Response Time Of Thermocouples Research Articles

Thermocouple response time estimation and temperature signal correction for an accurate heat flux calculation in inverse heat conduction problems

Many heat transfer processes do not allow temperature measurements with advanced instruments like IR cameras, and thus require the use of thermocouples – often inserted sheathed thermocouples – in order to measure the temperature in some key-regions. Nevertheless, thermocouples have an intrinsic response time that can dampen substantially temperature measurements and, when applicable, affect heat flux estimations by inverse methods. In this study, a thermocouple measurement correction method is proposed, especially for fast thermal transients like quenching, to avoid underestimation of the boundary heat fluxes due to delayed temperature responses. A simplified energy balance at the hot junction allowed modeling the temperature measurement by a thermocouple, in which heat losses and the response time were taken into account so they can be estimated using the least squares method on experimental data. A series of tests was performed to validate the present method with a heated jet impinging on a copper body instrumented with two thermocouples: one ungrounded and sheathed, then with high response time (”slow”); and another with exposed welded wires, hence with low response time (”fast”). After having estimated the slow thermocouple response time and heat loss parameters, the model allowed a good reconstruction of the fast thermocouple signal using the slow thermocouple measurements. Also, the heat flux estimation by the inverse method using the reconstructed signal resulted in practically the same results obtained using the fast thermocouple data. A parametric sensitivity analysis showed the heat loss parameters can be neglected for heat flux estimations, while simulations showed that temperature noises degrade the response time estimation but data filtering can mitigate this noise effect. Finally, the present method was applied to a large scale jet-cooling of a hot plate – near industrial conditions – and showed a substantially higher dissipated heat flux than the initially estimated. In fact, the results using reconstructed signals demonstrated that the heat flux dissipated by the jet has a higher dependence on the jet Reynolds number than the observed with the original thermocouple measurements, showing how important the temperature measurement correction is to evaluate fast thermal processes.

Impact of bed motion on the wall-to-bed heat transfer for powders in a rotary kiln and effect of built-ins

This study analyses experimentally the heat transfer from the wall to the particle bed in a rotating drum with and without addition of built-ins for improving the mixing of the powder. Two types of materials were employed, sand as a granular material (dp,mean=200μm) and cement raw meal as a powdery material (dp,mean=15μm). A rotating cylinder heated through the outer shell was operated in batch mode, with rotational speeds of 1, 2 and 3 rpm and fillings of 5 and 10 %. The sand achieved a rolling motion, which makes the measured heat transfer coefficient predictable with theoretical models. At the contrary, cement raw meal showed very little mixing in a cylinder without built-ins and the measured heat transfer coefficient was much lower than the model predictions. Using longitudinal strips along the wall caused the powder to collapse regularly, increasing its heat transfer by a factor of 2–3, up to the range of predictions from theoretical models. Thus, a motion comparable to the rolling motion can be established with such built-ins which only cause limited dust formation. Furthermore, the limitations of the measurement method with rotating thermocouples are pointed out which mostly stem from the thermocouple response time and convectional losses, which are increased once built-ins are employed.

Experimental Characterization of Heat Transfer Coefficients During Hot Forming Die Quenching of Boron Steel

The heat transfer coefficient (HTC) between the sheet metal and the cold tool is required to predict the final microstructure and mechanical properties of parts manufactured via hot forming die quenching. Temperature data obtained from hot stamping experiments conducted on boron steel blanks were processed using an inverse heat conduction algorithm to calculate heat fluxes and temperatures at the blank/die interface. The effect of the thermocouple response time on the calculated heat flux was compensated by minimizing the heat imbalance between the blank and the die. Peak HTCs obtained at the end of the stamping phase match steady-state model predictions. At higher blank temperatures, the time-dependent deformation of contact asperities is associated with a transient regime in which calculated HTCs are a function of the initial stamping temperature.

Detailed multiphysics modeling and validation of thermocouple readings in fires

Detailed CFD models of mineral-insulated metal-sheathed (MIMS) thermocouples are discussed and thermocouple readings in fire environment are validated against experimental thermocouple data for a Hencken burner flame. Thermocouple readings are presented as functions of equivalence ratio, thermocouple diameter, thermocouple orientation, position of the junction, depth of insertion into fire, and sheath surface emissivity. Effects of surface oxidation, sheath diameter, and insertion depth on thermocouple response time are analyzed in detail. Computational results are compared with published experimental data. Overall, predictions showed excellent agreement with experimental data. Small deviations occurred in the case of insertion depths due to axial heat losses.

