Abstract and Figures

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.
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Hysteresis Effects and Strain-Induced Homogeneity Effects in Base Metal
Thermocouples
P. Pavlasek
1,4
, C. J. Elliott
2
, J. V. Pearce
2
, S. Duris
1
, R. Palencar
1,3
, M. Koval
1
and G. Machin
2
1
Slovak Institute of Metrology, Bratislava,Karloveska 63, SK
2
Temperature and Humidity Group, National Physical Laboratory (NPL), Teddington, TW11 0LW,UK
3
Faculty of Mechanical Engineering, Slovak University of Technology, Bratislava, Nam.Slobody 17, SK
4
E-mail: peterpavlasek@gmail.com, Tel: +421904220382
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 effectsare major sources of
uncertainty in thermocouple measurements. To efficiently mitigate these effects, it is first necessary to
explore the impact ofstrain induced inhomogeneities andhysteresis, and their contribution to the uncertainty.
This article investigates homogeneity andhysteresis 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 ˚C and was found to be very small 0.09 °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 ˚C to 1000 ˚C. The hysteresis measurements presented
simulate the conditions that thermocouples may be exposed to in industrial applications through continuous
cycling over 136 hours. 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.
Keywords: Base metal; Homogeneity; Hysteresis; Thermocouple
Introduction
Temperature measurement directly affects the quality, effectiveness and safety of industrial manufacturing
processes. In many of these processes base metal thermocouples are used for monitoring and control.
Because of their wide use as a temperature sensor, it is necessary to understand their behaviour under
various influences and in different conditions. Two factors that contribute considerably to the uncertainty in
temperature measurement using such sensors are the hysteresis effect and the homogeneity effect: many
publications such as [1-3, 5-8, 10-12] have shown the need for further investigation into these effects.
For this investigation the two commonly used Type N and Type K thermocouples were investigated
Type N thermocouples are typically made with alloys consisting of 14.2% Cr, 1.4% Si, balanced Ni for the
positive wire branch and alloys that composition is 4.4% Si, 0.1% Mg and balanced Ni for the negative wire
branch [18]. Type K thermocouples are made with alloys that typically consists of 9.5% Cr, 0.4% Si and
balanced Ni for the positive wire branch and alloys that are made of 1.0% Si, 3.0% Mn, 2.0% Al, 0.4% Co,
0.015% Mg and balanced Ni for negative branch of wire [18].The typical composition of the alloys should
be consistent but it can vary by manufacturer. The thermocouples were all mineral insulated metal sheathed
(MIMS) format. Strain measurements were performed on thermocouples of diameter 1.0 mm and 1.5 mm,
whilst hysteresis measurements were performed on 3.0 mm diameter thermocouples. In all cases the sheath
material was Inconel 600. The overall objective of this study is to better quantify these effects to enable
realistic values to be included in uncertainty budgets for this type of thermocouple.
Mechanical strain effects in base metal thermocouples
The degree of homogeneity of the thermocouple wires directly influences the degree of uniformity in the
thermoelectric Seebeck coefficient and hence the voltage output generated by the application of a
temperature gradient to the thermocouple wires (thermoelements).
Thermocouples are active sensors. This means that unlike other temperature measuring devices, they do not
need a source of electricity to power them; instead they produce a thermoelectric signal by themselves. The
Seebeck effect is the basis on which thermocouples operate, and is an example of thermoelectricity; where
an electromotive force (emf) is produced whenever there is a heat flow along aconductor, i.e. it manifests
itself along those parts of the wire that are in a temperature gradient [4].
Thermocouple measurements in industry and their problems
Thermocouples are used in various industrial applications for temperature measurement. These
measurements are used for the monitoring and control of manufacturing processes, to uphold safety, ensure
product quality and regulate their properties. Thermocouples have an irreplaceable role in high temperature
contact thermometry. The most commonly used thermocouples in industry are made from base metal alloys.
As described above, the homogeneity of such thermocouples critically affects their uncertainty when used in
manufacturing process they control. The homogeneity of the thermoelements can be changed by various
influences, with the main sources being) structure changes, short-range-ordering (SRO), oxidation, phase
transformations, interstitial /vacancy movements at grain boundaries ii)diffusion at high temperature of
minor alloying elements such as Al, Mg, Mn, Fe, Co and, iii) cold work causes by dislocations and
acceleration of grain growth. Inhomogeneity in industrial conditions mainly arises from changing
composition and grain structure of the thermoelements. The latter effect is enhanced under mechanical strain
[4, 11, 12]. The most common source of mechanical strain is bending, as thermocouples are commonly bent
to reach various places within industrial processes that would be inaccessible otherwise. In the following
sections, an investigation into the effect on homogeneity of bending on Type N and Type K base metal
thermocouples through different diameters is described and discussed.
