Thermocouples are used to measure temperature in order to monitor and/or display the temperature reading. The sensing point of the TC is a junction of two dissimilar metals that produces a small voltage (current) proportional to the temperature of the measured area. By knowing the rate of change of voltage with temperature, the TC can be used to continuously measure temperature.
The simplest thermocouple designs are those made using insulated thermal wires. The usual insulation materials are glass fibers, mineral fibers, PVC, Silicone rubber, PFA, or Ceramic. They must be compatible with the installation requirements, which include chemical resistance, temperature resistance, moisture protection, etc.
A special design of insulated thermocouple wires is mineral insulated thermocouple cables.
Thermocouple types according to EN 60584/IEC 584
The thermocouples described in these standards are generally divided into two groups. The precious metal thermocouples Types S, R, and B, and the base metal thermocouples Types E, J, K, N, and T.
These standardized types are incorporated in many international standards and, relative to their basic thermal voltage values, are compatible. For example, it is possible to use a Type K according to EN 60584 as a Type K according to ANSI-MC 96.1, or even, as a Type K according to JIS C 1602. Only in the deviation limits of the accuracy classes may differences be found. Detailed information for each type is available in the corresponding standard.
Type S (Pt10%Rh-Pt)
Defined Temperature Range -50…1768 °C (-58…3214 °F).
The Type S thermocouple was developed and tested over 100 years ago by H. LeChatelier. These early investigations already indicated that the primary advantages of Type S were the reproducibility of its measurements, its stability, and its applicability to middle-high temperatures. This was the primary reason why it has been selected as the standard thermocouple since 1927 (ITS 27) until the introduction on 1st January 1990 of ITS 90.
The nominal composition of Type S consists of Platinum-10%Rhodium compared against Platinum. The positive conductor (SP) contains 10.00 ± 0.05 % Rhodium. For the alloy, Rhodium with a purity of ≥ 99.98 %, and Platinum with a purity of ≥ 99.99 % should be used. The negative conductor (SN) is made of Platinum with ≥ 99.99 % purity. The Type S thermocouple can be used in a temperature range from -50 °C (-58 °F) almost to the melting point of Platinum at 1769 °C (3216 °F). It should be noted that the output voltages for continuous operation are only stable to about 1300 °C (2372 °F).
The life span of the thermocouple is limited at higher temperatures due to the physical problem of grain growth in the wires. This reduces the mechanical strength, also impurities can diffuse into the wires and thereby change the thermal voltage. The thermocouple is most stable when it is operated in a clean, oxidizing environment (e.g., air), although short-term use in inert, gaseous atmospheres or in a vacuum is possible. Without suitable protection, it should not be used in reducing environments, in metallic or nonmetallic vapors containing, for example, Lead, Zinc, Arsenic, Phosphorous, or Sulphur, or in lightly reducing oxides.
Decisive for the stability at higher temperatures is furthermore the quality of the protection tube and insulation material. Ceramic, in particular Aluminum oxide (Al2O3) with a purity of ≥ 99 %, is best suited for this purpose. Metallic protection tubes should never be used at higher temperatures > 1200 °C (2192 °F).
Type R (Pt13%Rh-Pt)
Defined Temperature Range -50…1768 °C (-58…3214 °F).
At the beginning of the twentieth century, it was noticed that the Type S thermocouples used in the USA and in Europe showed large differences in their thermal voltages among each other. In some temperature ranges differences, up to 5 °C (9 °F) were noted. The reason was that in Europe the Rhodium used for the alloy was contaminated with 0.34 % iron. Since many instruments were already calibrated with these “contaminated Type S “thermocouples, the Type R was developed as a compromise, which has comparable thermal voltages.
The nominal composition of Type R consists of Platinum-13%Rhodium compared against Platinum. The positive conductor (RP) contains 13.00 ± 0.05 % Rhodium. For the alloy Rhodium with a purity of ≥ 99.98 %, and Platinum with a purity of ≥ 99.99 % should be used. The negative conductor (RN) is made of Platinum with ≥ 99.99 % purity.
