A thermocouple can be a commonly used type of sensor which is used to measure temperature. Thermocouples are popular in industrial control applications because of the relatively affordable and wide measurement ranges. In particular, thermocouples excel at measuring high temperatures where other common sensor types cannot function. Try operating an internal circuit (LM35, AD 590, etc.) at 800C.
Thermocouples are fabricated from two electrical conductors created from two different metal alloys. The conductors are usually included in a cable possessing a heat-resistant sheath, often with an integral shield conductor. At one end of the cable, both the conductors are electrically shorted together by crimping, welding, etc. This end of the thermocouple–the hot junction–is thermally coupled to the object to become measured. The other end–the cold junction, sometimes called reference junction–is linked to a measurement system. The goal, naturally, is to discover the temperature close to the hot junction.
It ought to be noted how the “hot” junction, that is somewhat of your misnomer, may in reality be at a temperature lower compared to the reference junction if low temperatures are increasingly being measured.
Since thermocouple voltage can be a function of the temperature distinction between junctions, it can be required to know both voltage and reference junction temperature to be able to determine the temperature with the hot junction. Consequently, a thermocouple measurement system must either study the reference junction temperature or control it to preserve it in a fixed, known temperature.
There is a misconception of methods thermocouples operate. The misconception is that the hot junction is the method to obtain the output voltage. This is wrong. The voltage is generated across the duration of the wire. Hence, if the entire wire length are at exactly the same temperature no voltage can be generated. If this were not true we connect a resistive load to your uniformly heated temperature controller inside an oven and make use of additional heat from the resistor to make a perpetual motion machine of the first kind.
The erroneous model also claims that junction voltages are generated with the cold end involving the special thermocouple wire and the copper circuit, hence, a cold junction temperature measurement is needed. This concept 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 opt to measure, as an alternative to control, the reference junction temperature. This is certainly mainly because that it must be typically more affordable to simply put in a reference junction sensor with an existing measurement system rather than add-on an entire-blown temperature controller.
Sensoray Smart A/D’s measure the thermocouple reference junction temperature through a dedicated analog input channel. Dedicating a special channel to this particular function serves two purposes: no application channels are consumed through the reference junction sensor, as well as the dedicated channel is automatically pre-configured for this function without requiring host processor support. This special channel is for direct link to the reference junction sensor that is standard on many Sensoray termination boards.
Linearization Within the “useable” temperature selection 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 really non-linear over their operating ranges. To be able to obtain temperature data from the thermocouple, it can be essential to convert the non-linear thermocouple voltage to temperature units. This thermocoup1er is called “linearization.”
Several methods are normally utilized to linearize thermocouples. With the low-cost end of your solution spectrum, anybody can restrict thermocouple operating range in a way that the thermocouple is nearly linear to throughout the measurement resolution. At the opposite end of the 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-designed for inexpensive, multipoint data acquisition systems.
Along with linearizing thermocouples in the analog domain, it is easy to perform such linearizations in the digital domain. This can be accomplished by means of 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 designed to hold costs to your practical level without sacrificing performance.
First, both the thermocouple and reference junction sensor signals are digitized to acquire thermocouple voltage Vt and reference junction temperature Tref. The thermocouple signal is digitized in a higher rate in comparison to the reference junction because it is assumed how 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.