Description of homogeneity measurement and results
Four 800 mm long MIMS Type N and Type K thermocouples, with an outer diameter of either 1.0 mm or
1.5 mm were tested. The thermocouples each contained thermoelements of diameter 0.18 mm (outer
diameter 1.0 mm) and 0.27 mm (outer diameter 1.5 mm). One of each diameter and type of thermocouple
was exposed to a series of bends around a cylinder. A further one of each diameter and type was exposed to
a series of bends around an angled surface. The scheme of the strain application and the used diameters and
angles of bending are illustrated in Fig. 1.
Fig. 1 Illustration of the bending procedure using circular (upper panel) and angular(lower panel) surfaces
together with the values of diameter and angle applied.
To determine the effect of mechanical strain on these thermocouples, the initial state of their homogeneity
was determined. This was measured by the single-gradient method which was realized by immersing the
thermocouple in an isothermal stirred oil bath and then incrementally raising it via a linear rail system. The
thermocouple emf depends on the thermoelement homogeneity over the region of the applied temperature
gradient and thus on the position of the thermocouple in the oil bath. The oil temperature was set to 150 °C,
high enough to see the changes in homogeneity along the thermocouple but low enough so as not to
introduce any thermally generated inhomogeneity. Although works of Fenton, Carr, Reed and Webster [19,
20, 21, 22, 23] indicate that this temperature could affect the changes caused by the deformation procedure
as can be seen in Figs. 2 to 9 the areas not exposed to this mechanical bending (0 mm to 50 mm) show more
or less constant thermoelectric response over multiple runs. The maximum immersion depth in the oil bath
was 580 mm, which was sufficient to detect inhomogeneities imprinted on the thermocouple due to bending
experiments. Furthermore, it was important to maintain proper immersion of the measurement junction. For
these reasons the mechanical strain tests were limited to the region between 30 mm to 580 mm from the
thermocouple measurement junction
Fig. 2 The temperature difference from initial state measured with a 1.0 mm outer diameter Type K
thermocouple, after bending around circular surfaces.
Fig. 3 The temperature difference from initial state measured with a 1.0 mm outer diameter Type K
thermocouple, after bending around sharp surfaces.
Fig. 4 The temperature difference from initial state measured with a 1.5 mm outer diameter Type K
thermocouple, after bending around circular surfaces.
Fig. 5 The temperature difference from initial state measured with a 1.5 mm outer diameter Type K
thermocouple, after bending around sharp surfaces.
Fig. 6The temperature difference from initial state measured with a 1.0 mm outer diameter Type N
thermocouple, after bending around circular surfaces.
Fig. 7 The temperature difference from initial state measured with a 1.0 mm outer diameter Type N
thermocouple, after bending around sharp surfaces.
Fig. 8 The temperature difference from initial state measured with a 1.5 mm outer diameter Type N
thermocouple, after bending around circular surfaces.
Fig. 9 The temperature difference from initial state measured with a 1.5 mm outer diameter Type N
thermocouple, after bending around sharp surfaces.
From maximum immersion, the tested thermocouple was raised incrementally in steps of 8.2 mm.
The measurements began after a 90s equilibrium time, which was needed at each step to stabilize the newly
emerged section of the thermocouple to the different thermal condition.The end-to-end size of the oil bath
gradient within the thermocouple during scanning was estimated to be 100 mm. This fairly broad gradient
causes that it is difficult to precisely localise the region of cold working, but it is sufficient to indicate the
effect on the thermocouple as a whole.
The mechanical strain was generated by bending a segment of the thermocouple around a surface.
Two surface types were used; circular and angular. The circular surfaces were used to expose the
thermocouples to different diameters of bending; all circular bends were done by bending the thermocouple
back on itself to form a 180° angle. The surfaces were used to expose the thermocouples to different angles
of bending. For each of the thermocouples exposed to circular bends, the circular surfaces used were 10 mm,
16 mm, 20 mm, 25 mm, 32 mm, 40 mm, 50 mm and 63mm in diameter. These bends were applied in turn
along the length of the thermocouple, beginning with the largest (away from the thermocouple measurement
junction) and ending with the smallest (close to the thermocouple measurement junction). For each of the
thermocouples exposed to sharp bends, the surfaces used resulted in bends over angles of 30°, 45°, 90° and
180° angles. These bends were also applied in turn along the length of the thermocouple, as described
above. All of the bends were separated from each other by a segment of the thermocouple on which no
mechanical strain was applied. This separation of individual bends ensured that the effect of each bend
could be easily identified. After each bend application, the thermocouples were then straightened and the
homogeneity re-measured.
The effect of the applied mechanical strains on the thermocouple can be seen in Figs.2-9. In these
figures the temperature difference form the initial temperature state(before strain application) is presented.
The uncertainty on each of these measurement point has been estimated and its values is 20.2µV (where the
coverage factor k = 2).
