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Drift in Type K Bare-Wire Thermocouples from Different Manufacturers

DOI:10.1007/s10765-017-2210-1
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Emile Stefan Webster at Robinson Research Institute
Emile Stefan Webster
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Abstract and Figures

Base-metal thermocouples play a significant role in industrial measurements, and among the many varieties and formats, bare-wire Type K is often preferred. The reason for this preference is its low cost, durability and tolerance of high temperature. Unfortunately, Type K, like all base-metal thermocouples, is made based on a temperature-to-emf relationship and not on a specific metallurgical formulation. The original Hoskins Chromel/Alumel couple was simple in composition, and it had known thermal drift characteristics associated with reversible crystallographic changes and irreversible oxidation, and these two drift mechanisms led to large instabilities in use. To improve the stability of Type K, most modern manufacturers now adopt compositions with alterations that can depart significantly from those of the original formulation. These alterations are usually made to improve the stability and/or manufacturing processing of their wire. So, although the wire is made to meet the limits of error and tables, this is only true at the time of manufacture. As soon as the wire is exposed to temperatures above \(150~{^{\circ }}\hbox {C}\), the supplier-dependent alloys can exhibit a wide range of drift behaviors that depend on composition and even the batch of the wire. This study investigates the change in Seebeck coefficient as a function of temperature for Type K bare-wires from different suppliers by using a linear-gradient furnace and a high-resolution homogeneity scanner. Wires were exposed to temperatures over the range \({\sim }20~{^{\circ }}\hbox {C}\) to \(950~{^{\circ }}\hbox {C}\) for time periods between 24 h and 500 h. The results show that most wires have very different drift behaviors, which the end user could not realistically predict or correct.
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Int J Thermophys (2017) 38:70
DOI 10.1007/s10765-017-2210-1
TEMPMEKO 2016
Drift in Type K Bare-Wire Thermocouples from
Different Manufacturers
E. S. Webster1
Received: 22 June 2016 / Accepted: 6 March 2017 / Published online: 14 March 2017
© Springer Science+Business Media New York 2017
Abstract Base-metal thermocouples play a significant role in industrial measure-
ments, and among the many varieties and formats, bare-wire Type K is often preferred.
The reason for this preference is its low cost, durability and tolerance of high tem-
perature. Unfortunately, Type K, like all base-metal thermocouples, is made based
on a temperature-to-emf relationship and not on a specific metallurgical formulation.
The original Hoskins Chromel/Alumel couple was simple in composition, and it had
known thermal drift characteristics associated with reversible crystallographic changes
and irreversible oxidation, and these two drift mechanisms led to large instabilities
in use. To improve the stability of Type K, most modern manufacturers now adopt
compositions with alterations that can depart significantly from those of the original
formulation. These alterations are usually made to improve the stability and/or man-
ufacturing processing of their wire. So, although the wire is made to meet the limits
of error and tables, this is only true at the time of manufacture. As soon as the wire
is exposed to temperatures above 150 ◦C, the supplier-dependent alloys can exhibit
a wide range of drift behaviors that depend on composition and even the batch of
the wire. This study investigates the change in Seebeck coefficient as a function of
temperature for Type K bare-wires from different suppliers by using a linear-gradient
furnace and a high-resolution homogeneity scanner. Wires were exposed to tempera-
tures over the range ∼20 ◦C to 950 ◦C for time periods between 24 h and 500 h. The
results show that most wires have very different drift behaviors, which the end user
could not realistically predict or correct.
Selected papers of the 13th international symposium on temperature, humidity, moisture and thermal
measurements in industry and science.
BE. S. Webster
emile.webster@callaghaninnovation.govt.nz
1Measurement Standards Laboratory, PO Box 31310, Lower Hutt 5040, New Zealand
123
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
... However, not all commercial formulations of Type N achieve this aim, and some are worse than typical Type K formulations (Webster 2017a). Similarly, many Type K formulations appear to have been designed for best performance in the mineral-insulated metal-sheathed (MIMS) format, and consequently may perform poorly as bare-wire thermocouples in air at high temperatures (Webster 2017d). ...