For the most part of their defined temperature range, Type R thermocouples have a temperature gradient about 12 % higher (Seebeck-Coefficient) than the Type S thermocouples. The remaining material properties are identical to the Type S.
Type B (Pt30%Rh-Pt6%Rh)
Defined Temperature Range 0…1820 °C (32…3308 °F).
The Type B thermocouple was introduced into the market in the fifties by Degussa/Hanau, Germany, and was called PtRh18, a name that is still used in some areas today. It was designed to satisfy the requirements for temperature measurements in the range 1200…1800 °C (2192…3272 °F).
The nominal composition for Type B consists of Platinum-30%Rhodium compared against Platinum-6%Rhodium. The positive conductor (BP) contains 29.60 ± 0.2 % and the negative conductor (BN) 6.12 ± 0.2 % Rhodium. For the alloy Rhodium with a purity of ≥ 99.98 %, and Platinum with a purity of ≥ 99.99 % should be used. They also contain a very small amount of Palladium, Iridium, Iron, and Silicon impurities.
Investigations have shown, that thermocouples, in which both conductors are made of Pt-Rh alloys, are suitable and reliable for measuring high temperatures. They have decided advantages over Types R and S, with regard to improved stability, increased mechanical strength, and higher temperature capabilities. The maximum application temperature range for Type B is essentially limited by the melting point of the Pt6%Rh conductor (BN) at approx. 1820 °C (3308 °F).
A Type B thermocouple can, if handled properly, be operated for a number of hours at temperatures to 1790 °C (3254 °F), and for a few hundred hours at temperatures to 1700 °C (3092 °F), without an appreciable change in the output thermal voltage values. The thermocouple operates most reliably when operated in a clean, oxidizing environment (air), a neutral atmosphere, or in a vacuum. Suitable protection is mandatory if it is to be used in reducing the environment as well as in environments with destructive vapors or other contaminants which might react with the Platinum materials.
The selections of suitable protection tubes and insulation materials are the same as for Type S.
Type J (Fe-CuNi)
Defined Temperature Range -210 …1200 °C (-346…2192 °F).
Because of its relatively steep temperature gradient (Seebeck-Coefficient) and low material costs, Type J, in addition to Type K, is one of the most commonly used industrial thermocouples today.
Nominally, Type J consists of Iron compared against a Copper-Nickel alloy. The positive conductor (JP) is made of commercially available Iron with a purity of approx. 99.5 % with approx. 0.25 % Manganese and approx. 0.12 % Copper, as well as smaller quantities of Carbon, Chromium, Nickel, Phosphorous, Silicon, and Sulphur.
The negative conductor (JN) is made of a Copper-Nickel alloy, which is called Constantan. It should be noted that alloys designated as Constantan which are available commercially, may have a Copper content between 45 % and 60 %. For negative conductor (JN) usually an alloy with approx. 55 % Copper, approx. 45 % Nickel and approx. 0.1 % each of Cobalt, Iron, and Manganese is used.
It should be stressed, JN conductors cannot generally be exchanged with conductors of Types TN or EN, even though all consist of Constantan. Manufacturers of Type J thermocouples usually combine one particular Iron melt with an appropriate Copper-Nickel batch in order to achieve the basic thermal voltage values of Type J.
Since the composition of both conductors (JP and JN) can vary from manufacturer to manufacturer, it is not advisable to use individual conductors from more than one manufacturer, otherwise, the required tolerance classes in some instances may be exceeded.
Although the basic values for Type J are defined in the standard for a temperature range from -210…1200 °C (-346…2192 °F), the thermocouples should only be used in a range of 0…750 °C (32…1382 °F) when operating continuously. For temperatures over 750 °C (1382 °F) the oxidation rate for both conductors increases rapidly.