Figures 2 and 3 show the results from mechanical strain testing on a 1.0mm diameter Type K
thermocouples; with circular and angular bends, respectively. Figures4 and 5show the results of the same
tests performed with 1.5 mm diameter Type K thermocouples. In the same way, Fig.6 and 7 show the results
of the same tests with 1.0 mm diameter Type N thermocouples, and Fig.8 and 9show the results of the same
tests with 1.5 mm diameter Type N thermocouples.
In all of these figures (Fig. 2-9), each line indicates the measurements for the same thermocouple,
after each bend was applied. The sensitivity of the Type N (Type K) thermocouples is taken to be 31
µV/°C(41µV/°C) at 150 °C. As can be seen from the Figures (Fig. 2 and 4) the mechanical strain applied
on the Type K thermocouple caused a change in its output by a small but distinct amount. The Type N
thermocouples showed no influence of the mechanical strain on homogeneity. The maximum difference
observed from the initial homogeneity state was 0.09 °C (3.58 µV) for the 1.0 mm Type K thermocouple
bent around a 63 mm diameter circular surface. This result was surprising as more pronounced
inhomogeneity due to crystalline structure changes was expected to occur with smaller bending radii, since
this is the most harsh impact on the wire. It is clear though, that use of larger bending radii (which will
introduce strain over a larger section of the thermocouple wire) is found (at least here) to have more of an
impact on the overall emf than the severity of the bend applied. For angular bending the maximum
difference was 0.02 °C (0.87 µV) around an angle of 90°.
For the 1.5 mm Type K thermocouple the maximum difference from the initial homogeneity state
was the same 0.09 °C (3.52 µV) around a 63 mm diameter bend and for angular bending the maximum
difference was 0.05 °C (2.07 µ V) around an angle of 90°. More detail information’s about the effects of
deformation on the thermocouples and the position of the applied deformations can be found in Tables 1 and
2. These effects are relatively small compared to the tolerances put on these type of thermocouples i.e.
moderate bending should not make the sensor go out of tolerance – particularly if the bent section is only in
a modest temperature gradient part of the process.
All of the results presented in Fig. 2-9 show the difference between the values obtained before the
mechanical deformation and the values obtain after mechanical deformation and strengthening. The results
do not quantify the magnitude of the EMF change, due to the straightening process that was needed to
perform the re-measurement of homogeneity. This process may have remove or reverse the stress that the
thermocouple wires were placed under.
Thermocouple
Bend applied
Central position
(mm)
Thermocouple
Bend applied
Central position
(mm)
1.0 mm Type K
10 mm circular
75
1.5 mm Type K
10 mm circular
75
16 mm circular
108
16 mm circular
108
20 mm circular
146
20 mm circular
146
25 mm circular
188
25 mm circular
188
32 mm circular
236
32 mm circular
236
40 mm circular
292
40 mm circular
292
50 mm circular
357
50 mm circular
357
63 mm circular
433
63 mm circular
433
90° sharp
140
90° sharp
140
45° sharp
200
45° sharp
200
30° sharp
260
30° sharp
260
180° sharp
320
180° sharp
320
Table 1The identified homogeneity changes and the position of deformation application for Type K
thermocouples.
Thermocouple
Bend applied
Central position
(mm)
Thermocouple
Bend applied
Central position
(mm)
1.0 mm Type N
10 mm circular
75
1.5 mm Type N
10 mm circular
75
16 mm circular
108
16 mm circular
108
20 mm circular
146
20 mm circular
146
25 mm circular
188
25 mm circular
188
32 mm circular
236
32 mm circular
236
40 mm circular
292
40 mm circular
292
50 mm circular
357
50 mm circular
357
63 mm circular
433
63 mm circular
433
90° sharp
140
90° sharp
140
45° sharp
200
45° sharp
200
30° sharp
260
30° sharp
260
180° sharp
320
180° sharp
320
Table 2The identified homogeneity changes and the position of deformation application for Type N
thermocouples.
Hysteresis effect in base metal thermocouples
Hysteresis effects in base metal thermocouples could be significant in industrial applications in which a
temperature increase and decrease needs to be monitored continuously. The main issue with hysteresis in
base metal thermocouples is that the temperature indicated when heating is not the same as indicated when
cooling. This effect was investigated in publications [3, 6-8, 13, 24] where the results suggest that the
hysteresis effects in Type K alloys occur at temperatures between 200°C and 600°C and in Type N alloys
this effect occurs at a temperature range of 200 °C to 1000 °C. Hysteresis in base metal thermocouples is
caused by reversible changes in thermoelement alloys. These changes affect the Seebeck coefficient of the
wires and thereby change the thermoelectric response (emf) of the thermocouples. The mechanism causing
this hysteretic behaviour in dilute nickel-chromium alloys can be described by electron spin-cluster
mechanism which is described by Pollock [10].
