Micro-beam sensor for detection of thermal conductivity of gases and liquids (original) (raw)
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International Journal of Thermophysics, 2010
A new method was proposed to measure the thermal conductivity of liquids with infinitesimal samples, which are much smaller than those required in conventional methods. The method utilizes a micro-beam-type MEMS sensor fabricated across a trench on a silicon substrate. Numerical analysis of heat conduction within and around a uniformly heated sensor showed that the temperature of a 10 µm long sensor reached a steady state within approximately 0.1 ms, after the start of heating. It was also revealed that the average temperature of the sensor at the steady state was higher in liquids with lower thermal conductivity. These results demonstrate a new idea of measuring the thermal conductivity of liquids within an extremely short time at a steady state before the onset of natural convection.
Thermal conductivity measurement of liquids in a microfluidic device
A new microfluidic-based approach to measuring liquid thermal conductivity is developed to address the requirement in many practical applications for measurements using small (microlitre) sample size and integration into a compact device. The approach also gives the possibility of high-throughput testing. A resistance heater and temperature sensor are incorporated into a glass microfluidic chip to allow transmission and detection of a planar thermal wave crossing a thin layer of the sample. The device is designed so that heat transfer is locally one-dimensional during a short initial time period. This allows the detected temperature transient to be separated into two distinct components: a short-time, purely one-dimensional part from which sample thermal conductivity can be determined and a remaining long-time part containing the effects of three-dimensionality and of the finite size of surrounding thermal reservoirs. Identification of the one-dimensional component yields a steady temperature difference from which sample thermal conductivity can be determined. Calibration is required to give correct representation of changing heater resistance, system layer thicknesses and solid material thermal conductivities with temperature. In this preliminary study, methanol/water mixtures are measured at atmospheric pressure over the temperature range 30–50°C. The results show that the device has produced a measurement accuracy of within 2.5% over the range of thermal conductivity and temperature of the tests. A relation between measurement uncertainty and the geometric and thermal properties of the system is derived and this is used to identify ways that error could be further reduced. http://dx.doi.org/10.1007/s10404-010-0652-x
A New Instrument for the Measurement of the Thermal Conductivity of Fluids
International Journal of Thermophysics, 2006
The transient hot-wire technique is at present the best technique for obtaining standard reference data for the thermal conductivity of fluids. It is an absolute technique, with a working equation and a complete set of corrections reflecting departures from the ideal model, where the principal variables are measured with a high degree of accuracy. It is possible to evaluate the uncertainty of the experimental thermal conductivity data obtained using the best metrological recommendations. The liquids proposed by IUPAC (toluene, benzene, and water) as primary standards were measured with this technique with an uncertainty of 1% or better (95% confidence level). Pure gases and gaseous mixtures were also extensively studied. It is the purpose of this paper to report on a new instrument, developed in Lisbon, for the measurement of the thermal conductivity of gases and liquids, covering temperature and pressure ranges that contain the near-critical region. The performance of the instrument for pressures up to 15 MPa was tested with gaseous argon, and measurements on dry air (Synthetic gas mixture, with molar composition certified by Linde AG, Wiesbaden, Germany, Ar-0.00920; O 2-0.20966; N 2-0.78114), from room temperature to 473 K and pressures up to 10 MPa are also reported. The estimated uncertainty is 1%.
Review of Scientific Instruments, 2018
This research work demonstrates an innovative technique to measure the thermal conductivity of a small volume of non-volatile liquids. The method utilizes a micro-pipette thermal sensor (MPTS) (tip diameter < 2 μm) and is based on laser point heating thermometry and transient heat transfer. A laser beam is irradiated at the sensor tip immersed in a few microliters of the test fluid and the transient temperature change is recorded with the sensor. This temperature change is dependent on the surrounding fluid’s thermal properties, such as thermal conductivity and diffusivity. The numerical solution for transient temperature profile for a point source is obtained using the finite element method in the COMSOL software. To determine the optimizing parameters such as thermal conductivity and power absorbed at the sensor tip, the multi-parameter fitting technique is used in MATLAB, which will fit the COMSOL simulation result with the experimental data. Three liquids with known thermal c...
Journal of Thermal Analysis and Calorimetry, 2008
Various techniques and methodologies of thermal conductivity measurement have been based on the determination of the rate of directional heat flow through a material having a unit temperature differential between its opposing faces. The constancy of the rate depends on the material density, its thermal resistance and the heat flow path itself. The last of these variables contributes most significantly to the true value of steady-state axial and radial heat dissipation depending on the magnitude of transient thermal diffusivity along these directions. The transient hot-wire technique is broadly used for absolute measurements of the thermal conductivity of fluids. Refinement of this method has resulted in a capability for accurate and simultaneous measurement of both thermal conductivity and thermal diffusivity together with the determination of the specific heat. However, these measurements, especially those for the thermal diffusivity, may be significantly influenced by fluid radiation. Recently developed corrections have been used to examine this assumption and rectify the influence of even weak fluid radiation. A thermal conductivity cell for measurement of the thermal properties of electrically conducting fluids has been developed and discussed.
