Thermal Conductivity Sensor Research Articles

Numerical Simulation of Water Movement in the Suction Equalization of a Thermal Conductivity Sensor

Abstract Laboratory and field measurements of soil suction using a thermal conductivity soil suction sensor involves installing the sensors into a soil mass. The sensors may be installed either in an initially wet or an initially dry state. In order to ensure a reliable suction measurement, it is important that the sensor reach suction equalization with the surrounding soil. The results of a numerical study have shown that, in most cases, the equalization time required for an initially dry sensor is less than the equalization time for an initially wet sensor. This difference is mainly due to the unsaturated zone that develops on the boundary of the initially wet sensor at the beginning of the drying process. The results of the numerical simulation have been substantiated by laboratory suction measurements.

Thermal and gas-sensing properties of a micromachined thermal conductivity sensor for the detection of hydrogen in automotive applications

Abstract This work describes the design and characterization of a micromachined thermal conductivity sensor for detection of hydrogen in automotive fuel cell systems. Thermal and gas-sensing properties are investigated via simulations and experiments. The manufactured sensors consist of a thin dielectric membrane as carrier structure for a platinum heater and temperature sensor. Membrane and heater size were varied to examine their influence on sensitivity, power consumption and thermal response. Based on a sensor element with a membrane size 1.5 mm ×1.5 mm a hydrogen sensor prototype with a detection limit of 0.2% hydrogen in air was achieved.

Measurement of xenon in the breathing circuit: Comparison of a thermal conductivity sensor and a mass spectrometer

European Society of Anaesthesiologists; 8th Annual Meeting with the Austrian International Congress; Vienna, Austria, 1-4 April 2000

Fully integrated thermal conductivity sensor for gas chromatography without dead volume

A complete detector based on the anemometer principle and sensing the thermal conductivity of gases for application in gas chromatography (GC) will be introduced. The micromachined die contains a reference and a measurement channel. The channels consist of integrated capillary connectors, a measurement chamber, hot-wire elements and elements for thermostatting the bulk. The sensor has been designed to be connected to 100 μm separation columns and due to novel attachment of the tubing, it has virtually no dead volume. Each measurement chamber has a volume of about 14 nl. The sensing elements are fabricated as gold resistors and suspended in the middle of the gas channel. A sturdy and easy-to-handle packaging has been developed. The sensor has been successfully tested in a commercial GC set-up and consumes an approximate power of 0.5 W.

Constant power operation of incandescent micromachined polysilicon microresistors for use as vacuum pressure sensors

As a proof of concept study, a micromachined polysilicon resistor was used as a vacuum pressure sensor. The resistor was heated to incandescence and operated with a constant electrical power input. A photodiode monitored light intensity changes as the pressure of the surrounding nitrogen gas was varied over the pressure range of 10−2–104 Pa. This optical method to measure pressure was nine times more sensitive than one based on electrical resistance variations. This measurement method is thus both more sensitive and it operates over a large pressure range than conventional thermal conductivity sensors.

The Effects of Forced Convection on the Power Dissipation of Constant-Temperature Thermal Conductivity Sensors

The effect of forced convection on the power dissipation of cylindrical and planar, constant temperature, thermal conductivity detectors (TCDs) is investigated theoretically. Such detectors can be used either for on-line continuous sensing of fluid thermal conductivity or for determining the sample concentrations in gas chromatography. A low Peclet number, asymptotic theory is constructed to correlate the TCD’s power dissipation with the Peclet number and to explain experimental observations. Subsequently, the effect of convection on the TCD’s power dissipation is calculated numerically for both time-independent and time-dependent flows. The theoretical predictions are compared with experimental observations.

Gas thermal conductivity sensor based on SAW

An uncoated surface acoustic wave (SAW) delay line integrated with a suitable film heater is used as a gas sensitive element based on thermal conductivity change. It detects concentrations from 0.1-100% of H/sub 2/, O/sub 2/, He, and Ar in N/sub 2/ in the temperature range from 20-150/spl deg/C. The value and sign of the SAW gas response are controlled by substrate material and reference gas.

CMOS as sensor technology

Abstract Sensor design and fabrication using industrial IC technologies has the advantages of batch fabrication and on-chip interface circuitry. Sensors made by CMOS or bipolar IC technology have been demonstrated for magnetic, temperature and radiation measurands. Certain thermal, mechanical and chemical sensors can be realized by combining IC technologies with additional, compatible processing. We distinguish the methods of multiple project wafers, single project wafers, post-processing IC chips or wafers, and IC fabrication merged with sensor processing performed before, after, and in-between the regular IC processing steps. This paper is focused on sensor prototypes realized by industrial CMOS IC technology with post-processing micromachining. Examples include thermally excited acoustic resonators, thermoelectric gas flow, infrared, and power sensors, and a thermal conductivity sensor.