Description of Experiment
The hysteresis effect was investigated for Type K and Type N base metal thermocouples. These were
700 mm in length and 3.0 mm in diameter. As the hysteresis is caused by changes in the Seebeck coefficient,
the dimensions of the thermocouples do not have any influence on the hysteresis [3]. To eliminate random
influences that could be caused by the manufacturing process of the thermocouple, three of each type were
tested.
A single zone horizontal tube furnace was used for the experiment. The furnace uniformity vas
measured by a reference Type R thermocouple according to Fig. 10. The tested thermocouples were
positioned alongside each other in the furnace, with their measurement junctions located at the centre–
ensuring that they experienced very similar thermal conditions. Knowledge of the temperature profile of the
furnace was essential for the correct placement of the reference thermocouple, which was inserted from the
opposite end of the furnace. Five points were measured along the centre of the furnace, providing a
temperature profile over the region of the thermocouple measurement junctions. The temperature
measurements are shown in Fig. 10, and the test arrangement is illustrated in Fig. 11.
Fig. 10 An illustration of the arrangement of the test and reference thermocouples. The temperature profile
points measured in the vicinity of thermocouple measurement junctions are also given.
Fig. 11 Measurement setup for thermocouple hysteresis determination.
A low uncertainty Type R thermocouple was used as a reference to obtain the temperature in the
vicinity of the measurement junctions of the tested Type N and Type K thermocouples. The uncertainty
according to the calibration of this Type R is ± 1.5 µ Vat 200 °C and ± 3.1µV at 1000 °C.The position of the
reference thermocouple was chosen after the determination of the temperature uniformity. Seven complete
measuring cycles were performed, where each cycle typically lasted for more than 34 hours. This represents
around 300 hours of exposure above 200 °C for each thermocouple. Each cycle consisted of initially heating
the collection of thermocouples from 200 °C to 1000 °C, and then cooling back to 200 °C. The heating rate
was ~5 °C/min and the cooling rate was ~9 °C/min. Increasing and decreasing the temperature was
completed in 50 °C temperature steps with one hour for stabilization at each temperature before
commencing measurements (this process was automated to ensure the highest possible level of repeatability
and comparability of the measured data).
After stabilisation the average of the measurements recorded every 10 s over 40 min for each
thermocouple was used to determine the emf of the thermocouple at each step. The reference junction
temperature was monitored throughout the measuring cycles and readings used to correct for any variation
in the temperature at the thermocouple reference junction. The temperature of the ice point in which the
reference junction was placed in was monitored via a type N thermocouple.
Evaluation of the measured data and results
All of the measured data show almost identical behaviour and a high level of repeatability, which is typically
better than 0.6 °C. The standard deviation of the measurements taken after the stabilization period was found
to be better than 76.3 µV for the tested thermocouples and better than17.9 µV for the reference
thermocouple. The emf values measured at each step of the seven cycles(for each thermocouple) were
averaged, and then converted to temperature using reference tables included in the following standard [9].
The repeatability of these measurements is taken as the standard deviation of the result for the seven cycles.
This was found to be better than 21.2µV for the tested thermocouples and better than6.2µV for the reference
thermocouple.
The values of temperature obtained for the Type N and Type K thermocouples were then compared
to the indicated temperature of the reference Type R thermocouple (similarly calculated). The reported
results are shown as the difference of the average temperature measured with the tested thermocouple from
the average temperature measured with the reference thermocouple, for both heating and cooling. These
results are presented in Fig. 12 (Type N) and 13 (Type K).
Fig. 12 Measured temperature difference from the reference Type R thermocouple, for three 3.0 mm
diameter Type N thermocouples, upon heating (closed points) and cooling (open points
Fig. 13 Measured temperature difference from the reference Type R thermocouple, for three 3.0 mm
diameter Type K thermocouples, upon heating (closed points) and cooling (open points).
In general, it has been found that the Type N thermocouples deviate from the reference temperature
measurement by between about +2 °C (at the lowest temperatures) and -4 °C (at the highest
temperatures).According to publication [14] the reversible changes occur in Ni-based thermocouples in
MIMS configuration at temperatures below 900°C. This corresponds to our measured data but with
hysteresis occurring at a higher upper temperature of 1000°C. The measured temperature difference between
heating and cooling at 1000°C was ~0.5°C for Type N and ~2°C for Type K. When confronting the results
with the measured hysteresis from publications [4, 17] slight differences are noticeable in the temperatures
by which the hysteresis occurs. The hysteresis for Type K thermocouples according to publications [4, 17]
happens at temperatures form about ~250°C and ends at ~650°C. Here presented measurement results show
that this temperature range starts from 200°C and ends at ~500°C. The hysteresis reappears at temperature
range from ~750°C and ends at ~1000°C. For the Type N thermocouples the hysteresis occurs at
temperatures regions of ~200°C to ~490°C and ~510°C to ~650°C [4, 17]. This meets the results from our
gathered data with only negligible differences. The results from publications [15, 16] are similar and the
region of hysteresis for Type K thermocouples is in a temperature range of ~350°C to ~650°C and for Type
N the hysteresis occurs at temperature ranges from ~200°C to ~490°C and from ~500°C to ~1000°C.The
Type K thermocouples show a consistent tendency to give a result higher than that of the reference
thermocouple: up to about +3.5 °C (at the highest temperatures). Although in each figure (12 and 13), a
difference between the cooling and heating cycles for each thermocouple is visible in the presented graphs,
the magnitude of these differences is small (up to only 1 °C) compared to the repeatability of the
measurements, which has been found to be 0.6 °C.