... The negative thermoelement (Alumel) usually exhibits a magnetic phase transition somewhere between room temperature and about 250 °C, which may cause deviations from the reference function between −3 °C and +1 °C, depending on wire composition (Burley et al. 1982). Alumel also oxidises rapidly in air above 700 °C (Bentley 1998a, Webster 2017d). ...
... In many cases, this will be close to its as-received state. For example, heating Type K thermocouples to 650 °C for as little as a few minutes erases any ordering effects, and the temperature is high enough to remove most cold work and low enough to avoid significant vacancy effects (Webster 2017d. ...
Guide to Secondary Thermometry: Thermocouple Thermometry Part I: General Usage
Technical Report
Full-text available
  • Sep 2021
The Guides on Secondary Thermometry are prepared by the Consultative Committee for Thermometry to provide advice on good measurement practice and making temperature measurements traceable to the International Temperature Scale of 1990. This guide is the first of two guides collating information and advice on thermocouple thermometry. This first guide, Part I, focuses on aspects of general usage: the principles of operation, a summary of common thermocouple types, performance characteristics and service conditions, sources of error, and guidance on the construction, installation and maintenance of thermocouples. Part II discusses reference thermocouples and thermocouple calibration.
... For example, in the two recent studies [17,23] investigating modern Types K and N thermocouple alloys, numerous chemical, structural and performance changes were identified as a function of temperature and exposure time. These two studies also confirmed through energy-dispersive X-ray spectroscopy (EDS) that the key compositional components of Types K and N have not changed greatly from the time of their development. ...
... The most probable cause of this discrepancy is a reduction in the quality of thermoelement alloys and is suspected to be linked to degradation in manufacturing capability. In the complementary study [23] investigating modern formulations of bare-wire Type K, a reduced performance at high temperatures was found in the K+ thermoelements and was linked to inadequate passivation properties. These failures may be related to the wires being preferentially developed for the MIMS format (where oxidation resistance is not an important consideration). ...
... In the two studies focusing on Types K and N [17,23], it was shown that current users of these thermocouples have no easy way of knowing how inhomogeneous or susceptible to temperature-induced changes their thermocouples will be. This variability in performance between thermocouples from different manufacturers and even between batches of the same nominal type and format, makes much of the published in situ data of limited utility. ...
Base-metal thermocouple tolerances and their utility in real-world measurements
Article
Full-text available
  • Feb 2021
The reference functions and tables for base-metal thermocouples, which relate temperature and electro-motive-force, are specified by thermocouple type, rather than by alloy composition. The common thermocouple types—E, J, K, N, and T—are required to meet the specifications within defined tolerance bands; referred to as either limits-of-error (ASTM, US) or class (IEC, UK and Europe). In this requirement hides a fundamental problem: the tolerances are a manufacturing specification. Despite this, the tolerances are often used as a proxy for expected behaviour. However, these thermocouple types are subject to rapid and irreversible changes after short exposure times to modest temperatures. Thus, tolerance statements are poor predictors of in-use behaviour. This paper initially reviews the development of the base-metal thermocouples before identifying many of the metallurgical processes that lead to departures from the accepted tolerance bands. Focus is specifically given to the processes that occur in several type K and N thermocouples, which exemplify the changes common in the other base-metal thermocouples. From these results, it is shown that the use of traditional calibration techniques can be a futile exercise. Comparisons are then made between as-received thermocouples and those given thermal treatments, that have been designed to allow meaningful calibration while also inhibiting drift. These results show that thermal treatments and calibration are a practicable means to minimise drift over typical periods of use while offering small uncertainties, well within the span of manufacturing tolerances. Lastly it is suggested manufacturers give more information on the testing used to establish conformance with tables and reference functions and that they follow standardised testing procedures.
... For Type K, these changes can be between 2 °C to 6 °C when the temperature is cycled or the immersion is changed [16,17]. In addition, it is now known that the magnitude of these changes is highly sensitive to minor compositional variations and the initial anneal state [18,19]. ...
... Consequently, the fundamental measurement data used for the reference tables and functions is now between 50 and 70 years old. This is a problem, as wire producers are now making wire that often deviates substantially from that described in the ASTM and IEC standards, either at the time of manufacture or after short usage times [19,22]. ...
