Base Metal Thermocouples Research Articles

Characterizing Drift Behavior in Type K and N Thermocouples After High Temperature Thermal Exposures

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.

Thermoelectric stability of dual-wall and conventional type K and N thermocouples

Mineral insulated metal sheathed (MIMS) base metal thermocouples experience thermoelectric drift over their lifetime caused by use at high temperatures and metallurgical changes, causing spurious measurement errors. CCPI Europe Limited and University of Cambridge have designed a MIMS thermocouple with an additional inner sheath, in order to protect the thermoelements from the effects that cause thermoelectric drift. The performance of these dual-wall thermocouples and conventional type N and type K thermocouples are assessed at six different National Metrology Institutes (NMIs) using two different testing regimes: isothermal testing at 1200 °C, and thermal cycling tests between 300 °C and 1150 °C. The investigation demonstrates that in both testing regimes, the type N dual-wall thermocouples showed a significantly reduced thermoelectric drift by about a factor of three compared to the conventional thermocouples. There was no significant difference between the type K dual-wall and conventional type K thermocouples in the isothermal tests, and the type K dual-wall thermocouples showed greater drift than the conventional thermocouples in the thermal cycling tests, but the conventional type K thermocouples were less robust than the dual-wall type K thermocouples. The results presented in this paper represent an impartial assessment of the thermoelectric stability of both dual-wall thermocouples and conventional thermocouples, which may provide assurance to potential users.

Base-metal thermocouple tolerances and their utility in real-world measurements

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.

A critical review of the common thermocouple reference functions

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.

Research on Calibration of Short Base Metal Thermocouples

Short and cheap metal thermocouple is a general term for detachable short base metal thermocouple (electrode length is about 300mm-500mm) and short base metal armored couple (metal sleeve length is about 300mm-500mm). Their electrodes are short in length. Traditional measuring equipment often fails to accurately measure the indication deviation in the high temperature region (above 300 °C). Through experiments, this paper finds out the reasons of inaccurate measurement. Then, this paper designs a thermostat at the terminal, which can solve this problem. The design can effectively reduce the extended uncertainty of short-form even value deviation.

An investigation of the base metal thermocouples uncertainty components in ILC comparison

This paper is about unique the interlaboratory large comparison organized under EA-2/14 - Procedure for Regional Calibration ILCs in Support of EA MLA and describe final results of the comparison and several pitfalls when measuring temperature with base metal thermocouple. Interlaboratory comparison (ILC) serves as a tool for comparison of measurement results carried out by accredited or non-accredited calibration laboratories in the relevant field of measurement. ILC represents very effective means to demonstrate technical competence of the participant and also serves as a technical base for accreditation. The subject of this comparison was calibration of several pieces the N-type thermocouples. The pilot laboratory of this ILC was the Department of primary metrology of thermo-technical quantities of the Czech Metrology Institute. The reference values of the test items were provided by the above mentioned organizational unit this company. The evaluation of the measurement results of participating laboratories was performed on the basis of En score (EN ISO/IEC 17043). In this ILC program 47 accredited calibration laboratories from different European countries and also from Bosnia and Hercegovina, Switzerland and Turkey have participated. The results of this comparison demonstrate satisfactory agreement of the majority of the participants measured values with the reference ones within their stated expanded uncertainties. The article also highlights the basic and often neglected components of uncertainty.

A comprehensive survey of thermoelectric homogeneity of commonly used thermocouple types

Thermocouples are widely used as temperature sensors in industry. The electromotive force generated by a thermocouple is produced in a temperature gradient and not at the thermocouple tip. This means that the thermoelectric inhomogeneity represents one of the most important contributions to the overall measurement uncertainty associated with thermocouples. To characterise this effect, and to provide some general recommendations concerning the magnitude of this contribution to use when formulating uncertainty analyses, a comprehensive literature survey has been performed. Significant information was found for Types K, N, R, S, B, Pt/Pd, Au/Pt and various other Pt/Rh thermocouples. In the case of Type K and N thermocouples, the survey has been augmented by a substantial amount of data based on calibrations of mineral-insulated, metal-sheathed thermocouple cable reels from thermocouple manufacturers. Some general conclusions are drawn and outline recommendations given concerning typical values for the uncertainty arising from thermoelectric inhomogeneity for the most widely used thermocouple types in the as-new state. It is stressed that these recommendations should only be heeded when individual homogeneity measurements are not possible. It is also stressed that the homogeneity can deteriorate rapidly during use, particularly for base metal thermocouples.

