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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 in...
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Context 1
... entrance design, shown Figure 1, provides thermal shielding to the thermocouple before entering the furnace, thus yielding a narrow temperature gradient. The apparatus incorporates a machinable ceramic housing with three 1 mm thick silicon wafer disks layered with platinum foil. ...
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... long aging periods, can be used to assess the linear scalability of inhomogeneity due to rhodium- oxide. Comparisons of high-resolution scans made at 100 °C with those made at the 600 °C to 900 °C show the depression has the same general shape. By plotting the characteristic peak- to-peak inhomogeneity in the 400 mm to 500 mm region, as seen in Fig. 10, the magnitude of the depression can be evaluated as a function of scanning temperature. The temperature dependence in these plots has been fitted using 2 nd order polynomials for 114422 R2 and 114422 ...
Citations
... Jahan and Ballico [4] first showed that, under certain conditions, a linear relationship between the thermoelectric inhomogeneity of type S and R thermocouples and the measured emf exists and the inhomogeneity ∆emf may be expressed as a percentage of the total emf measured. In another study [5], a linear scalability of irreversible thermoelectric inhomogeneities with temperature in type S and R thermocouples in the temperature range between 600 • C and 1000 • C was demonstrated. Thermoelectric inhomogeneities of type B and Land-Jewell thermocouples were investigated in [6] by using a high-resolution scanner operated between 600 • C and 900 • C. The results showed drift effects of between 0.2% and 0.6% for reversible oxidation effects, whereas drift caused by irreversible contamination effects was between 0.6% and 1.1%. ...
... The ∆emf max values measured at the different scanning temperatures are also shown for comparison. The linear extrapolation of the measured maximum deviation ∆emf max at an arbitrary scanning temperature T S to other temperatures according to equation (1) delivers good results and clearly confirms that this approach can be used for irreversible inhomogeneities [4][5][6][7]. ...
... Irreversible inhomogeneities can be scaled linearly according to equation (1) over the entire temperature range as shown with thermocouple No. 01/92 based on only one immersion profile measurement at a temperature that is in principle arbitrary (see figure 13). This confirms the common practice [4][5][6][7] of treating irreversible inhomogeneities. For reversible inhomogeneities, the ∆emf max value measured at a scanning temperature between 400 • C and 450 • C is sufficient to present a maximum estimate of the thermoelectric inhomogeneity in the whole temperature range. ...
Thermoelectric inhomogeneities of type S, type R and Pt-20%Rh/Pt thermocouples were determined in the temperature range between 200 °C and 1000 °C in temperature steps of 50 °C. Immersion profiles with each thermocouple at each of the sixteen scanning temperatures were measured. From the measured temperature dependencies of the inhomogeneities, methods were derived how thermoelectric inhomogeneities measured at only one or two scanning temperatures are quantitatively transferred to other temperatures or temperature ranges. For this purpose, thermoelectric inhomogeneities were classified as irreversible and reversible inhomogeneities, as they must be treated differently. Irreversible thermoelectric inhomogeneities can be extrapolated linearly with temperature or electromotive force from only one immersion profile measurement at an arbitrary temperature to other temperatures in the temperature range investigated. Reversible inhomogeneities in Pt/Rh alloyed thermocouples must be taken as a kind of unavoidable background inhomogeneity (noise) whose amplitude essentially depends on the alloy composition. The distinction between reversible and irreversible inhomogeneities is made by measuring immersion profiles at two scanning temperatures: first at a temperature between 400 °C and 450 °C, where reversible inhomogeneities have a maximum value and at a temperature between 600 °C and 700 °C, where reversible inhomogeneities have a minimum in contrast to irreversible inhomogeneities.
... In the last 20 years a significant advance has been made in understanding how noble-metal Types B, R and S vary from the reference function, as-supplied, and after exposure to various temperatures [70][71][72][73]. From this knowledge new Pt-Rh alloy pairs [74], able to replace Type B, and a new Pt-20%Rh vs Pt reference thermocouple, able to replace Types R and S, are being developed [43,75,76]. ...
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.
... For measuring thermoelectric inhomogeneity, it is necessary for the isothermal enclosure to have good temperature uniformity and stability. This was determined for the apparatus at NPL at 600 °C, over the whole length using an industrial PRT [47]. A linear slope spanning 3 °C was observed from the entrance to the exit of the furnace, which may be due to radiative heat transfer ('light piping') within the quartz tube. ...