... This ordering behaviour has been observed to occur at temperatures below ~600 °C in Pt/Rh10% (Type S+), Pt/Rh13% (Type R+), Constantan (Types T−, J− and E−), Chromel (Type K+), Nisil (Type N−) and below ~800 °C in Nicrosil (Type N+). Ordering effects in Alumel (Type K−) have been shown to be almost insignificant [19]. Figure 1 is provided to give the reader a sense of the magnitude and temperature range over which these SRO effects have been observed in a 24 h period. ...
A critical review of the common thermocouple reference functions
Article
Full-text available
  • Jan 2021
The use of thermocouples in many present-day applications can often occur with little consideration as to the inherited historical burden of the reference functions the thermocouples must meet. For base-metal thermocouples, the reference functions are specified by equations relating temperature to electro-motive-force and not by alloy composition. Most of the common thermocouples contain at least one alloyed thermoelement, the bulk of which are now known to be inherently unstable above 200 °C. As manufacturing technologies change, along with the material feedstock from which thermocouples are made, modern thermocouples can frequently give measurements that deviate significantly from the ASTM and IEC standards. This study first reviews the development of the thermocouple alloys and historical conditions under which the reference functions were derived and contrasts this with modern thermocouple alloys and new testing methods. From this comparison, it is shown that users of modern base-metal thermocouples need to be extremely cautious when anticipating likely behaviour, with even short exposures to modest temperatures revealing a myriad of manufacturer-dependent instabilities. Minor variations in composition are shown to strongly influence reversible crystallographic ordering effects in addition to passivation behaviour at high temperatures, in some instances leading to catastrophic failure. It is also shown that the initial anneal state given by the manufacturer has a significant effect on the stability and hence, drift rate, with inadequate anneal leading to unnecessarily large drift rates at less than 200 °C. Lastly, this review looks at recent attempts to develop more-stable thermocouples, based on state-of-the-art techniques able to identify specific causes of instability in many of the historic thermocouple alloys and demonstrates how these new thermocouples might better serve the end user’s needs.
... Table 3 clarified that K-is more sensitive to oxidation 3 times than K+. The total thermal exposure of type K was only 600 hours because one of its thermoelements (may be K-) was cut after 600 hours and this may be due to destructive corrosion at about 850 •C, where wires started failing at only 800 •C [12] and also embrittlement along grain boundaries which is a result of thermocycling-Alumel embrittles at lower temperatures than Chromel [13]. The decreasing of Cr to about 9% is in accordance with KRIŪKIENĖ et al., [14] which enhance the oxidation rate in K+. ...
... The decreasing of Cr to about 9% is in accordance with KRIŪKIENĖ et al., [14] which enhance the oxidation rate in K+. As a recommendation, 0.5 mm Type K should not be used above 750 °C as suggested in the ASTM manual (used at a temperature well below the 870 •C) as well as over 700 °C, the user should ensure sufficient clean air to avoid green rot [12]. Figure 2 exhibits the changes in its microstructure as received and after 300 hours and 1200 hours of thermal exposure at 1050 ͦ C. Table 4 shows the EDX analysis of positive (N+) and negative (N-) thermoelements of type-N thermocouple as received and after different thermal exposure periods at 1050 ͦ C in weight percent (%). ...
Characterizing Drift Behavior in Type K and N Thermocouples After High Temperature Thermal Exposures
Article
Full-text available
  • Aug 2022
Although of the widespread use of base metal thermocouples in the industry, many previous relevant researches have shown that the accuracy and stability of thermocouples are clearly influenced by any physical or chemical changes in their thermoelements. Among the most important of these changes are the inhomogeneity, pollution, oxidation and microstructure changes of the thermoelements, all of these changes and more leads to thermocouples drift after a prolonged thermal exposure. To study how these changes affect the drift and thermoelectric properties of thermocouples, in this work we subjected the base metal thermocouples of types K and N to successive thermal exposure periods at their maximum temperatures. Scanning electron microscopy (SEM) and Energy Dispersive X-ray (EDX) systems were used to monitor the change in the crystal structure and chemical composition of the thermocouple wires after each stage of the thermal heating, and then we studied the changes in the thermoelectric properties of thermocouple wires. The results showed type N thermocouples are more stable at high temperatures (up to 1050 ͦ C), even if used for long periods (for more than 1200 hours) at those temperatures, but K type thermocouples showed a rapid drift with first exposure to high temperatures and completely failed after 600 hours due to devastating corrosion.