Seebeck Changes Due to Residual Cold-Work and Reversible Effects in Type K Bare-Wire Thermocouples

Type K thermocouples are the most commonly used thermocouple for industrial measurements because of their low cost, wide temperature range, and durability. As with all base-metal thermocouples, Type K is made to match a mathematical temperature-to-emf relationship and not a prescribed alloy formulation. Because different manufacturers use varying alloy formulations and manufacturing techniques, different Type K thermocouples exhibit a range of drift and hysteresis characteristics, largely due to ordering effects in the positive (K+) thermoelement. In this study, these effects are assessed in detail for temperatures below $$700\, {^{\circ }}\hbox {C}$$ in the Type K wires from nine manufacturers. A linear gradient furnace and a high-resolution homogeneity scanner combined with the judicious use of annealing processes allow measurements that separately identify the effects of cold-work, ordering, and oxidation to be made. The results show most K+ alloys develop significant errors, but the magnitudes of the contributions of each process vary substantially between the different K+ wires. In practical applications, the measurement uncertainties achievable with Type K therefore depend not only on the wire formulation but also on the temperature, period of exposure, and, most importantly, the thermal treatments prior to use.

Limits of noble and base metal thermocouples

In the fallowing paper we will focus on one of the most frequently used temperature sensors, which are thermocouples. As these sensors are used in a variety of applications reaching from simple industrial ones up to aerospace measurements the knowledge of their behaviour and limitations is of high importance. Within this paper we will be focusing on both base and noble metal thermocouples which have their specific areas of use. The emphasis of the presented studies is on the hysteresis and homogeneity effects within base metal type K and N thermocouples and on the effect of new construction of noble metal type Au/Pt thermocouples.

Drift in Type K Bare-Wire Thermocouples from Different Manufacturers

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.

A System for High-Temperature Homogeneity Scanning of Noble-Metal Thermocouples

Noble-metal thermocouples are amongst the most widely used thermocouples for high-temperature process measurement and as references. Although they are less susceptible to inhomogeneity effects than the more-common base-metal thermocouples, inhomogeneity is still the major source of uncertainty. Currently, most estimates of the uncertainty due to inhomogeneity are based on thermocouple specifications or historical performance of similar thermocouples. It is not common for the inhomogeneity to be measured directly, in part because there is no accepted method for measuring the inhomogeneities, and in part because there is no conclusive evidence linking the magnitude of inhomogeneities determined at the scanning temperature to the effects of the same inhomogeneities at other temperatures. This paper describes an inhomogeneity scanner able to be fitted to sodium heat-pipe furnaces to operate between $$600 {\,{}^{\circ }}\hbox {C}$$ and $$1000 {\,{}^{\circ }}\hbox {C}$$ . Comparison of scans made at $$100 {\,{}^{\circ }}\hbox {C}$$ demonstrates the scalability of some types of inhomogeneity in Type S and R thermocouples. It is concluded that for Type R and S thermocouples, a robust uncertainty assessment can be obtained from a scan made at a single temperature.

