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Thermocouple Material Types

Suitability of Thermocouple Material Types

Most conducting materials can produce a thermoelectric output. However, when considerations like width of the temperature range, actual useful signal output, linearity and repeatability (the unambiguous relationship of output to temperature), are taken into account, there is a somewhat restricted sensible choice. Material selections have been the subject of considerable work over several decades, on the part of suppliers, the main calibration and qualifying laboratories and academia. So, the range of temperatures covered by usable metals and alloys, in both wire and complete sensor form, now extends from -270°C to 2,600°C.

Naturally, the full range cannot be covered by just one thermocouple junction combination. There are internationally recognised type designations, each claiming some special virtue. The International standard IEC 60584 refers to the standardised thermocouples (these are described by letter designation - the system originally proposed by the Instrument Society of America.

Rare and Base Metals Thermocouples

In general, these are divided into two main categories: rare metal types (typically, platinum vs platinum rhodium) and base metal types (such as nickel chromium vs nickel aluminium and iron vs copper nickel (Constantan)). Platinum-based thermocouples tend to be the most stable, but they’re also the most expensive. They have a useful temperature range from ambient to around 2,000°C, and short term, much greater (-270°C to 3,000°C). The range for the base metal types is more restricted, typically from 0 to 1,200°C, although again wider for non-continuous exposure. However, signal outputs for rare metal types are small compared with those from base metal types.

Temperature Related Instability


Another issue here is the inherent thermoelectric instability of the base metal thermocouple, Type K, with both time and temperature (although Types E, J and T have also come in for some criticism). Hence the interest in Type N thermocouples (Nicrosil vs Nisil), with the best of the rare metal characteristics at base metal prices, with base metal signal levels and a slightly extended base metal temperature range.

N Type Thermocouple

Instability and Long Term Drift

Instabilities come in a number of forms. First, there is long term drift with exposure to high temperatures, mainly due to compositional changes caused by oxidation - or neutron bombardment in nuclear applications. In the former case, above 800°C oxidation effects on Type K thermocouples in air, for example, can cause changes in conductor homogeneity, leading to errors of several percent. Then again, where the devices are mounted in sheaths with limited air volume, the `green rot’ phenomenon can be encountered - due to preferential oxidation of the chromium content. Meanwhile, with nuclear bombardment there is the problem of transmutation - leading to similar effects. 

Hysterisis


Second, there are short term cyclic changes in the thermal emfs (hysteresis) generated on heating and cooling base metal thermocouples, again notably Type K in the 250°C to 600°C range, causes being both magnetic and structural inhomogeneities. Errors of about 5°C and more are common in this temperature range, peaking at around 400°C. Third, with mineral insulated thermocouple assemblies there can be time-related emf variations due to composition-dependent and magnetic effects in temperature ranges depending on the materials themselves. This is due essentially to transmutation of the high vapor pressure elements (mainly manganese and aluminum) from the K negative wire through the magnesium oxide insulant to the K positive wire. Again, the compositional change results in a shifting thermal emf.

Type N Material


Type N materials obviate or dramatically reduce these instabilities because of the detailed structure of the alloys engineered for this novel thermocouple. This applies to time, temperature cycling hysteresis, magnetic and nuclear effects. Basically, oxidation resistance is superior because of the combination of a higher level of chromium and silicon in the NP (Nicrosil) conductor, and a higher level of silicon and magnesium in the NN (Nisil) conductor, forming a diffusion barrier. Hence, there is much better long term drift resistance.

Then again, the absence of manganese, aluminum and copper in the NN conductor increases the stability of Type N against its base metal competitors in nuclear applications. As for the transmutation problem in mineral insulated assemblies, this too is virtually eliminated since the two Type N conductors both contain only traces of manganese and aluminum.

Temperature Cycling

Looking at the temperature cycling hysteresis instabilities, these are also dramatically reduced due to the high level of chromium in the NP conductor and silicon in the NN conductor. In fact, the cycling spread is between 200°C and 1,000°C with a peak around 750°C - and figures of around 2°C to 3°C maximum excursion are quoted.

Thermocouple Selection

As for selection of a particular thermocouple type for a sensing application, physical conditions, duration of exposure, sensor lifetime and accuracy all have to be considered. Additionally, in the case of base metal types, there are the further criteria of sensitivity and compatibility with existing measuring equipment.


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