The sources of uncertainty which would contribute to an uncertainty budget for these tests are shown
in Table 3.This includes the influences of the ice-point and measuring equipment, the influences of the
thermocouple behaviour (homogeneity and repeatability) and the influence of the furnace environment. This
shows that beside the influence of the furnace, all other factors are small. The uniformity of the furnace is
the biggest factor and is caused by the construction of the one zone furnace itself. The values of repeatability
were established by repeated measurements with the thermocouples at fixed temperature. The value of
homogeneity was established before the strain application by using the one gradient method of homogeneity
measurement.
Source of uncertainty Uncertainty (k=2), °C
Uniformity of the furnace (over 12 cm) 3.88
Type N thermocouple 1.1
Type K thermocouple 1.1
Type R thermocouple 0.3
Ice-point 0.01
Multimeter 0.0045
Type A uncertainty for hysteresis measurements 0.0027
Table 3 Uncertainty budget of the experiment taking into account uncertainties of each element used in the
experiment. The uncertainty values of each source are presented in °C.
Conclusions
Homogeneity measurements have been performed on Type N and Type K thermocouples using a uniform
oil bath, both before and after each were bent around a series of circular or angular surfaces. The results
show that a small impact on the homogeneity of Type K thermocouples(up to 0.09 °C) is measureable.
Type N thermocouples, on the other hand, are found to be insensitive to the effects of mechanically induced
strain through bending (no effect was measureable). This test provides a useful method to inform a practical
uncertainty budget, when bending of thermocouples is performed and therefore must be accounted for in the
measurement uncertainty.
Although there are differences in measurement methods, the measured hysteresis show similar behaviour as
in publications [4, 15, 16, 17] with slight differences in the temperature range where the hysteresis occurs.
This is most likely caused by the different source of material used for the thermocouple thermoelements.
The hysteresis tests performed on three base-metal Type N and Type K thermocouples have shown a
tendency for both types to give a repeatable deviation of up to 4 °C from the reference function but the
measurement uncertainty has to be taken into account.
Acknowledgments
This work was completed in the framework of the European Metrology Research Project (EMRP) ENG08
“HiTeMS”. The EMRP is jointly funded by the EMRP participating countries within EURAMET and the
European Union. We thank CCPI, UK for supplying the type K and N thermocouples for testing.
This work was also supported by National Physical Laboratory (NPL), Slovak Institute of Metrology
(SMU), Slovak University of Technology, Faculty of Mechanical Engineering and the Slovak Research and
Development Agency, grant APVV-0096-10 and by the research grants VEGA 2/0038/12 and VEGA
1/0120/12.
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... Over the years as interest in TCs grew, variations in the voltage response of thermoelements due to differences in temperature, orchestrated the exploration and development of many different thermocouples with TC sensitivity and stability at elevated temperatures of the thermoelements being the factors of concern. A wide range of thermocouples made from selected materials ranging from base and noble metals, through refractory metals to thin films are known [7][8][9][10][11][12][13]. But only a few of them have found usefulness as temperature sensors with longer lifespans in extreme operating environments [13]. ...
... The operation of thermocouples constitutes a means for converting thermal energy to electrical energy. TCs are active sensors and therefore are capable of producing their own thermoelectric power [12]. The thermoelectric effect involves transforming temperature differences in material to voltage. ...
Article
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This work reports the performance characteristics of custom thermocouples developed for use in elevated temperatures such as metal casting operations. The scope of this research is limited to thermocouples designed using pyrolytic graphite (PG) as the primary thermoelement in connection with aluminum, copper, steel, and tungsten. The Seebeck coefficients of the sensors were determined from experimental data after heating to ~500 °C. Cooling from ~500 °C to room temperature enabled us to compare the characteristic behaviors of the sensors from the obtained near-linear responses in the voltage-temperature plots. Tungsten being a refractory metal produced the highest sensitivity of the sensors. The sensitivity of the PG-tungsten thermocouple upon heating measured up to 26 μV/°C and a slightly lower value of 24.2 μV/°C was obtained upon cooling. Conversely, the PG-steel thermocouple rather produced the lowest Seebeck coefficients of 13.8 μV/°C during heating and 14.0 μV/°C for the cooling experiments though steel has a high melting temperature than most of the other thermoelements.