Gas-Controlled Heat Pipes (GCHPs) are devices based on generating and maintaining, at millikelvin level, a thermodynamic liquid-vapour equilibrium of a fluid. For this reason, GCHPs have been studied for more than thirty years for research and applications in thermal metrology. Capabilities have been constantly improved and adapted by National Metrology Institutes (NMIs) and accredited laboratories. Activities include study of vapour pressure curves of pure elements and substances, thermometers’ non-uniqueness up to 960 °C, calibrations between -20 °C and 900 °C with millikelvin uncertainties, studies of innovative pressure controllers allowing regulation better than 10⁻⁶ from below 1000 Pa up to 400 kPa. GCHPs operating at different temperature ranges have also been connected to a common pressure line in the so-called “Temperature Amplifier” configuration. This review paper presents an almost complete report about the several models of GCHPs, materials and working fluids, techniques adopted in different temperature/pressure ranges. All involved NMIs using GCHP are here included, with detailed bibliography.
... A transformation of (2) leads to the relationship between the measured emf and the Seebeck coefficient in inhomogeneous thermocouples (6). ...
... Recent investigations of inhomogeneities of both type B thermocouples and Land-Jewell noble metal thermocouples in the temperature range between 600 °C and 900 °C [5] revealed a linear relation of emf changes with temperature caused only by irreversible inhomogeneities. Further investigation showed that this conclusion was also true of other Pt/Rh-alloyed thermocouples [6,7]. Problems arise when reversible, temperature-dependent inhomogeneities have to be considered. ...
This paper describes the results of low-temperature investigations of the thermoelectric inhomogeneity of conventional standardized noble metal thermocouples (types S and R, Au/Pt and Pt/Pd thermocouples) and of non-standardized Pt/Rh-alloyed thermocouples which have undergone initial annealing at 400 °C for 24 h. The investigations aim to verify the generally accepted practice of using a linear relationship between the inhomogeneity and the measured electromotive force (emf). Immersion profile measurements were carried out in a salt bath at temperatures of about 200 °C, 300 °C, 400 °C, and 500 °C. These temperatures are below the critical temperature range of the selective Rh oxidation of Pt/Rh-alloyed thermocouples. Therefore, the results of the homogeneity investigations should not be influenced by such reversible oxidations effects. Nevertheless, the results of the homogeneity tests at 500 °C showed a deviation from the linearity of the measured inhomogeneities of the thermocouples containing Pt/Rh alloys related to the measurements at 200 °C, 300 °C, and 400 °C. The results obtained with the Au/Pt and Pt/Pd thermocouples confirmed the above-mentioned assumption of the linear relationship between the magnitude of the inhomogeneities and the thermoelectric voltages measured. An additional investigation of a multi-wire thermocouple consisting of non-standardized Pt/Rh alloys (Pt5%Rh, Pt17%Rh, and Pt20%Rh) and a pure platinum thermoelement confirmed the results obtained with the standardized type S and R thermocouples and allowed conclusions to be drawn about re-ordering effects of the Pt/Rh-alloyed thermocouples, which are strongly related to the initial annealing temperature of 400 °C.
... It is generally accepted that the formation of Rhodium-oxide causes a small reduction in the Rh concentration for Pt-Rh thermocouples, resulting in changes to the emf [15,26]. For Types S and R these changes in Rh concentration have been found to scale well with temperature and lead to a reduction in emf at all temperatures [30]. This behaviour is best illustrated using Caldwell's data [4], which has been reproduced in figure 6(a). ...
... Thus, the effects of this type of inhomogeneity cannot be linearly scaled with temperature. An earlier study [30] showed for Type S the effect on the Seebeck coefficient due to rhodium depletion via rhodium oxidation obeyed a square law relationship, doubling between 100 °C and 900 °C. For the Pt-20%Rh thermocouple the effect of rhodium oxide on the Seebeck coefficient changes sign at ~300 °C. ...
