Inhomogeneity Of Thermocouples Research Articles

Real-Time Monitoring System for Thermocouple Inhomogeneity Equipment

Thermocouple inhomogeneity equipment was developed to measure the Seebeck coefficient characteristic in each wire position based on the effect of temperature exposure along the thermocouple wire. Temperature and distance sensors are used to measure the temperature output of the heating system and the movement of the thermocouple position. We developed a real-time measurement system to monitor the measurement of the thermocouple sensor based on a microcontroller. This monitoring system is needed to monitor changes in the ice bath during measurement, if the ice bath was unstable or being melting, it can affect the output voltage of the thermocouple. The program was compiled and uploaded to ESP 32 then monitoring data with an open-source internet of things called Thingspeak was used as a platform to retrieve and display the collected data. Real-time monitoring of distance from ultrasonic and temperature of heat source (heat gun) can be accessed through web applications and smartphones. The data were analyzed based on conditions for Seebeck value of inhomogeneity and validated with standard calibration using the liquid bath. This study has been considered successfully implemented that performs on IoT platforms and was adaptively ready to monitor thermocouple inhomogeneity equipment.

Design of a scanning thermocouple inhomogeneity

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.

DECREASING OF THERMOCOUPLE INHOMOGENEITY IMPACT ON TEMPERATURE MEASUREMENT ERROR

The subject of research is accurate temperature measurements by thermocouples whose legs undergone degradation during long term operation at high temperatures. The degradation rate is approximately proportional to the temperature of constant operation of the sections of thermocouple legs and time of operation. Degradation causes two types of errors – due to drift of the conversion characteristic (its gradual change during operation) and due to the acquired thermoelectric inhomogeneity of thermocouple legs (manifests itself as a change in thermocouple conversion characteristic when changing the temperature field even at constant temperatures of the measuring and reference junctions). The aim of the article is to study the method of increasing the accuracy of temperature measurement by thermocouples that have significant thermoelectric inhomogeneity acquired during operation. The aim is achieved by stabilizing the temperature field along the thermocouple legs, so error due to acquired thermoelectric inhomogeneity cannot manifest itself. The tasks of the paper are to estimate the error of temperature measurement using the thermocouple, develop the method for increasing accuracy of temperature measurements by inhomogeneous thermocouples, means of stabilization of the temperature field, methods of control of the temperature field, and experimental study the proposed control methods. There are well-known methods used such as the design of equipment with given parameters and limitations, calculations by solving a system of linear equations, the formation of the necessary functional dependence using a neural network, experimental studies. The obtained results are as follows: stabilization of the temperature field makes it possible to reduce the influence of acquired thermoelectric inhomogeneity of thermocouples, which, according to the literature, can reach 11°С to 1,3°С. It is shown that the developed multi-zone tubular furnace for stabilizing the temperature field due to a close thermal connection between the zones, requires a multi-channel controller that is not prone to self-excitation. There are two methods of control of the temperature field are proposed. One based on the solution of a system of linear equations. The second method is based on the neural network, in which the neural network is trained directly on a multizone furnace. Conclusions. As shown by experimental studies, the proposed methods do not allow the error of the acquired thermoelectric inhomogeneity of thermocouples to manifest itself, which provides an opportunity to increase the accuracy and metrological reliability of temperature measurements by existing types of thermocouples.

Temperature Dependence of Thermoelectric Homogeneity of Noble Metal Thermocouples

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.

Development and Investigation of the Method for Compensating Thermoelectric Inhomogeneity Error

The method proposed in this paper is one of the best methods for neutralizing the impact of error due to acquired thermoelectric inhomogeneity of thermocouples’ electrodes. This method’s main idea is stabilization of temperature along the thermoelectrodes of the thermocouple that measures temperature of an object. The description of a furnace design that provides a stable temperature field, as well as the methods for controlling the temperature field profile, together with their experimental evidence, is also provided.

Methods of Reducing the Effect of the Acquired Thermoelectric Inhomogeneity of Thermocouples on Temperature Measurement Error

The effect of the error of thermoelectric transducers caused by thermoelectric nonuniformity of the thermocouple electrodes, acquired during long-term operation, on the results of temperature measurements is analyzed using well-known correction methods. Ways of developing new correction methods and for compensating errors are proposed.

Thermocouple homogeneity scanning

The inhomogeneities within a thermocouple influence the measured temperature and contribute the largest component to uncertainty. Currently there is no accepted best practice for measuring the inhomogeneities or for forecasting their effects on real-world measurements. The aim of this paper is to provide guidance on the design and performance assessment of thermocouple inhomogeneity scanners by characterizing the qualitative performance of the various designs reported in the literature, and developing a quantitative measure of scanner resolution. Numerical simulations incorporating Fourier transforms and convolutions are used to gauge the levels of attenuation and distortion present in single- and double-gradient scanners. Single-gradient scanners are found to be far superior to double-gradient scanners, which are unsuitable for quantitative measurements due to their blindness to inhomogeneities at many spatial frequencies and severe attenuation of signals at other frequencies. It is recommended that the standard deviation of the temperature gradient within the scanner is used as a measure of the scanner resolution and spatial bandwidth. Recommendations for the design of scanners are presented, and include advice on the basic design of scanners, the media employed, operating temperature, scan rates, construction of survey probes, data processing, gradient symmetry, and the spatial resolution required for research and calibration applications.