... For this reason, it would not be significant to measure temperature just on the core of the sample. Furthermore, the process itself is so fast that a thermocouple would not be meaningfully responsive in such a narrow process window [20]. ...
Article
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Metal–metal composites represent a particular class of materials showing innovative mechanical and electrical properties. Conventionally, such materials are produced by severely plastically deforming two ductile phases via rolling or extruding, swaging, and wire drawing. This study presents the feasibility of producing metal–metal composites via a capacitive discharge-assisted sintering process named electro-sinter-forging. Two different metal–metal composites with CP-Ti/AlSi10Mg ratios (20/80 and 80/20 vol.%) are evaluated, and the effects of the starting compositions on the microstructural and compositional properties of the materials are presented. Bi-phasic metal–metal composites constituted by isolated α-Ti and AlSi10Mg domains with a microhardness of 113 ± 13 HV0.025 for the Ti20-AlSi and 244 ± 35 HV0.025 for the Ti80-AlSi are produced. The effect of the applied current is crucial to obtain high theoretical density, but too high currents may result in Ti dissolution in the Ti80-AlSi composite. Massive phase transformations due to the formation of AlTiSi-based intermetallic compounds are observed through thermal analysis and confirmed by morphological and compositional observation. Finally, a possible explanation for the mechanisms regulating densification is proposed accounting for current and pressure synergistic effects.
... For this reason, it would not be significant to measure temperature just on the core of the sample. Furthermore, the process itself is so fast that a thermocouple would not be meaningfully responsive in such a narrow process window [20]. ...
Preprint
Metal/metal composites represent a particular class of materials showing innovative mechanical and electrical properties. Conventionally, such materials are produced by severely plastically deforming two ductile phases via rolling or extruding, swaging, and wire drawing. This study presents the feasibility of producing metal/metal composites via a capacitive discharge-assisted sintering process named electro-sinter-forging. Two different metal/metal composites with CP-Ti/AlSi10Mg ratios (20/80 and 80/20 %vol) are evaluated, and the effects of the starting compositions on the microstructural and compositional properties of the materials are presented. Bi-phasic metal/metal composites constituted by isolated α-Ti and AlSi10Mg domains with a microhardness of 113 ± 13 HV0.025 for the Ti20-AlSi and 244 ± 35 HV0.025 for the Ti80-AlSi are produced. The effect of the applied current is crucial to obtain high theoretical density, but too high currents may result in Ti dissolution in the Ti80-AlSi composite. Massive phase transformations due to the formation of AlTiSi based intermetallic compounds are observed through thermal analysis and confirmed by morphological and compositional observation. Finally, a possible explanation for the mechanisms regulating densification is proposed accounting for current and pressure synergistic effects.
... For this reason, it would not be significant to measure temperature just on the core of the sample. Furthermore, the process itself is so fast that a thermocouple would not be meaningfully responsive in such a narrow process window [28]. ...
Article
Full-text available
In this study, the efficacy of an innovative ultra-fast sintering technique called electro-sinter-forging (ESF) was evaluated in the densification of Fe-Cr-C steel. Although ESF proved to be effective in densifying several different metallic materials and composites, it has not yet been applied to powder metallurgy Fe-Cr-C steels. Pre-alloyed Astaloy CrM powders have been ad-mixed with either graphite or graphene and then processed by ESF. By properly tuning the process parameters, final densities higher than 99% were obtained. Mechanical properties such as hardness and transverse rupture strength (TRS) were tested on samples produced by employing different process parameters and then submitted to different post-treatments (machining, heat treatment). A final transverse rupture strength up to 1340 ± 147 MPa was achieved after heat treatment, corresponding to a hardness of 852 ± 41 HV. The experimental characterization highlighted that porosity is the main factor affecting the samples’ mechanical resistance, correlating linearly with the transverse rupture strength. Conversely, it is not possible to establish a similar interdependency between hardness and mechanical resistance, since porosity has a higher effect on the final properties.
... Thermocouples are used in various industrial applications for temperature measurement, such as for the monitoring and control processes, to uphold safety, ensure product quality, and regulate their properties [1]. Measurement and compare is a very important part of physics. ...
