As well as linearizing thermocouples in the analog domain, it can be possible to perform such linearizations within the digital domain. This really is accomplished by means of either piecewise linear approximations (using look-up tables) or arithmetic approximations, or in some instances a hybrid of the two methods.
The Linearization Process Sensoray’s Smart A/D’s employ a thermocouple measurement and linearization procedure that was designed to hold costs into a practical level without having to sacrifice 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 the higher rate compared to reference junction as it is assumed the reference junction is comparatively stable in comparison to the hot junction. Reference junction measurements are transparently interleaved between thermocouple measurements without host processor intervention.