Intrinsic Thermocouple Research Articles

Local temperature surface measurement with intrinsic thermocouple

This paper reports on a technique based on the well-known Seebeck effect to measure local surface temperature. The aim of this work is to increase the spatial resolution of temperature measurement performed using thermocouples. The method relies on an intrinsic thermocouple consisting of a conducting tip put in contact with a conducting sample. The tip is made of nickel or constantan wires fabricated using an electrochemical etching process. This paper shows that the electric behavior of an intrinsic junction is similar to the one of a standard soldered thermocouple. The main difference between these two techniques is the localization of the measure. For a standard soldered thermocouple, the temperature value is the result of an average of the real temperature of the weld volume while for an intrinsic thermocouple the temperature information is localized in the contact area. The first experimental results actually confirm that a better spatial resolution can be obtained in the case of the intrinsic method, compared to a standard soldered thermocouple used in the same experimental conditions.

Symbolic Computation of Correction Function for Polynomial Time Response of Intrinsic Thermocouples

A new exact expression is obtained for the coefficients of a correction function for an intrinsic thermocouple reading, expressed in a polynomial form, also including half-order polynomial terms. The obtained expression gives as special cases the coefficients reported in the recent literature for a typical third order polynomial response in time. The solution is critically examined against the numerical Laplace transform inversion approach. All the symbolic and numerical computations are performed by making use of the Mathematica software system.

A Duhamel Integral Based Solution for the Sideways Heat Equation Applied to the Non-intrusive Temperature Measurement

In the present study, an inverse heat conduction problem is considered, the Sideways Heat Equation, which is a model of a problem, where one wants to determine the temperature on both sides of a thick wall, but where one side is inaccessible to measurements. Considering the problem of an intrinsic thermocouple attached to a slab, and no thermal contact, a transfer function is proposed. It is based on Duhamel Integral and the measured response is approximated by a quasi-periodical function. The derived transfer function is the product of two transfer functions, for the slab and for the intrinsic thermocouple respectively. To validate the present model, a comparison with actual measurements is carried out. It was found that the present model can predict the inner temperatures more accurately than the previous model, which did not take into account the effect of the thermocouple.

Actively Cooled Calorimeter Measurements and Environment Characterization In a Large Pool Fire

Quasi-steady-state heat fluxes absorbed on the calorimeter surface in ten vertical 0.1 m high×1 m wide zones were measured by means of water calorimetry. The calorimeter surface also included an array of intrinsic thermocouples to measure surface temperatures, and an array of Schmidt – Boelter radiometers for a second, more responsive, method of heat flux measurement. The pool fire environment characterization was done with measurements from velocity probes, directional flame thermometers (DFTs), and thermocouples. The initial measurements with a 1 m×1 m water cooled vertical flat plate calorimeter located 0.8 m above and inside a 6 m×6 m JP-4 pool fire are described. Water calorimetry measured absorbed surface heat fluxes of about 65–70 kW m−2 with a gradual decrease with increasing height above the pool. Intrinsic thermocouple measurements recorded typical calorimeter surface temperatures of about 500°C, with spatial variations of ±150°C. Gas velocities across the calorimeter face averaged 3.4 m s−1 with a predominant upward component, but with an off-vertical skew. Analysis of data collected in the fire environment in the vicinity of the calorimeter was performed to characterize the fire environment and to determine the input parameters required to calibrate analytical models. For this test, the emissive power distribution near the plate was essentially linear. Flux measurement in the fire environment ranged from 75 kW m−2 to 175 kW m−2. With temperature and heat flux data, effective absorption coefficients were determined by using a two-flux method to solve the inverse problem. The results show that the optical thickness increases with increasing distance from the calorimeter surface. The effective absorption coefficient is approximately 0.8 m−1 in the vicinity (0 – 1.85 m) of the calorimeter and is approximately 2 m−1 in the vicinity (1.85 – 2.8 m) of the plume centerline. The observed decrease in heat flux on the calorimeter surface with increasing vertical height is consistent with analytical fire models derived for constant temperature surfaces. Results from several diagnostics also indicated trends and provided additional insight into events that occurred during the fire. Some events are correlated, and possible explanations are discussed.

Measurements of temperature fluctuations in liquid sodium using fast-response thermocouples

Installed components within the primary circuit of a nuclear reactor can be thermally stressed by temperature fluctuations that occur within the coolant. This problem is more acute in the liquid-metal-cooled fast breeder ractor because of the much higher heat transfer coefficients between the sodium coolant and the immersed structures. Experiments to investigate this problem rely on thermocouples to monitor the fluctuations that occur in practice, but because of the finite time constatns of such devices the signals are attenuated. Because of this measurement limitation it is difficult to establish the real magnitude of the fluctuations, and although corrections can be applied there is a need for better instrumentation to improve the quality of the results obtained. This paper described work carried out at Berkeley Nuclear Laboratories to develop a fast-response thermocouple for this purpose. This thermocouple, because of its unique construction, has a junction mass approaching zero, and hence its frequency response should be very high. Some of the problems of manufacture are discussed, in particular the high quality of seal required to avoid ingress of liquid metal. The results obtained with the fast-response thermocouple are compared with those obtained with conventional stainless-stell-sheated thermocouples. The improved response of the new thermocouple is clearly visible, confirming that measurements made with sheathed thermocouples suffer attenuation. A comparison is also made between the fast-response thermocouple and an intrinsic thermocouple. Spectral information for each device is discussed, and it is shown that the cutoff gradients in the inertial subrange within liquid metals are Reynolds number dependent as predicted by theory. It is concluded that results obtained with the fast-response thermocouple are close to the real magnitude of temperature fluctuations present in turbulent flow. It is also demonstrated that with suitable corrections results obtained with sheathed thermocouples can be used to estimate the real signals present in the flow.