Gas analysis using a thermal conductivity method

A novel type of gas-sensing technique has been developed by combining a gas-diffusion membrane with a gas thermal conductivity sensor operating in a cyclic mode. The small volume enclosed between the diffusion membrane and the thermal conductivity sensor is periodically flushed with a purge gas. After each purge cycle, the thermal conductivity reading gradually changes from the value of the purge gas to that of the gas to be measured. The rate of change is continuously monitored and is a direct function of the concentration of the gas to be measured. Thanks to the hydrophobic diffusion membrane, the sensor can also be used for measuring dissolved gas in almost any liquid in a wide range of temperature and pressure. A simplified theoretical model has been established and experimental results will be presented for the measurement of CO 2 in N 2, N 2 in CO 2 and H 2 in N 2. Comparisons with an amperometric sensor will also be given.

A thermal conductivity sensor for the measurement of skin blood flow

The 'Hematron' is a non-invasive sensor (disc 25 mm o.d., 4 mm thick) for the measurement of skin blood flow (SBF) in the capillary layers from the surface of the skin. A constant temperature difference is maintained between the center and the periphery of the sensor by a PID control device. The heat power injected into the skin by a micro-fold heater is equal to the heat power washed out by the blood flow in the capillary network. SBF may be recorded from continuous measurement up to 1 Hz. The results are expressed in units of thermal conductivity (mW/cm °C): minimum value 2.5 mW/cm °C (null blood flow); maximum value 10 mW/cm °C (vasodilation). The calibration is carried out by physical models: media, the thermal conductivity of which is constant and well defined (vaseline, polyacrylamide gel), rotating water tank with low linear speed, perfused catheter bunch. The high repeatability of this quantitative method allows long-term measurement and inter- or intrasubject comparison. This sensor is used for the study of physiological tests on SBF (valsalva maneuver, effect of heat, posture, etc…), for the study of the vascularization of burnt skin and graft, the effects of pharmacological or cosmetic agents and for per and post operative monitoring. As SBF is related to brain activity, the sensor is used for the study of the vigilance status and the emotional response (sport, driving of vehicle, mental imagery).

Typical matric suction measurements in the laboratory and in the field using thermal conductivity sensors : Rahardjo, H; Loi, J; Fredlund, D G Proc Indian Geotechnical Conference, 1989, Geotechniques of Problematic Soils and Rocks, Visakhapatnam, 14–16 December 1989V1, P127–131. Publ New Delhi: Sarita Prakashan, 1989

Typical matric suction measurements in the laboratory and in the field using thermal conductivity sensors : Rahardjo, H; Loi, J; Fredlund, D G Proc Indian Geotechnical Conference, 1989, Geotechniques of Problematic Soils and Rocks, Visakhapatnam, 14–16 December 1989V1, P127–131. Publ New Delhi: Sarita Prakashan, 1989

Effect of Facial Cooling on Mucosal Blood Flow in the Mouth in Humans

1. To determine the effects of facial cooling on intraoral thermal events, we placed a thermal conductivity sensor on the buccal surface of the left cheek in six normal and six asthmatic subjects. Room temperature and cold stimuli were then applied to the integument surface of both sides of the face while mucosal surface temperature and thermal conductivity, as an index of blood flow, were recorded. 2. The room temperature challenge had no effect. Application of the cold stimulus to the exterior of the left cheek caused a monotonic decrease in temperature in the mouth in all subjects and was associated with a change in thermal conductivity in which blood flow increased and then fell to baseline despite a continued drop in temperature. These responses were purely local in that cooling of the right side of the face did not change the temperature or blood flow on the left side. No differences were noted between the asthmatic and normal subjects. 3. The data indicate that lowering the temperature of the skin of the face produces significant alterations in the thermal environment within the mouth. With facial cooling, buccal temperature falls and mucosal blood supply transiently rises. This effect appears to be a purely local thermally mediated event. Facial pressure and cutaneous reflexes do not play a role. The above changes may contribute to the conditioning of inspired air during oral breathing.

Design and operation of a thermal sensor with micrometer-spatial resolution

We have developed a simple superconducting thin film device that can be used as either a thermometer or a thermal conductivity sensor. Over a narrow range the device has somewhat better temperature resolution than a conventional germanium thermometer, but its main advantage is that it has very high spatial resolution. The fabrication and operation of the device are described, together with preliminary results of experiments in which it is used to study liquid4He very nearTλ. Potential improvements are also discussed.

Calibration of thermal conductivity sensors for measuring soil suction : Fredlund, D G; Wong, D K H Geotech Test J V12, N3, Sept 1989, P188–194

Calibration of thermal conductivity sensors for measuring soil suction : Fredlund, D G; Wong, D K H Geotech Test J V12, N3, Sept 1989, P188–194

Use of thermal conductivity sensors to measure matric suction in the laboratory. Note : Sattler, P; Fredlund, D G Can Geotech JV26, N3, Aug 1989, P491–498

Use of thermal conductivity sensors to measure matric suction in the laboratory. Note : Sattler, P; Fredlund, D G Can Geotech JV26, N3, Aug 1989, P491–498

Calibration of Thermal Conductivity Sensors for Measuring Soil Suction

Abstract The accuracy of suction measurements using thermal conductivity sensors is dependent upon their calibration. Therefore, a calibration study was undertaken by using a modified, commercially available, pressure plate extractor. The experimental setup along with the calibration procedure are described. Typical calibration results are presented. In general, results showed reasonable agreement between the calibration curve determined in this study compared to those provided by the manufacturer for matric suction ranging from 0 to 175 kPa. However, large deviations in the calibration curves were observed at suctions above 175 kPa. The sensors were found to be quite sensitive to the changes in matric suction in the range of 0 to 175 kPa. For matric suction above 175 kPa, the sensitivity of the sensor is reduced.

