Thermopile Sensor Research Articles

Detecting thermal asymmetry in microfluidics for sensor applications: Critical design considerations and optimization

This article discusses the design and performance of continuous-flow microfluidic thermal sensors that operate on the principle of heat generation through lateral mixing. The platform under study is a Y-channel microfluidic device in which two laminar streams are brought into side-by-side contact under precise flow control. For this work, the two merging microchannels carry water and ethanol respectively; heat is generated as the liquids diffuse together. While much exists in the literature regarding the magnitude of this heat of mixing, this article examines the spatial distribution of this heat – in particular, the extent of lateral asymmetry in the resultant temperature map. This asymmetry is detected with milli-Kelvin sensitivity using a thin-film thermopile integrated on the exterior surface of the microdevice. Since this experimental configuration is implemented by numerous research groups in the design of biosensors and microfluidic calorimeters, a particular emphasis of this present study is on maximizing the detection sensitivity. The impact of the experimental parameters of volumetric flow rate and flow ratio on detection sensitivity was examined, and the optimum axial location for the thermopile sensor under a range of conditions was determined. Experimental and numerical methods were used in this study.

Calorimetric sandwich-type immunosensor for quantification of TNF-α

We report a lab-on-a-chip immunosesnor for quantification of the inflammatory cytokine TNF-α with picomolar sensitivity. The feasibility of the technology was demonstrated via accurate measurement of the concentration of TNF-α in astrocytes cell culture media. The immunoassay was performed in a microfluidic device with an integrated antimony/bismuth thermopile sensor and had a limit of detection of 14 pg mL−1. The device was fabricated using rapid prototyping xurography technique and consisted of two inlets and single outlet. Anti-TNF-α monoclonal antibody was used to capture the analyte while the detection was performed using glucose oxidase-conjugated secondary antibody. Glucose (55 mM) was injected through a sample loop into the fluid flowing within the microfluidic device. A nanovolt meter connected to the thermoelectric sensor recorded the voltage change caused by the enzymatic reaction. Computer simulations using COMSOL Multiphysics were performed to analyze the effect of fluid velocity on the concentration of glucose within the reaction zone. A standard calibration curve was created using serial dilutions of synthetic TNF-α (0–2000 pg mL−1) by plotting the area under the curve of the signal versus the concentration of the analyte. The efficacy of the device was evaluated by quantifying TNF-α in the cell culture medium of lipopolysaccharide stimulated and non-stimulated astrocytes. The results demonstrated high accuracy of the calorimetric immunoassay when compared with gold standard commercial ELISA microplate reader. The immunosensor offers excellent reproducibility, accuracy, and versatility in the choice of the detection enzyme.

Development of a simple CO2 sensor based on the thermal conductivity detection by a thermopile

A simple and effective thermal conductivity based CO2 gas sensor has been successfully developed. The CO2 sensor consists of a modified IR thermopile sensor without an IR window. The thermopile is heated by the AC current that is applied through the output terminals of the device, which in turn leads to a temperature gradient generating a DC output voltage. The gas exposed to the thermopile simultaneously cool down the thermopile. Thus, the thermopile can simultaneously serve as a heater and temperature sensor. The built-in sensors possess low production costs, low power consumption and are easily manufactured. From the conducted experiments, the sensor can work well, and the results can be validated with the established theory, as evidenced by the RMS error of 0.787. The measurement results are also in a good agreement with the model predictions Mason and Saxena for the thermal conductivity of gas mixtures.