Comparison of computed frequency responses of intrinsic and encapsulated bead type thermocouples in liquid sodium

A comparative, theoretical study of intrinsic and conventional sheathed, beaded thermocouple frequency responses has been carried out for a limited range of heat impulses in order to highlight differences in their responses for measurements in liquid sodium flows such as are met in exit plenums of nuclear reactor sub-assemblies. The mathematical model consists of a thin impulse layer surrounding the thermocouple, all of which is immersed in a pool of sodium. Only axisymmetric cases are considered for ease of computation but different positions and thicknesses of the impulse layer are included in the study. In the range of normally occurring frequencies of temperature fluctuations in the exit plenum of reactor sub-assemblies, the intrinsic thermocouple is seen to have almost a perfect response but only if the heated layer is in contact with the wire attachment point of the couple. This is attributed to the fact that sodium surrounding the thermocouple and impulse layer dominates the heat diffusion due to its very high thermal conductivity. The sheathed, beaded thermocouple has a poor frequency response even in the range of normally occurring frequencies.

Intrinsic thermocouple analysis using multinode unsteady surface element method

The intrinsic thermocouple problem has received considerable attention over the last two or three decades. The most common geometry for the intrinsic thermocouple problem is a semi-infinite cylinder (wire) attached perpendicularly to a semi-infinite solid (substrate). In this paper the transient thermal behavior of the interface between the thermocouple wire and the substrate due to step change in the substrate temperature is investigated using the multinode unsteady surface element (MUSE) method. The method uses Duhamel's integral and involves the inversion of a set of Volterra integral equations, one for each surface element. The solution is obtained for a wide range of dimensionless times. The results show excellent agreement between the present solution and the other existing solutions. Very high accuracy is attainable with a relatively small number of surface elements for the complete time domain. This feature makes the method superior to alternative numerical techniques such as finite difference or finite element that involve whole-body discretization.

Simplified Laplace Transform Inversion for Unsteady Surface Element Method for Transient Conduction

The unsteady surface element method is an analytical procedure for solving certain types of transient heat conduction problems. The method is intended for thermally contacting bodies of similar or dissimilar geometries such as occur in the contact conductance, fin, and temperature measurement problems. The method utilizes Duhamel's integral. The two simplified procedures in this paper utilize an approximate inverse Laplace transform relation given by Schapery to obtain functional forms of the solutions. In certain cases the approximate inverse Laplace relations simplify the solution greatly and produce acceptable accuracy. Tauberian, series, and numerical tests define these successful cases. The results presented are useful for the intrinsic thermocouple problem and as background for more general heat conduction problems.

Unsteady Surface Element Method

A method for the solution of transient heat conduction problems, called the unsteady surface element (USE) method, is developed and applied to several problems. The method is intended for thermally contacting bodies of similar or dissimilar geometries such as occur in contact conductance and intrinsic thermocouple problems. The method utilizes Duhamel’s integral in several ways. Two different procedures are presented, one utilizing temperature-based kernels and the other uses heat flux-based kernels. One of the given applications is to the intrinsic thermocouple problem. Several solutions are given and the results agree very well with two finite difference solutions.

The effect of temperature dependent properties on transients measurement with intrinsic thermocouple

Solution of the transient response of intrinsic thermocouples by Henning and Parker are expanded to include temperature dependent properties. Finite difference and Runge-Kutta numerical solutions are compared with the analytical solution. Results for thermocouples for which the temperature variability is significant are presented. The approach allows the calculation of large temperature steps. An experimental method is described by which large step temperature increases can be simulated. The method also allows for large dimension substrate and thermowire, which reduce errors and slow-down real time performance to simplify measurements. Experiment and calculations agree well after the very initial response. It is suggested that the experiment described is suitable for the measurement of the prompt response of intrinsic thermocouples.

Transient Response of an Intrinsic Thermocouple

Where rapid heating is encountered, as in a prompt-burst nuclear reactor, a thermocouple with a rapid time-response is necessary to monitor the temperature of the specimen in question. Because all thermocouples have some mass, they cannot have an infinitely fast response. However, an intrinsic-type thermocouple has less thermal inertia than the usual welded-bead type. Thus it is a natural choice for the measurement of transient surface temperatures of conducting solids. In this report an analytical expression is obtained for the transient response of an intrinsic thermocouple. In support of the analysis, experimental data are presented which were obtained by means of a capacitor bank pulse-heating technique. It is concluded that thermocouple wires of small diameter and low thermal conductivity respond the fastest. As an example, a 1-mil constantan wire on a copper substrate produces 95 percent of the steady-state emf in 3 μsec.

Closure to “Discussions of ‘Transient Response of an Intrinsic Thermocouple’” (1967, ASME J. Heat Transfer, 89, pp. 152–154)

Closure to “Discussions of ‘Transient Response of an Intrinsic Thermocouple’” (1967, ASME J. Heat Transfer, 89, pp. 152–154)