Hysteresis Effects and Strain-Induced Homogeneity Effects in Base Metal Thermocouples

Thermocouples are used in a wide variety of industrial applications in which they play an important role for temperature control and monitoring. Wire inhomogeneity and hysteresis effects are major sources of uncertainty in thermocouple measurements. To efficiently mitigate these effects, it is first necessary to explore the impact of strain-induced inhomogeneities and hysteresis, and their contribution to the uncertainty. This article investigates homogeneity and hysteresis effects in Types N and K mineral-insulated metal-sheathed (MIMS) thermocouples. Homogeneity of thermocouple wires is known to change when mechanical strain is experienced by the thermoelements. To test this influence, bends of increasingly small radii, typical in industrial applications, were made to a number of thermocouples with different sheath diameters. The change in homogeneity was determined through controlled immersion of the thermocouple into an isothermal liquid oil bath at \(150\,^{\circ }\hbox {C}\) and was found to be very small at \(0.09\,^{\circ }\hbox {C}\) for Type K thermocouples, with no measureable change in Type N thermocouples found. An experiment to determine the hysteresis effect in thermocouples was performed on swaged, MIMS Type N and Type K thermocouples, in the temperature range from \(200\,^{\circ }\hbox {C}\) to \(1000\,^{\circ }\hbox {C}\). The hysteresis measurements presented simulate the conditions that thermocouples may be exposed to in industrial applications through continuous cycling over 136 h. During this exposure, a characteristic drift from the reference function has been observed but no considerable difference between the heating and cooling measurements was measureable. The measured differences were within the measurement uncertainties; therefore, no hysteresis was observed.

Measurement of Inhomogeneities in MIMS Thermocouples Using a Linear-Gradient Furnace and Dual Heat-Pipe Scanner

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

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 $$200\,^{\circ }\mathrm{C}$$ 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 $$200\,^{\circ }\mathrm{C}$$ . This study reports on measurements of inhomogeneities in base-metal thermocouples arising from heat treatment at temperatures in the region of $$200\,^{\circ }\mathrm{C}$$ . 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 $$100\,^{\circ }\mathrm{C}$$ to $$320\,^{\circ }\mathrm{C}$$ or uniform temperature zones of $$100\,^{\circ }\mathrm{C}$$ , $$150\,^{\circ }\mathrm{C}$$ , and $$200\,^{\circ }\mathrm{C}$$ . The experiments reveal noticeable drift in all base-metal types for temperatures as low as $$100\,^{\circ }\mathrm{C}$$ 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 $$200\,^{\circ }\mathrm{C}$$ 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 $$100\,^{\circ }\mathrm{C}$$ to $$200\,^{\circ }\mathrm{C}$$ . Additionally, thermoelectric scanning of base-metal thermocouples should be carried out at temperatures well below $$150\,^{\circ }\mathrm{C}$$ to avoid erasure of strain effects or imprinting of new thermal signatures.

Design of Temperature Control System for Thermocouple Verification Based on Virtual Instrument Technology

The thermocouple calibration temperature control system is constructed USB - 4716 200 ks/s, 16 multi-function USB module, JKH-C series thyristor phase shift trigger, PC machine as the main hardware, the U.S. National Instrument (NI) company Labview as the software development platform. According to the precious metal, base metal thermocouple wire thermoelectric EFM method to determine the temperature test point, the temperature control system using automatic control to test furnace temperature, and realize the visual operation interface. Temperature control accuracy: temperature deviation calibration point is less than ± 5 °C when testing the thermocouple, constant temperature stability: constant temperature's change is less than 0.2 °C /min in the thermocouple testing process.

Life Expectancy Study of Small Diameter Type E, K, and N Mineral-Insulated Thermocouples Above 1000 °C in Air

Very little if any current data is available for the life expectancy of very small diameter thermocouples operating at high temperatures, greater than 1000 °C. Over the past 10 years significant changes in the supply stream of the materials used to manufacture base metal thermocouples have occurred. In many industrial applications, small diameter thermocouples are the only solution for high-temperature measurements. This study has been undertaken to assess the performance of small diameter magnesium oxide insulated metal sheathed thermocouple sensors at or above 1000 °C. Three different American Society of Testing and Materials (ASTM) standard letter designation thermocouple types have been included in this study, Types E, K, and N. Inconel 600 and 316 stainless, for three different sizes, 0.5 mm, 1.0 mm, and 1.5 mm, have been tested for different thermocouple types. Each group of sensors was placed in an equalization block in air for a maximum of 500 h or until failure. The performance of 0.5 mm diameter thermocouples varies widely depending on the thermocouple type, sheath material, and test temperature. Larger diameter Type K and N thermocouples show very little drift up to 500 h at 1100 °C. The data for each test was collected at 10 s intervals for the entire duration of the test. Data for the sensor drift and subsequent failure are presented.