Further reasons for the restricted temperature range are to find in the special properties of the positive conductor (JP). Since Iron rusts in damp environments and becomes brittle, it is not advisable to operate Type J thermocouples at temperatures below 0 °C (32 °F) without suitable protection. In addition, Iron experiences a magnetic change at 769 °C (1462 °F) (Curie point) and at approx. 910 °C (1670 °F) an Alpha-Gamma crystal structure change occurs.
Both effects, particularly the latter, have a significant influence on the thermoelectric properties of the Iron and therefore on the Type J thermocouple. Should a Type J be operated above 910 °C (1670 °F), the output thermal voltages will change appreciably, especially when cooled quickly to lower temperatures.
In the temperature range 0…760 °C (32…1400 °F) the Type J can be used in vacuum, oxidizing, reducing, or inert atmospheres. In Sulphur containing environments, suitable protection should be employed at temperatures above 500 °C (932 °F).
Type K (NiCr-NiAl)
Defined Temperature Range -270…1372 °C (-454…2501 °F).
Since this thermocouple type for middle temperatures is more resistant against oxidation than Types J and E, it is used in many applications today for temperatures over 500 °C (932 °F). Nominally, the thermocouple contains a Nickel-Chromium alloy compared against a Nickel-Aluminum alloy. The positive conductor (KP) is identical to the material of Type E positive conductor and consists of 89 to 90 % Nickel, 9 to 9.5 % Chromium, approx. 0.5 % Silicon, approx. 0.5 % Iron and smaller amounts of Carbon, Manganese, and Cobalt. The negative conductor (KN) contains 95 to 96 % Nickel, 1 to 2.3 % Aluminum, 1 to 1.5 % Silicon, 1.6 to 3.2 % Magnesium, approx. 0.5 % Cobalt, as well as minimal traces of Iron, Copper, and Lead.
The basic values for Type K thermocouples are defined for the range from -270… 1372 °C (-454…2501 °F). It should be noted that at temperatures over 750 °C (1382 °F) the oxidation rate in the air for both conductors increases sharply. Also, it should not be installed without suitable protection at higher temperatures in Sulphur containing, reducing, or alternately oxidizing and reducing atmospheres.
There are also effects to be considered here which drastically change the output thermal voltages.
If a Type K is exposed for longer periods of time to higher temperatures in a vacuum, then the Chromium volatilizes out of the alloy of the KP conductor (“vacuum sensitivity “). If on the other hand, a smaller, but not negligible amount of oxygen or steam is present at the thermocouple, the KP conductor may be subjected to the so-called “green rot “. In these situations, the oxidation attacks only the easier to oxidize Chromium without oxidizing the Nickel. At temperatures between 800 °C and 1050 °C (1472…1922 °F), this is most severe. “Green rot“and “vacuum sensitivity“ produce irreversible effects on the composition of the conductor and thereby on the thermal voltage. Erroneous measurements of more than 100 °C (212 °F) are possible!
In addition, a magnetic change in the Nickel leg KN occurs at 353 °C (667 °F) (Curie point). The Nickel-Chromium alloy of the KP-conductor in the range from 400…600 °C (752…1112 °F) changes from an ordered to an unordered atomic distribution state, the so-called “K-Condition “. If a Type K is operated at temperatures over 600 °C (1112 °F) and subsequently cooled too quickly, these changes may not be reversible and can change the output thermal voltages by up to 5 °C (9 °F).
Both effects are reversible since they can be restored to their original condition by heating to over 600 °C (1112 °F) and then slowly cooling.
Type N (NiCrSi-NiSi)
Defined Temperature Range -270…1300 °C (-454…2372 °F).
Type N is the newest thermocouple defined in this standard. It was developed at the end of the sixties and offers distinct advantages due to its higher thermoelectric stability at temperatures over 870 °C (1598 °F) and less tendency to oxidize compared against thermocouples Types J, K, and E.
