A thermocouple is actually a commonly used sort of sensor that is used to measure temperature. Thermocouples are popular in industrial control applications because of their relatively low cost 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 from two different metal alloys. The conductors are normally that are part of a cable having a heat-resistant sheath, often with the integral shield conductor. At one end of your cable, the two conductors are electrically shorted together by crimping, welding, etc. This end from the thermocouple–the junction–is thermally linked to the object being measured. Other end–the cold junction, sometimes called reference junction–is associated with a measurement system. The objective, obviously, is to determine the temperature near to the hot junction.
It should be noted how the “hot” junction, that is somewhat of the misnomer, may in fact attend a temperature lower compared to the reference junction if low temperatures are being measured.
Since thermocouple voltage is a purpose of the temperature difference between junctions, it can be needed 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 keep it in a fixed, known temperature.
You will find a misconception of how thermocouples operate. The misconception would be that the hot junction may be the way to obtain the output voltage. This is wrong. The voltage is generated across the size of the wire. Hence, when the entire wire length is in a similar temperature no voltage can be generated. If this were not true we connect a resistive load into a uniformly heated thermocouplers inside an oven and employ additional heat from the resistor to make a perpetual motion machine from the first kind.
The erroneous model also claims that junction voltages are generated at the cold end between the special thermocouple wire along with the copper circuit, hence, a cold junction temperature measurement is necessary. This concept 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 prefer to measure, as opposed to control, the reference junction temperature. This is due to the fact that it is typically more affordable just to include a reference junction sensor to a existing measurement system rather than to add on a full-blown temperature controller.
Sensoray Smart A/D’s look at the thermocouple reference junction temperature through a dedicated analog input channel. Dedicating a particular channel to this function serves two purposes: no application channels are consumed with the reference junction sensor, and the dedicated channel is automatically pre-configured for this function without requiring host processor support. This special channel is ideal for direct connection to the reference junction sensor which is standard on many Sensoray termination boards.
Linearization Within the “useable” temperature selection of any thermocouple, there exists a proportional relationship between thermocouple voltage and temperature. This relationship, however, is by no means a linear relationship. In reality, most thermocouples are really non-linear over their operating ranges. So that you can obtain temperature data from a thermocouple, it is actually necessary to convert the non-linear thermocouple voltage to temperature units. This thermocoup1er is known as “linearization.”
Several methods are normally employed 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 throughout 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 within the analog domain. In general, neither of those methods is well-suited for cost-effective, multipoint data acquisition systems.
As well as linearizing thermocouples from the analog domain, it really is easy to perform such linearizations from the digital domain. This can be accomplished by way of either piecewise linear approximations (using look-up tables) or arithmetic approximations, or occasionally a hybrid of these two methods.
The Linearization Process Sensoray’s Smart A/D’s use a thermocouple measurement and linearization procedure that is made to hold costs to some practical level without having to sacrifice performance.
First, the 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 reference junction since it is assumed that this reference junction is fairly stable when compared to the hot junction. Reference junction measurements are transparently interleaved between thermocouple measurements without host processor intervention.