A thermocouple is actually a commonly used type of sensor that is used to measure temperature. Thermocouples are popular in industrial control applications due to their relatively low cost and wide measurement ranges. In particular, thermocouples do well 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 manufactured from two different metal alloys. The conductors are typically included in a cable possessing a heat-resistant sheath, often by having an integral shield conductor. At one end in the cable, both conductors are electrically shorted together by crimping, welding, etc. This end of the thermocouple–the hot junction–is thermally connected to the object to become measured. The other end–the cold junction, sometimes called reference junction–is connected to a measurement system. The objective, of course, is to determine the temperature nearby the hot junction.
It needs to be noted the “hot” junction, which can be somewhat of your misnomer, may actually be at a temperature lower than that of the reference junction if low temperatures are measured.
Since thermocouple voltage is actually a purpose of the temperature distinction between junctions, it is actually necessary to know both voltage and reference junction temperature to be able to determine the temperature in the hot junction. Consequently, a thermocouple measurement system must either look at the reference junction temperature or control it to preserve it at a fixed, known temperature.
There is a misconception of how thermocouples operate. The misconception is the hot junction is definitely the way to obtain the output voltage. This is wrong. The voltage is generated across the duration of the wire. Hence, in the event the entire wire length is at the same temperature no voltage could be generated. If the were not true we connect a resistive load to your uniformly heated temperature controller inside an oven and make use of additional heat in the resistor to create a perpetual motion machine from the first kind.
The erroneous model also claims that junction voltages are generated at the cold end in between the special thermocouple wire and also the copper circuit, hence, a cold junction temperature measurement is essential. This concept is wrong. The cold -end temperature is the reference point for measuring the temperature difference across the size of the thermocouple circuit.
Most industrial thermocouple measurement systems prefer to measure, rather than control, the reference junction temperature. This is because of the fact that it is usually more affordable to easily give a reference junction sensor to an existing measurement system instead of add-on a complete-blown temperature controller.
Sensoray Smart A/D’s appraise the thermocouple reference junction temperature through a dedicated analog input channel. Dedicating an exclusive 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 with this function without requiring host processor support. This special channel is for direct link to the reference junction sensor that may be standard on many Sensoray termination boards.
Linearization Within the “useable” temperature array of any thermocouple, you will find 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. As a way to obtain temperature data coming from a thermocouple, it is actually needed to convert the non-linear thermocouple voltage to temperature units. This thermocoup1er is referred to as “linearization.”
Several methods are commonly utilized to linearize thermocouples. On the low-cost end in the solution spectrum, one can restrict thermocouple operating range such that the thermocouple is almost linear to within the measurement resolution. At the opposite end of your 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 those methods is well-suited for cost-effective, multipoint data acquisition systems.
As well as linearizing thermocouples from the analog domain, it can be possible to perform such linearizations from the digital domain. This is certainly accomplished by using either piecewise linear approximations (using look-up tables) or arithmetic approximations, or sometimes a hybrid of such 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 some practical level without sacrificing performance.
First, the two thermocouple and reference junction sensor signals are digitized to get thermocouple voltage Vt and reference junction temperature Tref. The thermocouple signal is digitized with a higher rate compared to reference junction because it is assumed how the reference junction is fairly stable in comparison to the hot junction. Reference junction measurements are transparently interleaved between thermocouple measurements without host processor intervention.