Heat Transfer Control During Quenching

The aim of this article is to discuss the measures to control the dynamic of heat extraction by changing some parameters during quenching. This is possible for workpieces of not too small a cross-section size, because transformation of the microstructure proceeds gradually from the surface to the core only when a particular point attains the temperature A 1. Differences between calculation of the temperature dependent heat transfer coefficient (HTC) for laboratory specimens and for real workpieces, taking into account the damping effect, the time lag, and the thermocouple response time, have been discussed. High Pressure Gas Quenching (HPGQ) in vacuum furnaces is especially prone to changing some parameters during quenching. To increase its quenching intensity, in order to attain high enough hardness in the core, the gas pressure and/or its flow velocity can be increased. When this measure is combined with the transient spraying of liquid nitrogen, a new Controllable Heat Extraction (CHE) Technology can be developed. Moreover, if the furnace is provided with a necessary control system and software program, fully automatic control of the heat extraction during quenching, which guarantees repeatable hardness results, is possible.

Assessment of effective thermal product of surface junction thermocouples on millisecond and microsecond time scales

Surface junction thermocouples are used extensively for transient heat flux measurements, but their accuracy is dependent on the effective thermal product (TP) of the gauge and this can be a function of the time scale of interest. In the present work the response of surface junction k-type thermocouples was investigated experimentally using a water droplet calibration technique (for millisecond times scales) and a small shock tube (for microsecond time scales). Different junctions formed by scalpel blade scratches and abrasive paper were investigated. When scratches from scalpel blades were used to form the junction, the TP identified from the water droplet calibrations consistently differs by approximately 20% depending on whether the junction was made on the chromel or alumel substrate, in accord with existing thermal properties data. However, the shock tube calibrations indicate that for scalpel-scratched junctions there is considerable variability in thermocouple response time due to effective junction depth variations produced during construction. In contrast, junctions formed with abrasive paper produced rise times consistently less than 1s, but the water droplet and shock tube experiments both indicated significant variability in the effective TP for these gauges. The consistency in TP for scalpel-scratched junctions for millisecond time scales and the variability for junctions created with abrasive grit for both the millisecond and microsecond time scales is attributed to the differences in the effective proximity of the junction to the insulation between chromel and alumel substrates. For junctions created with abrasive grit, the effective TP is approximately 30% smaller for microsecond time scales than it is for millisecond time scales.

Local velocity measurements in a thermally-stratified sodium mixing layer using a permanent-magnet probe

Abstract Mean and fluctuating velocities have been measured in a sodium mixing layer experiment, i.e. in a fluid with very low Prandtl number ( Pr ⩽10 −2 ), with a miniature permanent-magnet velocity probe in the presence of strong temperature gradients. A mathematical model for the probe, based upon Faraday's law of induction and including thermoelectric as well as inertia effects due to the finite response time of thermocouples, is presented together with a new dynamic method to compensate for these effects. The sensitivity of the four different probes used in this experiment is in the range of 81–65 (μV/ms −1 ). Electrical pertubations arising from large-scale thermoelectric effects inside the test section and their influence on the velocity signal are also discussed. The electronic measurement system, combining low noise and high resolution, was specially developed to match the experimental requirements. With this system it was possible to measure velocity RMS-values down to 1 mm/s corresponding to a voltage of 100 nV, and mean velocities with an accuracy of about 6 mm/s. This paper deals with the peculiarities of the measurement technique and its performance, but does not analyze the experimental results, which will be presented in a separate publication.

Transient temperature measurement using embedded thermocouples

The response time of thermocouples is generally considered to be a limiting factor when transient temperature changes need to be assessed in solids. As an example, transient temperature changes which develop during dynamic straining of materials, adiabatic shear band formation, dynamic fracture and related fields are often investigated using sophisticated noncontact measurement techniques such as infrared detectors. In these phenomena, the time scale is of the order of the microsecond. In this paper, the authors revisit the application of thermocouples to such measurements using small embedded thermocouples (ETC). Experiments with dynamically loaded polymeric disks (characteristic strain rate of 103 s−1) show that the thermocouples record transient temperatures with a short typical rise time of 10 μs as a result of the conversion of plastic deformation into heat. This observation is corroborated by the solution of the temperature distribution in a sphere subject to constant surface temperature which predicts the same fast reaction. Specifically, considering a sphere which is representative of the sensing bead, the average temperature is shown to rise in a few microseconds. These theoretical results can be used to deconvolve the experimental results with respect to a calculated impulse response of the sensor to recover the actual temperature variations. The results show that small thermocouples can be embedded to yield useful information about the transient temperature evolution in a solid. This technique is easy to use and provides an important complement to other noncontact techniques.

Accurate measurement of airway thermal challenge.