Enhanced thermoelectric properties of Sb2Te3 and Bi2Te3 films for flexible thermal sensors

Improved thermoelectric properties of p-Sb2Te3 and n-Bi2Te3 films deposited by thermal co-evaporation at 503 K and 543 K, respectively, are reported, by using two types of substrate (borosilicate glass and 25 μm - Kapton® polyimide). Seebeck coefficients of 194 μV K−1 and - 233 μV K−1 and electrical conductivities of 3.2 × 104 (Ω m)−1 and 5.0 × 104 (Ω m)−1, lead to optimized power factors of 1.2 × 10−3 W K−2 m−1 and 2.7 × 10−3 W K−2 m−1, measured at 298 K, for flexible Sb2Te3 and Bi2Te3 films, respectively. The power factor value increases until a maximum of 2.3 × 10−3 W m−1 K−2 for the p-type film, and to 5.9 × 10−3 W m−1 K−2 for the n-type film, at 373 K, respectively. An improvement of more than two times on the power factor value is observed when compared with the actual state-of-art for flexible Sb2Te3 and Bi2Te3 films, at room temperature, with thickness below 1 μm. A flexible thermopile sensor based on p - n films is presented, into possible thermal sensing. The thermoelectric device shows a responsivity of 0.05 V W−1 and a specific detectivity of 1.6 × 107 cm √Hz W−1.

Measurement of Core Body Temperature Using Graphene-Inked Infrared Thermopile Sensor.

Continuous and reliable measurements of core body temperature (CBT) are vital for studies on human thermoregulation. Because tympanic membrane directly reflects the temperature of the carotid artery, it is an accurate and non-invasive method to record CBT. However, commercial tympanic thermometers lack portability and continuous measurements. In this study, graphene inks were utilized to increase the accuracy of the temperature measurements from the ear by coating graphene platelets on the lens of an infrared thermopile sensor. The proposed ear-based device was designed by investigating ear canal geometry and developed with 3D printing technology using the Computer-Aided Design (CAD) Software, SolidWorks 2016. It employs an Arduino Pro Mini and a Bluetooth module. The proposed system runs with a 3.7 V, 850 mAh rechargeable lithium-polymer battery that allows long-term, continuous monitoring. Raw data are continuously and wirelessly plotted on a mobile phone app. The test was performed on 10 subjects under resting and exercising in a total period of 25 min. Achieved results were compared with the commercially available Braun Thermoscan, Original Thermopile, and Cosinuss One ear thermometers. It is also comprehended that such system will be useful in personalized medicine as wearable in-ear device with wireless connectivity.

Strain gauges debonding fault detection for structural health monitoring

Physical redundancy is a common approach in applications with safety-critical systems. But that is not always possible in some structural health monitoring (SHM) applications whether for economic, spatial, or safety reasons. Many SHM sensor validating applications only rely on analytical redundancy. In such circumstances, sensor faults and structural damage need to be assuredly discriminated. A self-diagnosis strain sensor operating in a continuous online SHM scenario is considered. The sensor is based on a full electric resistance strain gauge Wheatstone bridge. The state of the art shows that such a sensor has not yet been developed. The loop current step response (LCSR) is a well-known method to detect strain gauge debonding. However, applying the LCSR method to a full strain gauge Wheatstone bridge has some limitations analyzed in this paper. Two new methods for detecting strain gauge debonding are proposed and evaluated. These methods are based on consistency checking of the strain gauges grids temperature measurements—employing an array of (a) digital contact temperature sensors or (b) quasi-contact microelectromechanical system thermopile sensors. The experimental results reveal that both methods are suitable for application in an SHM self-diagnosis sensor scenario. However, the quasi-contact measuring method showed to be more sensitive to the strain gauge grid debonding fault though.

MEMS for biofuel composition measurement based on thermal impedance spectroscopy

Continuous monitoring of the composition of E85 biofuel is essential for a quick start and clean and efficient operation of Flex-Fuel Vehicles. The actual ethanol concentration in E85 fuel is in the range 50%–85% and fuel-line sensors are used for ethanol-gasoline composition measurement. However, also a small amount of water is typically present, which cannot be reliably detected using state-of-the-art capacitive fuel-line sensors. Thermal impedance spectroscopy has been investigated as a non-destructive technique to determine the composition of ternary mixtures of biofuels. The principle of the thermal conductivity detector has been extended for measuring both the thermal conductivity and the thermal capacity of biofuel in the range up to 10 kHz using an AC-operated polysilicon heater for injecting a sinusoidal heat flux, and another polysilicon strip at a well-defined spacing or thermopile sensors for measuring the in-phase and quadrature components of the resulting AC temperature difference. Measurements on the components are in reasonable agreement with simulations, with a −3 dB cut-off frequency at 422.5 Hz and 340.8 Hz for ethanol and gasoline, respectively. However, the cut-off frequency of water was found to be significantly lower than simulations due to its high surface tension, thus limiting access to the detector.