Assessment of Temperature Measurements Using Thermocouples at NIS-Egypt

Thermocouples are increasingly used in industry and research. For many industrial heating processes, particularly those carried out at high temperatures, a thermocouple is the most convenient and simple instrument for temperature measurement. In some instances, it is the only feasible method. The aim of this study is to select and recommend the best thermocouples from both base and noble metals to users in industrial and scientific institutions. Different types of thermocouples and calibration methods are described. From this work, the Nicrosil–Nisil thermocouple has been proposed as the best base metal thermocouple and the Au/Pt thermocouple is the most recommended as a substandard up to 1,000 °C.

Change in inhomogeneity with temperature between 180 °C and 950 °C in base-metal thermocouples

In order to investigate the temperature dependence of thermoelectric inhomogeneity in base-metal thermocouples, thermoelectric scan tests were performed at several temperatures using mineral-insulated metal-sheathed thermocouples of types K, N, E and J. To induce a severe inhomogeneity in the thermoelements, the thermocouples were heat-treated for 1000 h: types K and N at 950 °C, E and J at 750 °C. After the heat treatments, type K and N thermocouples were scanned at 180 °C in a stirred-liquid bath, and at 550 °C, 750 °C and 1000 °C in a sodium heat-pipe furnace. The measurement for the type E and J thermocouples was done only up to 750 °C. From the scan results, we found that the thermoelectric inhomogeneity, expressed as a ratio of maximum emf change within the working segment to the total emf generated, decreased at high temperature. We concluded that some base-metal thermocouples might have temperature-dependent inhomogeneity; therefore, applying the inhomogeneity value obtained at low temperature to assess the calibration uncertainty at higher temperatures may result in overestimation of the uncertainty.

A method for evaluation of the inhomogeneity of thermoelements

Thermoelectric inhomogeneity is the spatial variation of the Seebeck coefficient along a thermoelement. It is often considered to be the largest contribution of an uncertainty budget in thermocouple calibration. In this work, investigations of inhomogeneities were performed on single thermoelements, using a heater with short movable heating zones (two-gradient method). We chose platinum wires which are commonly used in noble metal thermocouples and copper wires which are available in good homogeneity and which are commonly used in base metal thermocouples. The results of different investigations of the thermoelements are presented.

Final report on SIM supplementary comparison 3.9: Type K thermocouple wire over the range 100 °C to 1100 °C

Type K thermocouples are one of the most commonly used temperature sensors in industry. The skills, personnel, and facilities necessary for calibrating type K thermocouples are also applicable to the calibration of other base metal thermocouples, and, to a lesser extent, calibration of platinum–rhodium alloy thermocouples. Under the auspices of the Inter-american Metrology System (SIM), the National Institute of Standards and Technology (NIST) initiated a regional comparison for type K thermocouples from 100 °C to 1100 °C, with 11 participating countries. The use of type K material above approximately 200 °C is considered destructive. Therefore, each participating laboratory was sent new, unused wire from a lot of material characterized by NIST. The uniformity of the lot was remarkable, especially at temperatures above 500 °C; the standard deviation of the thermocouple emf values of multiple cuts tested at NIST was 2.7 µV or less over the full temperature range. The high uniformity eliminated any need to correct for variations of the transfer standard among the laboratories, greatly simplifying the analysis. The level of agreement among the laboratories' results was quite good. Of the 380 total bilateral combinations of the data at the eight test temperatures, only 13 (i.e., 3.4% of all combinations) are outside the k = 2 limits, and of these 13, only 3 are outside k = 3 limits. All of the outliers occur at temperatures of 800 °C and below, which suggests that drift of the type K wire due to high-temperature oxidation did not cause changes in thermocouple emf comparable to or larger than the claimed uncertainties.Main text.To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.The final report has been peer-reviewed and approved for publication by the SIM, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).