... The common base-metal thermocouples employ at least one thermoelement containing a Ni alloy: Ni-Cr in Types K and N, and Ni-Cu in Types E, J, K and T. During use, these Ni alloys display a range of behaviors due to cold-working, metallic structure changes, chemical diffusion and oxidation, all of which are discussed in detail elsewhere [1,[6][7][8][9][10]. These localized variations in the Seebeck coefficient, or inhomogeneity, are often observed as drift [4]. ...
... Although Si (atomic number 14) is soluble in Ni (atomic number 28), the two elements have significantly different atomic radii and electronic configuration, and when combined with the low concentration of Si, the formation of regular lattice structures seems physically improbable. Atomic SRO effects are best exemplified in the positive Type K thermoelement (K+) [10,33], an alloy of roughly 90 % Ni and 10 % Cr (atomic number 24). Unlike Si, Cr is more mobile in Ni for the very reasons that Si is not. ...
A Performance Assessment of Current Formulations of Bare-Wire and Mineral-Insulated-Metal-Sheathed Type N Thermocouples
Article
Full-text available
The Type N thermocouple, at its introduction in the early 1980s, was intended to radically improve base-metal thermocouple measurements and would render other base-metal types obsolete, or so it was claimed. Almost 40 years on, Type K persists in being the thermocouple of choice, despite adequate opportunity for the uptake of Type N. The reasons for this may be many; however, recent research at is showing Type N, at least at low temperatures, is not nearly as stable as early claims made out. This study reports on the inhomogeneities in Type N thermoelements, which develop as a function of temperature and time in a selection of mineral-insulated-metal-sheath (MIMS) and bare-wire samples sourced from a range of manufacturers. Measurements were made using a linear-gradient furnace and high-resolution homogeneity scanner. It was found that Type N thermocouples, in both the bare-wire and MIMS format, are susceptible to significant deviations in Seebeck coefficient from the reference functions at temperatures between 100 °C and 950 °C. In fact, use at temperatures below 500 °C can result in measurement errors equal to or worse than for equivalently specified Type K. Consequently, the benefits of using as-supplied Type N, in terms of accuracy and longevity, are only fully realized at temperatures greater than 900 °C. Despite these findings, additional experiments revealed drift rates can be reduced by about a factor of four if thermal preconditioning between 600 °C and 900 °C is used on MIMS Type N, which is similar to the improvements seen for thermally preconditioned MIMS Type K.
... For simulation, the thermocouple in the reagent is assumed to be at the same distance from the exchange site. In this case, the initial value for T AI sim was taken and T AI eksp was changed until the experimentally measured temperature in the air was equal [16][17][18][19] . ...
Mathematical Study of Temperature Measurement by Thermocouple in Chemical Processes
Article
Full-text available
  • Jun 2022
Processing of oil, gas and gas condensate is one of the most widespread areas of modern chemical-technological processes. Due to the safety of chemical and technological process management, strict requirements have been set for the quality of processes in the oil and gas refining and petrochemical industries. During chemical-technological processes in the oil and gas industry, the fractional composition of the product, the typical boiling point, the octane number of gasoline, the cetane number of diesel fuel, the evaporation temperature, the ignition temperature and other parameters must be determined with special accuracy. This set of characteristics is used in the laboratory assessment of the quality of oil, oil products and gas condensate. Temperature measurements performed during such processes must have maximum accuracy. For this reason, the determination of temperature by thermocouples in the above-mentioned chemical processes has been mathematically studied and evaluated during the research.
... The effect of temperature on the drift rate for MI thermocouples is shown in Fig. 3 and is considered for all thermocouple formats in Fig. 4 at 1000 °C, 1100 °C and 1200 °C. It can be seen that as the temperature increases, the reported drift rates initially within the ± 0.06 °C h −1 range become more dispersed and the drift rate becomes progressively more negative, with increasing magnitude [9,[26][27][28][29][30][31][32][33][34][35][36][37][38][39]. Between temperatures of 200 °C and 1000 °C, the drift rate broadly tends to be positive and (with a few exceptions) in the ± 0.025 °C h −1 range. ...