Unobtrusive Sensor-Based Occupancy Facing Direction Detection and Tracking Using Advanced Machine Learning Algorithms

Facing direction detection plays a critical role in human–computer interaction and has a wide application in surveillance systems, driving awareness recognition, smart home appliances, computer games, and so on. Current detection methods are mainly focused on extracting specific patterns from user’s optical images, which raises concerns on privacy invasion and these detection techniques do not usually work in a dark environment. To address these concerns, this paper proposes an activity recognition system guided by an unobtrusive sensor (ARGUS). By using a low pixel infrared thermopile array sensor, ARGUS is capable of identifying five facing directions (left 45°/90°, right 45°/90°, and front) through the support vector machine classifier. Also two feature extraction methods are compared. One is manually-defined and the other is based on a pre-trained convolutional neural network (CNN) model. The facing direction detection accuracy resulting from manually-defined features reaches 85.3%, 90.6%, and 85.2% at detection distance of 0.6, 1.2, and 1.8 m, respectively. The level of accuracy resulted from using pre-trained CNN features demonstrates a much more reliable performance (89.1%, 95.3%, and 95.1% at distance of 0.6, 1.2, and 1.8 m, respectively). In addition, ARGUS has been successfully applied for occupancy tracking with a root mean square error of 0.19 m.

Measuring heat flux to a porous burner in microgravity

Flame heat flux absorbed by a porous gas-fueled burner is measured in microgravity. The burner's perforated copper plate serves as a slug calorimeter in which two heat flux thermopile sensors are embedded. The slug calorimeter provides the average heat flux over the burner surface as a function of time. The 25 mm diameter burner is calibrated as a slug calorimeter in normal gravity using a known radiative heat flux with step changes. Microgravity diffusion flames were observed in NASA Glenn's 5.18-s Zero Gravity Research Facility, and average heat fluxes measured with the calorimeter agree with the locally measured heat fluxes through a theoretical distribution function. The results show that the average slug calorimeter heat flux and the two local heat flux measurements are in harmony over a wide range of microgravity flame fluxes ranging from 5–20 kW/m2, with the edge heat flux much higher. Transient and nearly steady results are presented.

Simulation Results of Prospective Next Generation 3-D Thermopile Sensor and Array Circuitry Options

This article presents the simulation results and design rules of a new sensor for infrared (IR)-detection using the thermoelectric effect. Within the Seebeck effect, thermopiles generate a voltage based on a temperature gradient inside the structure. State-of-the-art thermopiles are manufactured as 2-D structures directly on a substrate. Here, a possible method of 3-D integration is shown, where the thermoelectric materials are fabricated as thin tubes using an atomic layer deposition process. These tubes are connected to an IR-absorber on top, where the IR-radiation causes a temperature gradient relative to the substrate. This has the advantage to achieve a fill factor of nearly 100%. In comparison to microbolometers, the 3-D thermopile is a passive structure, which does not need complex readout and supply circuits. Furthermore, the usage of energy harvesting is possible. Additionally, new array circuitry options are discussed to achieve a better signal-to-noise ratio. An electrical series connection of multiple sensors effects a rising specific detectivity and the noise equivalent temperature difference decreases analogously. With this technique, the groups of pixels of the detector can be merged to one “super-pixel” for detecting even marginal temperature changes of an object.

