Editing Thermocouple (section)

==Practical concerns==
Thermocouples ideally should be very simple measurement devices, with each type being characterized by a precise <math>\scriptstyle E(T)</math> curve, independent of any other details.
In reality, thermocouples are affected by issues such as alloy manufacturing uncertainties, aging effects, and circuit design mistakes/misunderstandings.

===Circuit construction===
A common error in thermocouple construction is related to cold junction compensation. If an error is made on the estimation of <math>T_\mathrm{ref}</math>, an error will appear in the temperature measurement. For the simplest measurements, thermocouple wires are connected to copper far away from the hot or cold point whose temperature is measured; this reference junction is then assumed to be at room temperature, but that temperature can vary.<ref>{{cite web|title=How to Prevent Temperature Measurement Errors When Installing Thermocouple Sensors and Transmitters|url=https://www.acromag.com/sites/default/files/How_to_prevent_temperature_measurement_errors_when_installing_thermocouple_sensors_and_transmitters_926A.pdf|website=acromag.com|publisher=[[Acromag]]|access-date=3 February 2017}}</ref> Because of the nonlinearity in the thermocouple voltage curve, the errors in <math>T_\mathrm{ref}</math> and <math>T_\mathrm{sense}</math> are generally unequal values. Some thermocouples, such as Type B, have a relatively flat voltage curve near room temperature, meaning that a large uncertainty in a room-temperature <math>T_\mathrm{ref}</math> translates to only a small error in <math>T_\mathrm{sense}</math>.

Junctions should be made in a reliable manner, but there are many possible approaches to accomplish this.
For low temperatures, junctions can be brazed or soldered; however, it may be difficult to find a suitable [[flux (metallurgy)|flux]] and this may not be suitable at the sensing junction due to the solder's low melting point.
Reference and extension junctions are therefore usually made with screw [[terminal block]]s.
For high temperatures, the most common approach is the [[spot weld]] or [[crimp connection|crimp]] using a durable material.<ref name="wang"/>

One common myth regarding thermocouples is that junctions must be made cleanly without involving a third metal, to avoid unwanted added EMFs.<ref>Pyromation, Inc. [http://www.pyromation.com/Downloads/Doc/Training_TC_Theory.pdf "Thermocouple theory"] (2009).</ref>
This may result from another common misunderstanding that the voltage is generated at the junction.<ref>Rowe, Martin (2013). [http://www.edn.com/design/test-and-measurement/4423409/Simple--but-misunderstood "Thermocouples: Simple but misunderstood"], EDN Network.</ref> In fact, the junctions should in principle have uniform internal temperature; therefore, no voltage is generated at the junction. The voltage is generated in the thermal gradient, along the wire.

A thermocouple produces small signals, often microvolts in magnitude. Precise measurements of this signal require an amplifier with low [[input offset voltage]] and with care taken to avoid thermal EMFs from self-heating within the voltmeter itself. If the thermocouple wire has a high resistance for some reason (poor contact at junctions, or very thin wires used for fast thermal response), the measuring instrument should have high [[input impedance]] to prevent an offset in the measured voltage. A useful feature in thermocouple instrumentation will simultaneously measure resistance and detect faulty connections in the wiring or at thermocouple junctions.

===Metallurgical grades===

While a thermocouple wire type is often described by its chemical composition, the actual aim is to produce a pair of wires that follow a standardized <math>\scriptstyle E(T)</math> curve.

Impurities affect each batch of metal differently, producing variable Seebeck coefficients.
To match the standard behaviour, thermocouple wire manufacturers will deliberately mix in additional impurities to "dope" the alloy, compensating for uncontrolled variations in source material.<ref name=wang>Wang, T. P. (1990) [http://www.dugantech.com/Product_Group-Temperature/Technical%20Articles/TE-Thermocouple%20Materials%20Paper.pdf "Thermocouple Materials"] {{Webarchive|url=https://web.archive.org/web/20140819091318/http://www.dugantech.com/Product_Group-Temperature/Technical%20Articles/TE-Thermocouple%20Materials%20Paper.pdf |date=2014-08-19 }} in ''ASM Handbook'', Vol. 2. {{ISBN|978-0-87170-378-1}}</ref>
As a result, there are standard and specialized grades of thermocouple wire, depending on the level of precision demanded in the thermocouple behaviour.
Precision grades may only be available in matched pairs, where one wire is modified to compensate for deficiencies in the other wire.

A special case of thermocouple wire is known as "extension grade", designed to carry the thermoelectric circuit over a longer distance.
Extension wires follow the stated <math>\scriptstyle E(T)</math> curve but for various reasons they are not designed to be used in extreme environments and so they cannot be used at the sensing junction in some applications.
For example, an extension wire may be in a different form, such as highly flexible with stranded construction and plastic insulation, or be part of a multi-wire cable for carrying many thermocouple circuits.
With expensive noble metal thermocouples, the extension wires may even be made of a completely different, cheaper material that mimics the standard type over a reduced temperature range.<ref name="wang"/>

===Aging ===

Thermocouples are often used at high temperatures and in reactive furnace atmospheres. In this case, the practical lifetime is limited by thermocouple aging. The thermoelectric coefficients of the wires in a thermocouple that is used to measure very high temperatures may change with time, and the measurement voltage accordingly drops. The simple relationship between the temperature difference of the junctions and the measurement voltage is only correct if each wire is homogeneous (uniform in composition). As thermocouples age in a process, their conductors can lose homogeneity due to chemical and metallurgical changes caused by extreme or prolonged exposure to high temperatures. If the aged section of the thermocouple circuit is exposed to a temperature gradient, the measured voltage will differ, resulting in error.

Aged thermocouples are only partly modified; for example, being unaffected in the parts outside the furnace. For this reason, aged thermocouples cannot be taken out of their installed location and recalibrated in a bath or test furnace to determine error. This also explains why error can sometimes be observed when an aged thermocouple is pulled partly out of a furnace—as the sensor is pulled back, aged sections may see exposure to increased temperature gradients from hot to cold as the aged section now passes through the cooler refractory area, contributing significant error to the measurement. Likewise, an aged thermocouple that is pushed deeper into the furnace might sometimes provide a more accurate reading if being pushed further into the furnace causes the temperature gradient to occur only in a fresh section.<ref>{{cite book|author1=Kerlin, T.W.  |author2=Johnson, M.P. |name-list-style=amp |title=Practical Thermocouple Thermometry (2nd Ed.)|year=2012|publisher=ISA|location=Research Triangle Park|isbn=978-1-937560-27-0|pages=110–112|url=http://www.isa.org/Template.cfm?Section=Books3&Template=/Ecommerce/ProductDisplay.cfm&ProductID=12178}}</ref>