A Comprehensive Survey of Reported Thermocouple Drift Rates Since 1972
Article
Full-text available
Thermocouples are widely used in industry, but a commonly encountered difficulty is the thermoelectric drift which causes an unknown change to the thermovoltage over time. A comprehensive survey of the published information on drift rates for Types B, C, D, K, N, R, S, Au/Pt, and Pt/Pd thermocouples is presented. The data that are available for thermocouple drift rates are reviewed for each major thermocouple type, and any large gaps in the reported data are stated. The effects of different parameters on the drift rate are summarized, with a particular focus on what happens at different temperatures. The effects of temperature, thermoelement diameter, sheath and/or insulation type, atmosphere, and thermocycling on the drift rate are considered (where data are available). This will allow users to assess the likely magnitude of the drift rate of different thermocouples for a given application.
... Both issues can lead to faulty surface temperature measurements. An extensive amount of literature on the biases and problems associated with Type K thermocouples has been produced [52][53][54][55][56]. Therefore, an upgrade to a Type N thermocouple is projected for the HAIRL to avoid these systematic errors. ...
Updated measurement method and uncertainty budget for direct emissivity measurements at the University of the Basque Country
Article
  • Apr 2020
This work reports on the upgrades made to the direct emissivity measurement facility of the University of the Basque Country (UPV/EHU). The instrumental improvements consist of, among others, a high-vacuum system and a wider temperature range (300−1273 K). Methodological developments include a refined measurement equation with updated parameters and a reworked ISO-compliant uncertainty budget, and a Monte Carlo procedure for accurate calculations of total emissivities from spec-tral data. These upgrades have been demonstrated and validated in measurements of both metallic and ceramic materials. The results obtained in this work are applicable to similar experimental devices for emissivity measurements in order to report reliable emissivity data.
... Both issues can lead to faulty surface temperature measurements. An extensive literature on the biases and problems associated with Type K thermocouples has been produced [52][53][54][55][56]. Therefore, an upgrade to a Type N thermocouple is projected for the HAIRL to avoid these systematic errors. ...
Updated measurement method and uncertainty budget for direct emissivity measurements at UPV/EHU
Preprint
Full-text available
  • Oct 2019
This work reports on the upgrades made to the direct emissivity measurement facility of the UPV/EHU. The instrumental improvements consist of, among others, a high-vacuum system and wider temperature range (300-1273 K). Methodological developments include a refined measurement equation with updated parameters and a reworked ISO-compliant uncertainty budget, and a Monte Carlo procedure for accurate calculations of total emissivities from spectral data. These upgrades have been demonstrated and validated in measurements of both metallic and ceramic materials. The results obtained in this work are applicable to similar experimental devices for emissivity measurements in order to report reliable emissivity data.
Characterization of Tool–Chip Interface Temperature Measurement With Thermocouple Fabricated Directly on the Rake Face
Article
The objective of this work is to fabricate thermocouples directly on the rake face of a commercially available tungsten carbide cutting insert for accurately measuring the tool–chip interface temperature during metal cutting. The thermocouples are sputtered onto the cutting insert using micromachined stencils, are electrically isolated with layers of Al2O3, and receive a top coating of AlTiN for durability. The result is a nonsacrificial thermocouple junction that is approximately 1.3 µm below the rake face of the tool and 30 µm from the cutting edge. Experimental and numerical characterization of the temperature measurement accuracy and response time are presented. The instrumented cutting tool can capture the tool–chip interface temperature transients at frequencies of up to 1 MHz, which enables the observation of serrated chip formation and adiabatic shear events. Temperature measurements from oblique machining of 4140 steel are presented and compared with three-dimensional, transient numerical simulations using finite element analysis, where cutting speed and feed are varied. This method of measuring the tool–chip interface temperature shows promise for future research and smart manufacturing applications.