Detection of intruders in warehouses using Infrared based Thermopile Sensor Array

The infrared thermopile array sensor detects the object at motion based on the infrared radiation radiated by the human body. The infrared radiation radiated is related to the human body temperature. Temperature difference exist between the human body and room temperature. Temperature values are obtained as the person approaches the field of view of a sensor. In this paper a mechanism is developed to track the moving object based on their centroid values. From the each temperature frame the foreground region is extracted and from the obtained centroid value of each region, the object is tracked by representing the bounding box across the tracked foreground region.

Improvement in Sensitivity of Bio-Calorimeter with MEMS Thermopile Sensor

Improvement in Sensitivity of Bio-Calorimeter with MEMS Thermopile Sensor

Infrared Scanning Cloud Monitor for the Indian Astronomical Observatory, Hanle

A large number of clear nights at a given location is one of the key prerequisites for establishing an astronomical observatory. This parameter needs to be reliably determined before the site location is finalized. For an already exiting observatory site, monitoring of clouds on real-time basis helps automating many observing procedures, thus optimizing the scientific returns from the facility. We have developed an infrared (IR) scanning cloud monitor which generates a local sky-map and provides reliable information about the changing sky conditions. Our device measures the IR sky brightness temperature using a circular array of thermopile sensors mounted on a rotating system. In this paper, we give a detailed description of the instrument hardware, laboratory and on-site calibration procedures and further tests carried out to check the reliability of the device. The cloud monitor has been in regular use since its installation at the Indian Astronomical Observation (IAO), Hanle, Ladakh in 2015 December. We also present the analysis of a year long sky data to demonstrate the usefulness of the device.

Human location estimation using thermopile array sensor

Utilization of Thermopile sensor at an early stage of human detection is challenging as there are many things that produce thermal heat other than human such as electrical appliances and animals. Therefrom, an algorithm for early presence detection has been developed through the study of human body temperature behaviour with respect to the room temperature. The change in non-contact detected temperature of human varied according to body parts. In an indoor room, upper parts of human body change up to 3°C whereas lower part ranging from 0.58°C to 1.71°C. The average changes in temperature of human is used as a conditional set-point value in the program algorithm to detect human presence. The current position of human and its respective angle is gained when human is presence at certain pixels of Thermopile’s sensor array. Human position is estimated successfully as the developed sensory system is tested to the actuator of a stand fan.

Highly Sensitive Microfluidic Chip Sensor for Biochemical Detection

Chip calorimetry offers a power tool for fast and high throughput analysis of biochemical process. However, it is challenging to realize an inexpensive, easy to fabricate microfluidic chip-based calorimeter with high sensitivity. This paper describes the design of a novel, highly sensitive, and continuous flow microfluidic chip sensor with an integrated antimony (Sb)–bismuth (Bi) thin-film thermopile heat detection element. The geometry and the design of the microfluidic device facilitate hydrodynamic flow focusing, and the integration and design of the thermopile sensor into the microfluidic device eliminates the need for reference temperature control. The device contains a single flow channel that is $120~\mu \text{m}$ high and 10-mm wide with two fluid inlets and one fluid outlet. An Sb-Bi thin film thermopile is fabricated on the inner surface of the bottom channel wall using thermal evaporation and was passivized with a $3~\mu \text{m}$ SU-8 photoresist layer. The device has been successfully used to measure the dynamic temperature changes resulting from heat generation following the mixing of glycerol and water. The effect of flow rates on the sensor’s response was measured. The sensor can detect dynamic temperature changes in the order of 10-6 K. The limit of detection of heat power of the device was calculated to be 8.8 pW. With the obtained remarkable sensitivity and heat power detection limit, the microfluidic chip sensor can potentially be used to investigate biochemical processes, such as enzyme-catalysed reactions, and metabolic activity of cells.