New portable instrument for the measurement of thermal conductivity in gas process conditions
Review of Scientific Instruments, 2016
The development of high temperature gas sensors for the monitoring and determination of thermophysical properties of complex process mixtures at high temperatures faces several problems, related with the materials compatibility, active sensing parts sensitivity, and lifetime. Ceramic/thin metal films based sensors, previously developed for the determination of thermal conductivity of molten materials up to 1200 • C, were redesigned, constructed, and applied for thermal conductivity measuring sensors. Platinum resistance thermometers were also developed using the same technology, to be used in the temperature measurement, which were also constructed and tested. A new data acquisition system for the thermal conductivity sensors, based on a linearization of the transient hot-strip model, including a portable electronic bridge for the measurement of the thermal conductivity in gas process conditions was also developed. The equipment is capable of measuring the thermal conductivity of gaseous phases with an accuracy of 2%-5% up to 840 • C (95% confidence level). The development of sensors up to 1200 • C, present at the core of the combustion chambers, will be done in a near future.
Obtaining accurate thermal properties of biomaterials plays an important role in the field of cryobiology. Currently, thermal needle, which is constructed by enclosing a manually winded thin metal wire with an insulation coating in a metallic sheath, is the only available device that is capable of measuring thermal conductivity of biomaterials. Major drawbacks, such as macroscale sensor size, lack of versatile format to accommodate samples with various shapes and sizes, neglected effects of heat transfer inside the probe and thermal contact resistance between the sensing element and the probe body, difficult to mass produce, poor data repeatability and reliability and labor-intense sensor calibration, have significantly reduced their potential to be an essential measurement tool to provide key thermal property information of biological specimens. In this study, we describe the development of an approach to measure thermal conductivity of liquids and soft bio-tissues using a proof-of-concept MEMS based thermal probe. By employing a microfabricated closely-packed gold wire to function as the heater and the thermistor, the presented thermal sensor can be used to measure thermal conductivities of fluids and natural soft biomaterials (particularly, the sensor may be directly inserted into soft tissues in living animal/plant bodies or into tissues isolated from the animal/plant bodies), where other more standard approaches cannot be used. Thermal standard materials have been used to calibrate two randomly selected thermal probes at room temperature. Variation between the obtained system calibration constants is less than 10%. By incorporating the previously obtained system calibration constant, three randomly selected thermal probes have been successfully utilized to measure the thermal conductivities of various solutions and tissue samples under different temperatures. Overall, the measurements are in agreement with the recommended values (percentage error less than 5%). The microfabricated thermal conductivity sensor offers superior characteristics compared to those traditional macroscopic thermal sensors, such as, (a) reduced thermal mass and thermal resistivity, (b) improved thermal contact between sensor and sample, (c) easy to manufacture with mass production capability, (d) flexibility to reconfigure sensor geometries for measuring samples with various sizes and shapes, and (e) reduced calibration workload for all sensors microfabricated from the same batch. The MEMS based thermal conductivity sensor is a promising approach to overcome the inherent limitations of existing macroscopic devices and capable of delivering accurate thermal conductivity measurement of biomaterials with various shapes and sizes.
Single element thermal sensor for measuring thermal conductivity and flow rate inside a microchannel
Sensors and Actuators A-physical, 2021
The increasing development of continuous-flow applications in the field of microfluidics generates demand for in-line monitoring methods. The thermal conductivity (κ) of a liquid has been proven to be a valuable measurand for quality control, process monitoring, and analytical testing. However, most available methods for measuring κ of microliter-sized samples are limited for use on stagnant samples. In this work, a novel method and associated prototype device for measuring κ under flow conditions is presented. The so-called Transient Thermal Offset (TTO) method requires only a single metal resistive structure that is excitated with direct current (DC) pulses. To demonstrate the working, proof-of-principle experiments are performed on liquids with various κ under different flow rates. The results show that, after calibration, the presented microfluidic device can be used for accurately measuring κ of liquids under flow, as well as for determining the flow rate of liquids with a known κ. Within the explored ranges, both parameters can be determined with an average error of approximately 2.6%. The results confirm that, also under flow conditions, uncertainties concerning probing depth are eliminated with the TTO method.
Microsystems & Nanoengineering, 2015
Micro-thermal conductivity detector (µTCD) gas sensors work by detecting changes in the thermal conductivity of the surrounding medium and are used as detectors in many applications such as gas chromatography systems. Conventional TCDs use steady-state resistance (i.e., temperature) measurements of a micro-heater. In this work, we developed a new measurement method and hardware configuration based on the processing of the transient response of a low thermal mass TCD to an electric current step. The method was implemented for a 100-µm-long and 1-µm-thick micro-fabricated bridge that consisted of doped polysilicon conductive film passivated with a 200-nm silicon nitride layer. Transient resistance variations of the µTCD in response to a square current pulse were studied in multiple mixtures of dilute gases in nitrogen. Simulations and experimental results are presented and compared for the time resolved and steady-state regime of the sensor response. Thermal analysis and simulation show that the sensor response is exponential in the transient state, that the time constant of this exponential variation was a linear function of the thermal conductivity of the gas ambient, and that the sensor was able to quantify the mixture composition. The level of detection in nitrogen was estimated to be from 25 ppm for helium to 178 ppm for carbon dioxide. With this novel approach, the sensor requires approximately 3.6 nJ for a single measurement and needs only 300 µs of sampling time. This is less than the energy and time required for steady-state DC measurements.
A computer-controlled instrument for the measurement of the thermal conductivity of liquids
International Journal of Thermophysics, 1987
A new instrument for the measurement of the thermal conductivity of liquids by the transient hot-wire method is described. The instrument has features in common with earlier versions but employs a novel technique for the determination of the transient temperature rise of the hot wire during the course of a measurement. New determinations of the thermal conductivity of toluene confirm the accuracy of the instrument to be better than 05 %.