Thermal Preconditioning of MIMS Type K Thermocouples to Reduce Drift
Article
Full-text available
Type K thermocouples are the most widely used temperature sensors in industry and are often used in the convenient mineral-insulated metal-sheathed (MIMS) format. The MIMS format provides almost total immunity to oxide-related drift in the 800 \({^{\circ }}\hbox {C}\)–1000 \({^{\circ }}\hbox {C}\) range. However, crystalline ordering of the atomic structure causes drift in the range 200 \({^{\circ }}\hbox {C}\)–600 \({^{\circ }}\hbox {C}\). Troublesomely, the effects of this ordering are reversible, leading to hysteresis in some applications. Typically, MIMS cable is subjected to a post-manufacturing high-temperature recrystallization anneal to remove cold-work and place the thermocouple in a ‘known state.’ However, variations in the temperatures and times of these exposures can lead to variations in the ‘as-received state.’ This study gives guidelines on the best thermal preconditioning of 3 mm MIMS Type K thermocouples in order to minimize drift and achieve the most reproducible temperature measurements. Experimental results demonstrate the consequences of using Type K MIMS thermocouples in different states, including the as-received state, after a high-temperature recrystallization anneal and after preconditioning anneals at 200 \({^{\circ }}\hbox {C}\), 300 \({^{\circ }}\hbox {C}\), 400 \({^{\circ }}\hbox {C}\) and 500 \({^{\circ }}\hbox {C}\). It is also shown that meaningful calibration is possible with the use of regular preconditioning anneals.
Thermocouple homogeneity scanning
Article
Full-text available
  • Feb 2015
The inhomogeneities within a thermocouple influence the measured temperature and contribute the largest component to uncertainty. Currently there is no accepted best practice for measuring the inhomogeneities or for forecasting their effects on real-world measurements. The aim of this paper is to provide guidance on the design and performance assessment of thermocouple inhomogeneity scanners by characterizing the qualitative performance of the various designs reported in the literature, and developing a quantitative measure of scanner resolution. Numerical simulations incorporating Fourier transforms and convolutions are used to gauge the levels of attenuation and distortion present in single- and double-gradient scanners. Single-gradient scanners are found to be far superior to double-gradient scanners, which are unsuitable for quantitative measurements due to their blindness to inhomogeneities at many spatial frequencies and severe attenuation of signals at other frequencies. It is recommended that the standard deviation of the temperature gradient within the scanner is used as a measure of the scanner resolution and spatial bandwidth. Recommendations for the design of scanners are presented, and include advice on the basic design of scanners, the media employed, operating temperature, scan rates, construction of survey probes, data processing, gradient symmetry, and the spatial resolution required for research and calibration applications.
Measurement of Inhomogeneities in MIMS Thermocouples Using a Linear-Gradient Furnace and Dual Heat-Pipe Scanner
Article
  • Dec 2014
This paper describes a linear-gradient furnace and a thermocouple homogeneity scanner that, together, measure changes in the Seebeck coefficient as a function of time and temperature. The furnace first exposes the test thermocouple to all temperatures in the range spanned by the furnace gradient. The homogeneity scanner then measures the Seebeck coefficient along the length of the thermocouple. By correlating the position on the thermocouple with the temperature in the furnace, changes in the Seebeck coefficient can be correlated with the temperature to which that part of the thermocouple was exposed. Repeat exposures for different durations allow the rapid accumulation of data describing drift versus temperature and time. The known profile of the furnace combined with the high resolution of the dual heat-pipe scanner enable the detection of Seebeck coefficient changes of less than 0.02 % over sub-millimeter distances. The high resolution of the scanner also minimizes the underestimation of short-range changes in the Seebeck coefficient. With the addition of other treatment processes, such as annealing, quenching, and cold work, the system can assess the full variety of reversible and irreversible effects in thermocouples. Preliminary experiments on base-metal thermocouples confirm much of the known long-term behavior. However, the system has also exposed the rapid onset of some of these effects at low temperatures, the large amount and variability of cold work in new thermocouples, and large variations between different thermocouples of the same type.