Rapid and accurate measurement of air temperature is needed to determine both heat and water losses from the respiratory tract during breathing. We have designed, constructed and validated a low dead-space, two-channel valve/mouthpiece which prevents mixing of inspired and expired air outside the body. The effects of airflow on the thermocouple response time and of thermocouple position within the airstream on the temperature measured were assessed. The 63% thermocouple response time fell from 159 to 78.8 ms as airflow rose from 23.7 to 297 1 min-1, a range including the peak flows observed while breathing at rest and during exercise. Measurement of temperature profiles across the valve/mouthpiece at different flows indicated that thermocouples were best positioned at the centre of the airstream, though small deviations did not adversely affect accuracy.

In-core coolant flow monitoring of pressurized water reactors using temperature and neutron noise

Noise measurements were performed at the Loss-of-Fluid-Test (LOFT) and Sequoyah-1 pressurized water reactors (PWRs) in order to investigate the possibility of inferring in-core coolant velocities from cross-power spectral density (CPSD) phases of core-exit thermocouple and in-core neutron detector signals. These noise measurements were used to investigate the effects of inlet coolant temperature, core flow, reactor power, and random heat transfer fluctuations on the noise-inferred coolant velocities. The effect on the inferred velocities of varying in-core neutron detector and core-exit thermocouple locations was also investigated. Theoretical models of temperature noise were developed, and the results were used to interpret the experimental measurements. Results of these studies indicate that the neutron detector/thermocouple phase is useful for monitoring core flow in PWRs. Our results show that the interpretation of the phase between these signals depends on the source of temperature noise, the response times and locations of the sensors, and the neutron dynamics of the reactor. At Sequoyah-1 we found that the in-core neutron detector/core-exit thermocouple phase can be used to infer in-core coolant velocities, provided that the measurements are corrected for the thermocouple response time.

In-core coolant flow monitoring of pressurized water reactors using temperature and neutron noise

Noise measurements were performed at the Loss-of-Fluid-Test (LOFT) and Sequoyah-1 pressurized water reactors (PWRs) in order to investigate the possibility of inferring in-core coolant velocities from cross-power spectral density (CPSD) phases of core-exit thermocouple and in-core neutron detector signals. These noise measurements were used to investigate the effects of inlet coolant temperature, core flow, reactor power, and random heat transfer fluctuations on the noise-inferred coolant velocities. The effect on the inferred velocities of varying in-core neutron detector and core-exit thermocouple locations was also investigated. Theoretical models of temperature noise were developed, and the results were used to interpret the experimental measurements. Results of these studies indicate that the neutron detector/thermocouple phase is useful for monitoring core flow in PWRs. Our results show that the interpretation of the phase between these signals depends on the source of temperature noise, the response times and locations of the sensors, and the neutron dynamics of the reactor. At Sequoyah-1 we found that the in-core neutron detector/core-exit thermocouple phase can be used to infer in-core coolant velocities, provided that the measurements are corrected for the thermocouple response time.

Summary of the session

Noise measurements were performed at the Loss-of-Fluid-Test (LOFT) and Sequoyah-1 pressurized water reactors (PWRs) in order to investigate the possibility of inferring in-core coolant velocities from cross-power spectral density (CPSD) phases of core-exit thermocouple and in-core neutron detector signals. These noise measurements were used to investigate the effects of inlet coolant temperature, core flow, reactor power, and random heat transfer fluctuations on the noise-inferred coolant velocities. The effect on the inferred velocities of varying in-core neutron detector and core-exit thermocouple locations was also investigated. Theoretical models of temperature noise were developed, and the results were used to interpret the experimental measurements.Results of these studies indicate that the neutron detector/thermocouple phase is useful for monitoring core flow in PWRs. Our results show that the interpretation of the phase between these signals depends on the source of temperature noise, the response times and locations of the sensors, and the neutron dynamics of the reactor. At Sequoyah-1 we found that the in-core neutron detector/core-exit thermocouple phase can be used to infer in-core coolant velocities, provided that the measurements are corrected for the thermocouple response time.

Measurement of rapid temperature changes by thermocouples

Measurements have been made of the response time of thermocouples to sudden temperature changes. A simple circuit has been designed to further improve the response time, so that changes occurring within a few milliseconds can be observed with a mechanically robust junction.

The Response of Thermocouples to Rapid Gas-Temperature Changes

Abstract This paper presents practical data which were taken from the main results of a research program dealing with the response of thermocouples to changing gas temperatures. Experimental and theoretical data for the response time of thermocouples ranging in wire size from 0.01 to 0.0005 in. are presented. The experiments were performed over a range of temperatures from 70 F to 950 F, and for air velocities ranging from 0 to 125 fps. All of the experiments were carried out in such a way that the hot junction of the thermocouple was always cooled. An equation is presented for computing the response time for thermocouples fabricated from fine wires and subjected to sudden air-temperature changes. Heat transfer by conduction and radiation were negligible in the experiments performed.