A thermocouple is actually a widely used form of sensor which is used to measure temperature. Thermocouples are popular in industrial control applications due to their relatively low cost and wide measurement ranges. Particularly, 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 created from two different metal alloys. The conductors are generally built into a cable developing a heat-resistant sheath, often by having an integral shield conductor. At one end in the cable, the 2 conductors are electrically shorted together by crimping, welding, etc. This end of the thermocouple–the recent junction–is thermally attached to the object to be measured. Another end–the cold junction, sometimes called reference junction–is associated with a measurement system. The objective, obviously, is to discover the temperature nearby the hot junction.
It needs to be noted that the “hot” junction, which happens to be somewhat of any 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 is really a function of the temperature distinction between junctions, it can be essential to know both voltage and reference junction temperature as a way to determine the temperature on the hot junction. Consequently, a thermocouple measurement system must either measure the reference junction temperature or control it to preserve it in a fixed, known temperature.
You will discover a misconception of how thermocouples operate. The misconception is the hot junction may be the supply 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 in the same temperature no voltage will be generated. If this type of were not true we connect a resistive load to your uniformly heated thermocouple sensors inside an oven and use additional heat through the resistor to produce a perpetual motion machine of your first kind.
The erroneous model also claims that junction voltages are generated in the cold end between the special thermocouple wire and the copper circuit, hence, a cold junction temperature measurement is essential. This idea is wrong. The cold -end temperature will be the reference point for measuring the temperature difference across the duration of the thermocouple circuit.
Most industrial thermocouple measurement systems choose to measure, rather than control, the reference junction temperature. This really is mainly because that it is more often than not more affordable just to include a reference junction sensor for an existing measurement system instead of add on a full-blown temperature controller.
Sensoray Smart A/D’s study the thermocouple reference junction temperature by means of a dedicated analog input channel. Dedicating a special channel to the function serves two purposes: no application channels are consumed through the reference junction sensor, and also the dedicated channel is automatically pre-configured for this particular function without requiring host processor support. This special channel is for direct connection to the reference junction sensor which is standard on many Sensoray termination boards.
Linearization Throughout the “useable” temperature variety of any thermocouple, you will find a proportional relationship between thermocouple voltage and temperature. This relationship, however, is in no way a linear relationship. In reality, most thermocouples are extremely non-linear over their operating ranges. To be able to obtain temperature data from a thermocouple, it really is required to convert the non-linear thermocouple voltage to temperature units. This thermocoup1er is known as “linearization.”
Several methods are commonly employed to linearize thermocouples. On the low-cost end in the solution spectrum, anybody can restrict thermocouple operating range in a way that the thermocouple is almost linear to in the measurement resolution. With the opposite end of the spectrum, special thermocouple interface components (integrated circuits or modules) are offered to perform both linearization and reference junction compensation from the analog domain. Generally, neither of those methods is well-best for cost-effective, multipoint data acquisition systems.
In addition to linearizing thermocouples inside the analog domain, it really is possible to perform such linearizations within the digital domain. This is accomplished by using either piecewise linear approximations (using look-up tables) or arithmetic approximations, or in some cases a hybrid of such two methods.
The Linearization Process Sensoray’s Smart A/D’s use a thermocouple measurement and linearization procedure that was created to hold costs to some practical level without sacrificing performance.
First, both the 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 in comparison to the reference junction because 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.