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Drift in Type K Bare-Wire Thermocouples from Different Manufacturers

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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. ...
Technical Report
Full-text available
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. ...
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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. ...
Article
Full-text available
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 (%). ...
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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. ...
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... 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] . ...
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... 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. ...
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... 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. ...
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... 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. ...
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