... Similar measurements made with Type S thermocouples resulted in hysteresis of between 100 mK and 200 mK. The hysteresis value is the maximum difference in emf between the ramp up and the ramp down in [30,37], then, µT (µV) = emf T ·[emf p-p /(emf Tscan − emf Tamb )]/√ 12, where emf p-p is the peak-to-peak emf observed during a homogeneity scan following 100 h exposure at 950 °C, emf Tscan is the average emf at the scanning temperature (~100 °C), emf Tamb is the emf when at the ambient laboratory temperature (~20 °C) and emf T is the typical emf at the calibration temperature, T (962 °C). ...
For over a century the Type S (Pt-10%Rh) and R (Pt-13%Rh) thermocouples have been used as primary or secondary reference thermometers. However, the measurement uncertainties of both types at the silver point (~962 °C) are typically limited to about 0.5 °C, due to drift caused by crystallographic ordering between 200 °C and 500 °C and rhodium oxidation between 500 °C and 900 °C. Although both processes can be reversed using an 1100 °C anneal, regular annealing can be inconvenient or impracticable. This paper follows up a prior study indicating that an alternative noble-metal thermocouple comprised of Pt-20%Rh and Pt is relatively insensitive to both drift mechanisms. Four thermocouples, assembled using wire from four different manufacturers, were evaluated using a gradient-furnace and homogeneity scanner. Measurements were also made using a salt-bath (200 °C to 500 °C) and fixed points between the indium and silver points. The experiments indicate that individual thermocouples are stable at the silver point to within 0.18 °C for periods of up to 100 hours and all four thermocouples have emf vs temperature characteristics within 0.3 °C of each other. This intrinsic stability and similarity, coupled with cost, assembly and use conditions identical to a Type R or S thermocouple make the Pt-20%Rh vs Pt thermocouple an attractive alternative. Recommended annealing procedures enabling the greatest stability are also given.
... Our report aims to build on previous studies and provides new data on the recovery of thermocouple inhomogeneity by electrical annealing. This work may also be important for those trying to improve the calibration and measurement capability (CMC) of thermocouple calibration by reducing the value of thermocouple inhomogeneity (9)(10)(11)(12). ...
The thermoelectric inhomogeneity as a function of position along wires is one the significant uncertainty of measurement using thermocouples. Here we report development of an electrical annealing system for thermoelectric inhomogeneity treatment. Two inhomogeneous typeS thermocouples, which had the inhomogeneity greater than 0.04%emf, are successfully recovered using the system. An improvement on thermocouple performance as large as 0.28 °C (at temperature of 1000 °C) can be obtained using the system. This article provides detailed information and may help the reader to obtain a quick grasp about the system.
... Our report aims to build on previous studies and provides new data on the recovery of thermocouple inhomogeneity by electrical annealing. This work may also be important for those trying to improve the calibration and measurement capability (CMC) of thermocouple calibration by reducing the value of thermocouple inhomogeneity (9)(10)(11)(12). ...
The thermoelectric inhomogeneity as a function of position along wires is one the significant uncertainty of measurement using thermocouples. Here we report development of an electrical annealing system for thermoelectric inhomogeneity treatment. Two inhomogeneous type-S thermocouples, which had the inhomogeneity greater than 0.04%emf, are successfully recovered using the system. An improvement on thermocouple performance as large as 0.28 °C (at temperature of 1000 °C) can be obtained using the system. This article provides detailed information and may help the reader to obtain a quick grasp about the system.
... It has been well documented that the vapour pressure of the oxides of both platinum and rhodium increases exponentially with temperature [7] and that the evaporation results in significant transport effects [6] which give rise to local changes in the wire composition, which are irreversible [8] and which can be detected with homogeneity scanning apparatus [20]. It has recently been demonstrated [5] that, for a given temperature, there is an optimum composition for which this evaporation leaves no net change to the wire composition; this amounts to about 37% at the Co-C point (1324 °C) and about 42% at the Pd-C point (1492 °C), although the uncertainty on this composition is relatively large, being about ±5% (coverage factor k = 2, i.e. 95% probability). ...