The Mechanism of the Occurrence of Acquired Thermoelectric Inhomogeneity of Thermocouples and its Effect on the Result of Temperature Measurement

The occurrence of an error due to the thermoelectric inhomogeneity of thermocouple electrodes, acquired during prolonged use, is considered, and the properties of this error are formulated. A mathematical model of the error of chromel and alumel electrodes is constructed, the error in measuring the temperature is estimated, and the most dangerous mode of operation of the thermocouples is revealed.

Final report of APMP.T-S5: APMP regional comparison of Au/Pt thermocouples from 0 °C to 960 °C

A regional supplementary intercomparison, APMP.T-S5, of Au/Pt thermocouples using ITS-90 fixed points from 0 °C to 960 °C was carried out, involving six National Metrology Institutes from the Asia-Pacific region: NMIA, SCL, NMIJ, KRISS, NMC and NMISA. This is a round robin comparison with two Au/Pt thermocouples designed by the pilot lab (NMIA). Both of these thermocouples were calibrated by the pilot lab twice: initially and finally at the end of circulating among other participating laboratories.Reference values computed using the simple mean, median and weighted mean were mutually consistent, but since the Birge ratio criterion was safely satisfied, the weighted mean was adopted, achieving a k = 2 uncertainty for the reference value ranging from 10 mK to 16 mK. The thermocouple inhomogeneity, which is the dominant component of the calibration, was measured by the pilot lab twice: initially and at the end of all measurements. The participating labs were asked to supply the temperature profile of the furnaces used to realize the fixed point calibrations. These data were convolved with the measured thermocouple inhomogeneity to estimate the systematic error in the temperature difference measured between the participants, due to the different temperature gradient of the furnaces. With this analysis, the uncertainty component due to the inhomogeneity was reduced from ±0.004% to below ±0.0015% and consequently allowed some participating laboratories to demonstrate agreement with the reference value to within an uncertainty of 16 mK at the Ag point.This paper is the final report of the comparison including analysis of the uncertainty of the reference value. This comparison demonstrated that the Au/Pt thermocouples can be used to compare the calibration capabilities of the laboratories at the 16 mK level at the silver (Ag) point and 10 mK at the aluminium (Al) point. The results from all laboratories were consistent with the reference value within the participants' stated 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 APMP, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

Evaluating the Inhomogeneity of Thermocouples Using a Pressure-Controlled Water Heat Pipe

There exists various research reports concerning the evaluation methods for the measurement uncertainty due to inhomogeneity of thermocouples; however, the universal method is still waiting to be established. This article considers the evaluation methods for the measurement uncertainty due to inhomogeneity of thermocouples based on comparison between results of two measurement methods. The first method is to estimate the uncertainty from the immersion characteristics of a thermocouple within a fixed-point furnace during its realization. The second method is to estimate the uncertainty from the immersion characteristics of a thermocouple within a heat-pipe furnace with a long uniform region. A pressure-controlled water heat-pipe furnace with an immersion depth of 1000 mm is developed to enable this work. It overcomes the technical difficulties that existed in applying conventional sealed heat pipes to such applications. From the immersion characteristics of a thermocouple measured by the above two methods, we have introduced three measurement parameters. Estimating the measurement uncertainty due to the inhomogeneity from our measurement results as examples is discussed.

Quantitative determination of the uncertainty arising from the inhomogeneity of thermocouples

The increasing prevalence of elemental thermocouples, Au/Pt and Pt/Pd, in place of those using alloys of platinum and rhodium, has dramatically reduced measurement uncertainties at temperatures above the range of platinum resistance thermometers, to the extent that the single largest contribution to the uncertainty budget is often that due to the inhomogeneity of the Seebeck coefficient in the thermoelements along the length. This uncertainty must be estimated as an upper limit because the temperature gradient experienced by the thermocouple during calibration may be very different to that used in the field. In the worst case uncertainties of hundreds of mK are possible, although often the profiles are quite similar and the uncertainties are much lower than this. Here a method is presented for quantitatively determining the uncertainty in emf arising from inhomogeneity, by employing an experimentally determined sensitivity as a function of distance along the thermocouple, and numerically evaluating the effect of this sensitivity by considering its influence on all possible linear temperature gradients that the thermocouple can experience. It is then possible to determine the uncertainty due to the inhomogeneity within a desired confidence interval.