Evaluation of empirical heat transfer models for HCCI combustion in a CFR engine

The heat transfer from the bulk gases to the combustion chamber walls has a strong effect on the combustion and emission formation process in an HCCI engine. In this work, the empirical heat transfer models of Annand, Woschni, Hohenberg, Bargende, Chang et al. and Hensel et al. are evaluated at various engine operating conditions. The modelled heat flux is compared to the measured heat flux in a CFR engine with a thermopile sensor. The shape of the heat flux trace, the maximum heat flux and the total heat loss are evaluated and different model calibration procedures are investigated. It is found that all models require calibration and need to be recalibrated if the fuel type and certain engine settings are changed. A better model fit can be obtained if different model coefficients are applied for the compression and the expansion phase.

Properties of Non-dispersive infrared Ethanol Gas Sensors according to the Irradiation Energy

A nondispersive infrared (NDIR) ethanol gas sensor was prototyped with ASIC implemented thermopile sensor, which included a temperature sensor and two ellipsoidal waveguide structures. The temperature dependency of the two ethanol sensors (with partially blocked and intact structures) has been characterized. The two ethanol gas sensors showed linear output voltages initially when varying the ambient temperature from 253 K to 333 K. The slope of the temperature sensor presented a constant value of 15 mV/K. After temperature compensation, the ethanol gas sensor estimated ethanol concentrations with larger errors of 20 to 25% below 200 ppm. However, the estimation errors were reduced to between -10 and +1 % from 253 K to 333 K above 200 ppm ethanol gas concentration in this research.

The impact of C/H on the radiative and thermal behavior of liquid fuel flames and pollutant emissions

The present experimental study is performed to document the effects of carbon-to-hydrogen mass ratio (C/H) in liquid hydrocarbon fuels, on the luminosity, thermal radiation rate, and temperature of flame. Furthermore, extraction of the thermal and radiative characteristics of flame, and measurement of exhaust gases including CO, CO2, and NO x have been carried out. The CHNS elemental analyzer system was employed to ascertain fuel composition. The thermopile sensor and the lux meter were utilized to measure the flame thermal radiation and luminosity. The measured flame radiation spectrum included all possible wavelengths (i.e., flame thermal radiation) and visible wavelengths (i.e., luminosity). The flame photography technique, as a non-intrusive method, depicted the flame visible radiation, IR spectral emittance, and high temperature zone. The results revealed that the luminosity and thermal radiation of the flame increased as we increase C/H. A rise in C/H made the temperature along the flame axis more uniform. The NO x emissions were within standard levels (i.e., under 200 ppm). It was also shown that the C/H variation has a greater effect on luminosity than thermal radiation of the flame. The flame luminosity was increased by 66% when the C/H was increased from 5.47 to 5.68, whereas the flame thermal radiation increased by 26%. In addition, a new correlation is proposed to predict the linear relation of the increase in the thermal radiation and the increase in the luminosity of the flame as a function of C/H.

Measurement issues associated with surface mounting of thermopile heat flux sensors

Thermopile is one of the most commonly used heat flux sensor, usually attached to the surface of interest for measurement of wall heat flux. One of the major drawbacks of such surface mounted sensors is that they potentially disturb the very thermo-physical phenomena responsible for heat transfer, altering the heat transport through the medium on which it is mounted, resulting in erroneous measurements. This paper closely examines the intrusion due to surface mounting of such sensors. Parametric simulations have been performed for commonly employed sensor mounting configurations. Furthermore, the attenuation in heat transfer through the plate as a function of its thermophysical properties (absolute as well as relative to the sensor material) is evaluated. It is found that matching the impedance of sensor and heat conduction medium is important for minimizing the intrusive effects. Simulations are substantiated by a case-study involving heat flux measurement during filmwise condensation of steam on a flat plate in the presence of air. The experimental design ratifies the recommendations of the simulation study, minimizing the intrusive effects after proper impedance matching and in situ calibration. The resulting heat transfer coefficient estimated using the measured wall heat flux matches well with those obtained using Terasaka-Makita correlation.

Thermal Methods and Non-Destructive Testing Instrumentation

This article describes several developed modifications to heat flux sensors. A common element in these devices is a specially designed thermopile sensor that acts as a thermoelectric converter of heat flow.