Use of thermal conductivity sensors to measure matric suction in the laboratory

The measurement of soil suction is pivotal to the application of soil mechanics principles in geotechnical engineering practice related to unsaturated soils. Volume change, shear strength, and seepage analyses all require an understanding of the matric suction in the soil. This note summarizes the use of thermal conductivity sensors to measure matric suction in the laboratory. The thermal conductivity sensor is described along with its mode of operation. A brief description is given of the procedure for calibrating thermal conductivity sensors using a pressure plate apparatus. The measurement of matric suction can be performed in the laboratory on Shelby tube samples. The laboratory measurements of matric suction can be adjusted for the effect of overburden pressure in the field. The required equilibration time for suction measurements is discussed along with details of the test procedure. The applications of the measured suction values to design are briefly discussed.Key words: matric suction, negative pore-water pressure, thermal conductivity sensor, laboratory, undisturbed samples.

Soil water potential measurement: An overview

Soil water potential is measured either by measuring some property of the soil which changes with the water potential, or by equilibrating the liquid or gas phase of the water in some reference medium with the liquid phase of the soil and measuring some property of the water in the reference medium. Devices commonly used to measure soil water potential are tensiometers, thermocouple psychrometers, electrical resistance sensors, thermal conductivity sensors, and correlations with water contents of the soil or of filter paper which has been equilibrated with the soil. The principles governing the operation of each of these devices will be discussed.

Polymer thick films: what are they, how are they used and where? : C. I. Wall. Circuit Wld12 (2), 42 (1986)

Sensors are important devices for alerting to the presence of leaked hydrogen in any application involving the production, storage, or use of hydrogen. Key missions for the sensor test laboratories in the U.S. Department of Energy, National Renewable Energy Laboratory and in the European Commission Joint Research Centre, Institute for Energy and Transport are to assure the availability and proper use of hydrogen safety sensors. As an integral element in a safety system, sensor performance should not be compromised by operational parameters. For example, safety sensors may be required to operate at reduced oxygen levels relative to air, such as that which would exist for nitrogen purges. Some sensor platforms are amenable for anaerobic operation, whereas other platforms will be deactivated and possible permanently altered with anaerobic operation. The NREL and JRC sensors laboratories assessed the ability of a number of sensor platforms to detect hydrogen under conditions of varying oxygen concentration. The performance of three common hydrogen sensor platforms, the thermal conductivity sensor, combustible gas sensor, and a palladium thin-film (metallic resistor) sensor, to operate under anaerobic conditions is presented.

Integrated Intravenous Pressure Transducer

Fusion machine operation requires high-vacuum conditions and does not tolerate water or gas leak in the vacuum vessels, even if they are micrometric. Tore Supra, as a fully actively cooled tokamak, has got a large leak management experience; 34 water leaks occurred since the beginning of its operation in 1988. To handle this issue, after preliminary machine protection phases, the current process for leak localization is based on water or helium pressurization network by network. It generally allows the identification of a set of components where the leakage element is located. However, the unique background of CEA-IRFM laboratory points needs of accuracy and promptness out in the leak localization process. Moreover, in-vessel interventions have to be performed trying to minimize time and risks for the persons. They are linked to access conditions, radioactivity, tracer gas high pressure and vessel conditioning. Remote operation will be one of the ways to improve these points on future fusion machines. In this case, leak sensors would have to be light weight devices in order to be integrated on a carrier or to be located outside with a sniffing process set up.A leak localization program is on-going at CEA-IRFM Laboratory with the first goal of identifying and characterizing relevant concepts to localize helium or water leaks on ITER. In the same time, CEA has developed robotic carrier for effective in-vessel intervention in a hostile environment.Three major tests campaigns with the goal to identify leak sensors have been achieved on several CEA test-beds since 2010. Very promising results have been obtained: relevant scenario of leak localization performed, concepts tested in a high volume test-bed called TITAN, and, in several conditions of pressure and temperature (ultrahigh vacuum to atmospheric pressure and 20–120 °C), “ppm” traces of helium or water have been measured. To improve these first promising results, a new test-bed called ROVE (remote operation vacuum equipment) has been set up in 2012 to qualify leak sensors and remote operation.A list of concepts has been set out from contacts with manufacturers and laboratories. For sniffing solutions it includes laser absorption measurement, mass spectrometer analyses and vibrating quartz. For embeddable concepts thermal conductivity and capacitive sensors are considered. Choice of concepts to detect water or helium traces is based on major criteria, such as weight, high sensibility and fast response time and recovery time. They also have to be operational under primary vacuum conditions or atmospheric pressure and from ambient temperature to 60 °C.