Low-Temperature Drift in MIMS Base-Metal Thermocouples
Article
  • Feb 2014
Inhomogeneities are known to develop within thermoelements exposed to elevated temperatures, resulting in temperature measurement errors. While the Seebeck coefficient drift in base-metal thermocouples due to aging at temperatures over 200° C has been extensively investigated, there have been very few investigations into possible Seebeck changes at lower temperatures. Despite warnings about possible effects, most practitioners assume changes in homogeneity are either not significant or not able to develop at temperatures less than 200° C . This study reports on measurements of inhomogeneities in base-metal thermocouples arising from heat treatment at temperatures in the region of 200° C . Thermoelectric scans of thermocouples were carried out following exposure of a range of mineral-insulated metal-sheathed base-metal thermocouples, from two large manufacturers, of Types E, J, K, N, and T, to either a linear-gradient furnace within the range of 100° C to 320° C or uniform temperature zones of 100° C , 150° C , and 200° C . The experiments reveal noticeable drift in all base-metal types for temperatures as low as 100° C and exposure times as short as 1 h. The most sensitive thermoelement alloys appear to be Constantan, Alumel, and Nicrosil. It is concluded that the common working assumption that base-metal thermocouples suffer no thermally induced changes in the Seebeck coefficient below 200° C is false. This observation has significant implications for many high-stability monitoring and control systems reliant on base-metal thermocouples that operate in the range of 100° C to 200° C . Additionally, thermoelectric scanning of base-metal thermocouples should be carried out at temperatures well below 150° C to avoid erasure of strain effects or imprinting of new thermal signatures.
Temperature-electromotive force reference functions and tables for the letter-designated thermocouple types based on the ITS-90
Article
  • Mar 1993
The International Temperature Scale of 1990 (ITS-90) and the new representation of the volt came into effect on 1 January 1990. Those changes required that the then-existing IPTS-68 based temperature-electromotive force reference functions and tables for thermocouples be revised to give values in terms of the ITS-90 and the SI volt. This monograph gives the new reference functions and tables for the eight letter-designated thermocouple types: noble-metal types B, R, and S; and base-metal types E, J, K, N, and T. Also, for these thermocouple types, reference functions and tables of their thermoelements versus the NIST platinum thermoelectric reference standard, Pt-67, are given. The computational methods used to derive each of the new reference functions are described.
Thermoelectric hysteresis in nickel-based thermocouple alloys
Article
  • Nov 2000
Hysteresis in the Seebeck coefficient has been studied for type K and type N thermocouples. The temperature range was 0 to 1200 degrees C, heating periods were up to 530 h and the alloys were supplied from a number of different sources (five for type K and two for type N). In both types, changes of about 1% in Seebeck coefficient were found but the hysteresis extended over a larger temperature range for the type N thermocouple. Thermoelectric hysteresis is a major cause of instability in Ni-based thermocouples, especially in the more stable, Nicrosil-sheathed mineral-insulated configuration. It is shown that it is possible to produce type K thermocouples with a performance comparable with that previously reported for type N.
Temperature measurement errors with type K (Chromel vs Alumel) thermocouples due to short‐ranged ordering in Chromel
Article
  • Dec 1975
After annealed, type K (Chromel vs Alumel) thermocouples were heated above 200 °C, their temperature measurements were in error up to 1.3%, as determined by comparison calibrations to working standard 90% Pt–10% Rh/Pt thermocouples or platinum resistance thermometers. Reannealing the type K thermocouples removed the errors. The errors were due to changes in the thermal emf vs temperature relationship of the type K thermocouples, which from previous work of others can be attributed to short‐ranged ordering of the Chromel thermoelements. Reannealing the thermocouples removed the errors because the order–disorder transformation is reversible; that is, short‐ranged ordering of the Ni and Cr atoms of the Chromel alloy occurs between 200° and 600 °C, and disordering occurs above 600 °C. The traveling gradient method was used to determine the effects of heat treatment on the thermal emf of type K thermocouples, to investigate the kinetics of ordering of Chromel, and to determine the amount of order produced by heat treatments. Though the order–disorder transformation could not be stopped, our results demonstrate that there exists an optimum amount of order in the Chromel thermoelement to yield a repeatable thermal emf vs temperature relationship for a type K thermocouple in a specific application. When the experimental conditions of the application were controlled carefully, isothermal heat treatment to produce the optimum order in the Chromel thermoelements and calibration of the type K thermocouples prior to use essentially eliminated the temperature measurement errors due to order. The optimum amount of order depended on the application and was determined experimentally. Variations from the optimized heating or cooling cycles, changes in the depth of thermocouple immersion during use, or modification of the temperature gradient on the thermocouple caused temperature measurement error- s of up to 1% with ordered and calibrated type K thermocouples.