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Inhomogeneities are known to develop within thermoelements exposed to elevated temperatures, resulting in temperature measurement errors and reduced energy efficiency. During process, it is important to determine how much the inhomogeneities affect the measurement result. Thermoelectric inhomogeneity is important factor of the calibration for budget uncertainty calculation. It is normally assessed by gradual, insertion of a thermocouple into a furnace or liquid bath. But, the length that can be scanned by using this equipment typically limited. It need an equipment to scan more length of wire thermocouple to see characteristic of thermocouple. This paper described development design double gradient method for measurements of inhomogeneities in type K and T thermocouples with various diameter and temperature scanning using hot air that can produce heating process up to 600 °C with some sensor to detection real temperature output from hot air source. Therefore, an equipment with a short, movable heating zone to scan along the thermocouple while both the measuring and reference junctions using ice bath are kept at 0 °C. The design of the equipment system was focused on measuring the stability of hot air and controlling the movement of the stepper motor control using a microcontroller. Interpretation of the measurement results and calculation of inhomogeneity using movable heating zone of type K and T thermocouples of the uncertainty budget is presented.
... Thermocouples function on the Seebeck effect-the thermoelectric signals created along temperature gradients in a wire. Thermocouples place wires of two different metal types into contact, and differences in each materials' rate of change in thermoelectric signals as the temperature of each wire changes scales in a predictable manner [1,2]. This self-generated voltage difference is detectable by electronic dataloggers. ...
Article
Thermocouple probes have long been standard equipment for wildland fire scientists. But despite substantial advancements in the electronic datalogger technology necessary to read and store data from thermocouples, the effective cost per thermocouple sensor of commercial systems has not decreased such that most researchers can afford to deploy enough sensors to account for the high degree of variability in wildland fire behavior. Because the equipment must endure the extreme conditions of wildland fire, is unlikely that any thermocouple datalogger system will be considered “cheap.” However, the growing number of applications of open-source, do-it-yourself (DIY) microcontroller systems in scientific research suggests these products might be employed in thermocouple datalogging systems if (1) their performance can be shown to be comparable to commercial systems and (2) they can be protected from exposure in the wildland fire environment. In this paper, we compare the performance of an Arduino MEGA microcontroller board relative to a Campbell Scientific CR1000, reading standard K-type metal overbraided ceramic fiber insulated thermocouple probes, under the constant temperature of a drying oven and the variable flame of a Bunsen burner. In both comparisons, we found that the variability among individual thermocouples, which are known to have a \(\pm\, 2\,^{\circ }\hbox {C}{-}6\,^{\circ }\hbox {C}\) margin of error, was greater than between the dataloggers. We also describe a compact and mobile Arduino-based system capable of recording wildland fire flame temperatures in agris. In considering these three systems, it is clear that Arduino-based open-source, DIY components can support a compact, low-cost datalogger that accommodates more sensors for lower cost than proprietary commercial systems with no sacrifice in data quality. The combination of low-cost, multi-sensor units can contribute to better understanding of variability in wildland fire behavior.
... Thermocouples sense temperature on the basis of the Seebeck effect thermoelectric signals are created along temperature gradients in a wire. Thermocouples create temperature gradients by putting two wires made of different metals into contact: as the temperature of the surrounding medium rises, the different metals heat at different rates and the thermoelectric signals between them scale with temperature (Shannon and Butler 2003;Pavlasek et al. 2015). Because the voltage difference between the metal types is selfgenerated, thermocouples have low power demands. ...
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Plant ecologists have long been interested in the effects of fire on vegetation. Thermocouples have been in their proverbial toolbox for decades, despite temperature not being a direct product or measure of wildland fire behaviour or fire effects. To better represent the cumulative impact of high-temperature exposure on organisms, ecologists often use temperature–time curves from thermocouples to calculate residence time—the duration of heat exposure above a threshold temperature—which can be used to calculate another popular metric, degree ·seconds. A systematic literature review of 105 published papers shows that residence time, especially, and degree · seconds are common metrics derived from raw temperature–time data. While several errors in thermocouple readings have been previously identified and addressed—responsiveness to heating, discrepancy between thermocouple temperature and actual temperature of the medium surrounding the thermocouple—this paper highlights a previously unconsidered source of error that must be reconciled for metrics like residence time to be biologically valid: the disproportionately long time it takes for thermocouples to cool once heat input is complete. Using an array of thermocouples in a fume hood over a Bunsen burner before and after the flame is extinguished, this paper shows that after being exposed to flame, 30-gauge K-type thermocouples require 80–100 s to register ambient temperatures despite taking only about 5 s to respond to heating. The review indicates ecologists give no consideration for this disproportionately slow cooling response. These findings indicate that residence time (and therefore degree · seconds) have been over-estimated in the fire ecology literature. The proposed solution is to simply truncate temperature–time curves at the point temperature begins to decline, which indicates a shift from the biologically relevant effect of heat input to the biologically irrelevant, physical properties (heat diffusivity) of the thermocouple itself. Conceptual models present these biologically relevant portions of the temperature–time curve and identify parts of the biologically relevant curve that might be useful in quantifying components of flammability.
... 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. ...
Article
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.
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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.