By using a simple model to relate the electromotive force drift rate of Pt-Rh thermoelements to dS/dc, i.e. the sensitivity of the Seebeck coefficient, S, to rhodium mass fraction, c, the composition of the optimal pair of Pt-Rh wires that minimizes thermoelectric drift can be determined. The model has been applied to four multi-wire thermocouples each comprising 5 or 7 Pt-Rh wires of different composition. Two thermocouples were exposed to a temperature of around 1324 °C, one thermocouple to around 1492 °C, i.e. the melting points of the Co-C and Pd-C high temperature fixed points, respectively, and one thermocouple to a series of temperatures between 1315 °C and 1450 °C. The duration of exposure at each temperature was several thousand hours. By performing repeated calibrations in situ with the appropriate fixed point during the high temperature exposure, the drift performance has been quantified with high accuracy, entirely free from errors associated with thermoelectric homogeneity. By combining these results it is concluded that the Pt-40%Rh versus Pt-6%Rh is the most stable at the temperatures investigated. A preliminary reference function was determined and is presented. © 2018 Crown copyright. Reproduced with the permission of the Controller of Her Majesty's Stationery Office 2018 BIPM & IOP Publishing Ltd.
... Numerous studies to estimate its magnitude at various temperatures have been carried out, and a wellwritten review article summarizing the reported test methods was recently published [3]. Webster et al studied the high temperature scanning of noble metal thermocouples of type R and S up to 1000 °C and concluded that a single test temperature is enough to estimate the inhomogeneity for the whole calibra tion temper ature range [4]. This conclusion was consistent with a previous report by Jahan and Ballico, which was the first report on the temperature dependence of the inhomoge neity of the type R and S thermocouples [5]. ...
... As shown in the figure, the cooling jacket was made of two parts, which had coolant flowing paths at each side, and they were welded together to be waterproof. This concept was suggested by Webster et al [4], where they used Pt foils and silicon wafers as the radiation shunts. We slightly modified the design by fully immersing the cooling jacket into the furnace entrance neck as shown in figure 2. Two Cu tubes with diameter of 6 mm were soldered to the jacket, through which water at 20 °C was continuously flowed using a circulating bath. ...
For noble metal thermocouples, thermoelectric inhomogeneity is the most important factor of the calibration uncertainty. For type R and S thermocouples, it is possible to perform a scan at low temperature of around 200 °C, and extrapolate the calculated inhomogeneity to a higher temperature range. However, there has been little experimental study on the thermoelectric scan of type B thermocouples covering low temperature to high temperature of about 1000 °C. In order to determine the practical temperature dependence of the inhomogeneity of type B thermocouples, two used thermocouples were selected and tested from 180 °C to 960 °C based on the single temperature gradient immersion technique. Two liquid baths, one a silicon oil bath at 180 °C and the other a salt bath at 400 °C, were used, and a sodium-heat pipe furnace was used above 600 °C with intervals of around 120 °C. The tested thermocouples generated large emf variations with the immersion depth, and the amount of the variation increased with the test temperature. The calculated inhomogeneity showed the largest value at the lowest temperature, and decreased gradually with increasing temperature to 600 °C. Above this temperature, the inhomogeneity was nearly constant with temperature change, indicating that it was possible to extrapolate the uncertainty due to the inhomogeneity at 600 °C to higher temperatures. From the measured results, it was recommended that a thermoelectric scan be performed at a temperature of at least 600 °C using the sodium-heat pipe furnace to obtain a small calibration uncertainty of a type B thermocouple.
... However, to the authors' knowledge the selection of Pt-Rh alloys of neither type has been subjected to optimization. The aim of this study is to evaluate the influence of Pt-Rh thermoelement composition on the stability of the thermocouple [3,4] with unprecedented accuracy, by using a Co-C high-temperature fixed point (HTFP) at 1324 • C. ...
A simple model is presented which relates the electromotive force drift rate of Pt–Rh thermoelements to dS/dc, the sensitivity of the Seebeck coefficient, S, to rhodium mass fraction, c. The model has been tested by repeated measurements of a Pt–Rh thermocouple assembly consisting of five thermoelements, using a Co-C high-temperature fixed point (\(1324{\,}^{\circ }{\mathrm{C}}\)) for a total duration of 500 h. By considering various thermocouples from the assembly, it is demonstrated that in this case, remarkably, there is a linear relationship between the measured drift rate and the combined dS/dc, where the combination is determined by addition of the individual values for each wire. Particular emphasis is placed on evaluation of the uncertainties associated with the calculations. This result supports previous findings that the thermoelectric stability of Pt–Rh thermoelements improves as the rhodium mass fraction increases. Within this paradigm, it is shown that for a selected Pt–Rh thermoelement of any given composition, there exists a second thermoelement having a composition that yields a minimum drift when combined with the first to form a thermocouple.