Nominally, the thermocouple consists of a Nickel-Chromium-Silicon alloy compared against a Nickel-Silicon alloy. The positive conductor (NP) contains approx. 84 % Nickel, 13.7 to 14.7 % Chromium, 1.2 to 1.6 % Silicon, <0.15 % Iron, <0.05 % Carbon, <0.01 % Magnesium, as well as minimal traces of Cobalt. The negative conductor (NN) contains approx. 95 % Nickel, 4.2 to 4.6 % Silicon, 0.05 to 0.2 % Magnesium, <0.15 % Iron, <0.05 % Carbon, as well as small amounts of Manganese and Cobalt. These conductors are also known by their trade names Nicrosil (NP) and Nisil (NN).
Of all the base metal thermocouples, Type N is best suited for applications with oxidizing, damp, or inert atmospheres. As a result of its relatively high Silicon content, oxidation occurs on the surface of the conductor. Tightly adhering and protective oxides are formed which minimize further corrosion.
In reducing atmospheres or air in the range of 870…1180 °C (1598…2156 °F) the thermocouple exhibits a decidedly higher thermoelectric stability than a Type K thermocouple under the same conditions. Also, the “K-State“which occurs in Type K is almost completely suppressed due to the Silicon content. At higher temperatures in Sulphur containing, reducing or alternately oxidizing and reducing atmospheres suitable protection is still necessary.
The “Green rot “and “vacuum sensitivity “phenomena described for the Type K thermocouple do also occur in the Type N, where, however, both the Chromium and the Silicon volatilize in a vacuum.
Attention: Type K and N cannot be exchanged for each other!
Type T (Cu-CuNi)
Defined Temperature Range -270…400 °C (-454…752 °F).
This is one of the oldest thermocouples for low-temperature measurements and is still commonly used in the triple point range for Neon at -248.5939 °C (-415.4690 °F) up to 370 °C (698 °F).
Type T nominally contains Copper compared against a Copper-Nickel alloy. The positive conductor (TP) consists of approx. 99.95 % pure Copper with an Oxygen content of 0.02 to 0.07 % dependent on the Sulphur content of the Copper. The remaining impurities amount to approx. 0.01 % in total. The negative conductor (TN) consists of a Copper-Nickel alloy, also called Constantan with approx. 55 % Copper and 45 % Nickel, as well as approx. 0.1 % each of Cobalt, Iron, and Manganese. The TN conductor is identical to and can be interchanged with an EN conductor. It is, however, generally not identical to Type JN conductors.
The Type T thermocouple exhibits good thermoelectric homogeneity. Due to the good heat conductivity of the conductors, problems can occur when used for precision measurements, resulting from heat abstraction, particularly if the conductor diameter is very large. Type T can be used in vacuum, oxidizing, reducing, or inert atmospheres.
It should be noted that above 370 °C (698 °F) the oxidation rate of the TP-conductor increases dramatically. It is not recommended to use the thermocouple in hydrogen-containing environments above 370 °C (698 °F) without suitable protection, because the TP-conductor could become brittle.
Type E (NiCr-CuNi)
Defined Temperature Range -270…1000 °C (-454…1832 °F).
The thermocouple has a relatively small heat conductivity, very high resistance in humid atmospheres, good homogeneity, and a relative steep temperature gradient (Seebeck-Coefficient) at extremely low temperatures. For these reasons, it has become the most common thermocouple for low-temperature measurements. Above 0 °C (32 °F) it has the steepest temperature gradient of all the thermocouples defined in the standard.