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The voltage of thermocouple during process contribution is determined by the absolute thermal electromotive force coefficient in each wire and the temperature difference across the section. This paper described the development of a system as a local heating method for measurements of inhomogeneities in type T thermocouples with various temperature scanning using hot air. The system could control automatically measurement voltage, temperature, distance, and speed of the stepper motor for moving the hot air scanning along with the thermocouple and control with temperature monitoring. Both the measuring and reference junctions were using an ice bath kept at 0 °C. In this paper calculation of interpretation of the measurement results of inhomogeneity using movable heating zone system of type T thermocouples and the system could detected inhomogeneity of thermocouple and there have different characteristics of voltage and seebeck effect.
Article
Following is a continuation of the list of titles and authors: Use of Superconductors to Provide Fixed Points on a Cryogenic Temperature Scale. By J. F. Schooley and R. J. Soulen Jr. Reproducibility of the Triple Point of Equilibrium Hydrogen. By M. Takahashi and T. Mochizuki. Realization of Low Temperature Fixed Points on the NBS Acoustical and the NBS-1955 Temperature Scales. By George Cataland and H. H. Plumb. Realization of Low Temperature Fixed Points. By J. P. Compton. Purification of Oxygen by Zone Melting. By J. Ancsin. Transition Temperatures of Solid Oxygen. By W. R. G. Kemp and C. P. Pickup. Apparatus for Realizing the Triple Point of Oxygen. By W. Thomas and W. Blanke. Triple Point of Argon. By George T. Furukawa, William R. Bigge and John L. Riddle.
Article
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.
Article
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.
Article
The irreversible changes in the Seebeck coefficient that occur in mineral-insulated Nicrosil-sheathed thermocouples were measured during 530 h of heating at temperatures up to 1200 degrees C. Type K and type N alloys from seven sources were examined. Most change occurred in the negative thermoelements whose behaviour depended on whether Mn and Al was present and not on whether they were type K or type N alloys. The presence of Mn and Al gave the more complex behaviour but reduced instability at high temperatures. It is shown that Nicrosil-sheathed temperature probes of 6 mm diameter and having Mn- and Al-bearing type K alloys would drift less than about 3 degrees C during a approximately 1000 h use at temperatures up to 1200 degrees C. Similar probes with type N thermoelements would drift up to 5 degrees C.
Article
EMF drift and changes in Seebeck coefficient were examined in metal-sheathed Ni-based thermocouples held at temperatures in the range 500-1100 degrees C for up to 1000 h. In 200 h, EMF drifts of up to the equivalent of 10 degrees C were observed and a subsequent reduction in immersion caused a drop of 80 degrees C. The analysis of X-ray spectra showed the presence and migration of Mn to be the main contributor to instability. The most stable configuration examined was the Inconel-sheathed Nicrosil/Nisil thermocouple. It is suggested that the optimum Ni-based thermocouple is one having Nicrosil and Nisil thermoelements and a Nicrosil sheath.
Article
Hysteresis in the Seebeck coefficients of the thermocouple alloys Nicrosil and Nisil was studied over the temperature range, 200 to 1200 degrees C, for heating periods up to 300 h. The alloys were thermoelements in Nicrosil-sheathed mineral-insulated cable. For both alloys hysteresis peaks were found near 400 and 700 degrees C. In Nicrosil the lower temperature peak was small (<0.05 mu V K-1) and the peak near 700 degrees C increased rapidly to about 0.3 mu V K-1) for 50 h of heating and changed little thereafter. Both peaks in Nisil were about 0.2 mu V K-1 in magnitude after 100 h of heating and increased steadily with time, reaching 0.3 mu V K-1 at 300 H. The study showed that hysteresis was the main cause of instability in such thermocouples and suggested that hysteresis in Nisil could be greatly reduced during manufacture.
Article
A criticism is made of the methods available for selecting metallic wires to be used in the construction of thermoelectric thermometers. Methods are described which increase the reliability, rapidity and convenience of selection tests. Techniques are introduced for the construction of more homogeneous thermocouples and multi-junction thermoelements from a test cable of the wire under consideration, thus extending the range of application of the thermoelement to the precise measurement of larger temperature differences and the validity of its calibrations to more varied conditions. Examples are given of the application of these procedures to the construction of low temperature copper/constantan thermometers.
Aging of Chromel Alumel Thermocouples
  • V A Callcum
V.A. Callcum, Aging of Chromel Alumel Thermocouples, (UKAEA) Report 1021 (United Kingdom Atomic Energy Authority, Culham, UK, 1965)
Mechanical Stability of Pt/Pd Thermocouples, CCT/03-10, BIPM open-access document
  • F Edler
  • H Lehmann
F. Edler, H. Lehmann, Mechanical Stability of Pt/Pd Thermocouples, CCT/03-10, BIPM open-access document (2009), http://www1.bipm.org/cc/CCT/Allowed/22/CCT03-10.pdf