Type E nominally consists of a Nickel-Chromium alloy compared against a Copper-Nickel alloy. The materials of the positive conductor (EP) are identical to those already described for the KP-conductor in Type K, and the negative conductor (EN) is the same as the TN-conductor in Type T. The Type E thermocouple can be used in a temperature range from -270…1000 °C (-454…1832 °F). For temperatures over 750 °C
(1382 °F) the oxidation rate in the air for both conductors is high. Since the EP-conductor is identical to the KP-conductor, the same effects of “vacuum sensitivity “, “K-State “and “Green rot “already described are also applicable to this thermocouple.
Type E is essentially insensitive to oxidizing or inert atmospheres. In Sulphur containing, reducing, or alternately oxidizing and reducing atmospheres suitable protection is still necessary.
Thermocouples according to DIN 43710
The thermocouples Type U (Cu-CuNi) and Type L (Fe-CuNi) defined in this standard are no longer included in any current national or international standards. This has not precluded the continued use of these thermocouples in many applications. They were not included in EN 60584 or IEC 584 but replaced by the Types J and T.
DIN 43710 recommends that these thermocouples should not be used for any new applications and if existing installations are updated or reworked, the thermocouples should be replaced by Types J and T.
Attention: They cannot simply be exchanged for one another!
Type U (Cu-CuNi)
Defined Temperature Range -200…600 °C (-328…1112 °F).
Type U nominally consists of copper compared against a Copper-Nickel alloy. The positive conductor (UP) is made of the same Copper composition as the positive conductor described for Type T earlier in this section. The negative conductor (UN) is made of a Copper-Nickel alloy (Constantan) with approx. 55 % Copper, approx. 44 % Nickel and approx. 1 % Manganese.
As a result of these very small differences in their compositions, the basic values for the thermal voltages for Type U are different from those for Type T. The remaining material properties are however essentially the same as those for Type T.
Type L (Fe-CuNi)
Defined Temperature Range -200…900 °C (-328…1652 °F).
Type L nominally consists of Iron compared against a Copper-Nickel alloy. The positive conductor (LP) is made of the same Iron composition as the positive conductor of Type J. The negative conductor (LN) is made of the same Copper-Nickel alloy (Constantan) as the negative conductor of Type U. Therefore, the basic values for the thermal voltages for Type L are different from those for Type J. The remaining material properties are however essentially the same as those for Type J.
In addition to the standardized thermocouples, there is a whole set of non-standard thermocouples for special applications, whose basic values are not included in any current standard. The basic values for these thermocouples must be established by the manufacturer using individual calibrations.
The most well-known include:
Iridium-Iridium rhodium (Ir-Ir40%Rh)
For laboratory measurements in neutral or weak oxidizing atmospheres at temperatures to 2000 °C (3632 °F). The thermocouple consists of very brittle cold-rolled steel wires which may not be bent. They are insulated using capillary tubes made of pure Aluminum oxide (Al2O3). The thermal voltage is approx. 10 mV at 2000 °C (3632 °F).
Tungsten-Tungsten Rhenium (W-W26%Rh), Tungsten Rhenium-Tungsten Rhenium (W5%Rh-W26%Rh) and Tungsten Rhenium-Tungsten Rhenium (W3%Rh-W25%Rh)
These thermocouples, identified in the USA by the letters “G“, “C“, and “D“, are designed for use in high vacuums and for inert gases to 2320 °C (4200 °F). The thermal voltage is at 2320 °C (4208 °F) for W-W26%Rh approx. 38.6 mV, for W5%Rh-W26%Rh approx. 37.1 mV and for W3%Rh-W25%Rh approx. 39.5 mV.
This thermocouple can be used to 1200 °C (2192 °F) in the air but is not suitable for environments containing Silicon or Carbon. It combines the stability of precious metal with the high thermal voltages of a base metal thermocouple. The thermal voltage is approx. 55.4 mV at 1200 °C (2192 °F).
Gold Iron-Chromium (AuFe-Cr)
his thermocouple is used primarily for low-temperature measurements in a range from -270…-200 °C (-454…-328 °F). At -270 °C (-454 °F) the thermal voltage is approx. 4.7 mV.
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