Development of a low-cost NDIR system for ppm detection of carbon dioxide in exhaled breath analysis

The composition of exhaled breath contains important information regarding the health of our body. Measurements of the level of exhaled carbon dioxide can help both diagnose respiratory diseases and determine metabolic rate. A low-cost NDIR sensor has been developed that offers the detection of CO2 from the ppm range up to 5% level in human breath. An innovative lock-in amplifier system allows a 10 Hz drive signal to be recovered from the high frequency noise associated with a silicon thermopile infra-red detector. Laboratory experiments have demonstrated excellent stability (±0.10% in 25% RH) and repeatability between dry and humid conditions (±1.2% for 25% humidity increase). The response time is typically 2.4s, limited by the low drive frequency necessary for the MEMS-based wideband infra-red source. The current system has a resolution of ca. 10 ppm of CO2. Further refinement in signal processing and a higher drive frequency should permit even lower concentrations of CO2 to be detected with an ultimate target of 1 ppm. Existing performance has been shown to be suitable for breath analysis using a side-stream analyser

Ultrasensitive WO3 gas sensors for NO2 detection in air and low oxygen environment

We report here on the results of a study into the response of a tungsten oxide based low power MEMS gas sensor to ppb of nitrogen dioxide at low levels of ambient oxygen. It was found that the resistive gas sensors not only had a high sensitivity to NO2 (3.4%/ppb vs. 0.2%/ppb obtained for commercial MOX) but can still operate reliably at lower oxygen levels (down to 0.5%) - albeit with slightly longer response and recovery times. The optimal operating temperature was determined to be ca. 350 °C and so easily within the range of a MEMS based SOI CMOS substrate. The response was sensitive to significant changes in ambient humidity, but was found to have low cross-sensitivity to CO, hydrogen, methane, and acetone even at much higher ppm levels. We believe that these tungsten oxide gas sensors could be exploited in harsh applications, i.e. with a low oxygen (lean) environment often associated in the exhaust gases from combustion systems

Molecular Wires and Molecular Rectification from Ionically Coupled Srtuctures

EThOS - Electronic Theses Online ServiceGBUnited Kingdo

H2S sensing in dry and humid H2 environment with p-type CuO thick film gas sensors

In this study we report for the first time detailed analysis of a p-type copper oxide based MEMS gas sensor to low ppm levels of hydrogen sulfide in a pure hydrogen environment and under various operating temperatures and humidity conditions. The p-type metal oxide sensing response to hydrogen sulfide seems to be reasonably stable and reversible under both dry and humid hydrogen ambiences. The response was sensitive to significant changes in ambient humidity, but was found to have no cross-sensitivity to carbon monoxide in dry and humid hydrogen. We believe that these copper oxide gas sensors could be exploited in harsh applications, i.e. in a gas contamination detector for testing the quality of hydrogen fuel

Identification of H2S impurity in hydrogen using temperature modulated metal oxide resistive sensors with a novel signal processing technique

We report here on the results of a study on the response of copper oxide (p-type and mixed) based, lowpower MEMS gas sensors to ppm levels of hydrogen sulfide in a hydrogen atmosphere. It is shown that a thermallymodulated metal oxide gas sensor can be used for the identification of the H2S impurity in a reducing environment without the need for a reference gas or baseline signal. It was found that using this material with a method of transient frequency analysis, a resistive gas sensor can operate reliably in a harsh environment, e.g. pure hydrogen and elevated humidity levels. A linear response in gas concentration was obtained from the analysis of transient sensor signal. This novel temperature modulation technique is more effective than an isothermal method where H2 background is introduced, due to the elimination of drift and additive noise in the sensor response that limits the practical utilization of these low cost sensors

Prediction of impurities in hydrogen fuel supplies using a thermally-modulated CMOS gas sensor : experiments and modelling

We report here on the results of a study on the response of copper oxide based low-power MEMS thermally modulated gas sensor to low ppm levels of hydrogen sulphide (H2S) in a hydrogen environment. It was found that by using this material with a method of transient frequency analysis, this resistive gas sensor can operate reliably in a harsh environment including hydrogen atmosphere and high humidity levels. We implemented a Bayesian method for data analysis to predict the concentration of H2S in hydrogen supplies used in fuel cells

Integrated manufacture of polymer and conductive tracks for real-world applications

The present study demonstrates for the first time a unique UK-designed and built Additive Manufacturing (AM) hybrid system that combines polymer based structural deposition with digital deposition of electrically conductive elements. This innovative manufacturing system is based on a multi-planar build approach to improve on many of the limitations associated with AM, such as poor surface finish, low geometric tolerance and poor robustness. Specifically, the approach involves a multi-planar Material Extrusion (ME) process in which separated build stations with up to 5 axes of motion replace traditional horizontally-sliced layer modelling. The construction of multi-material architectures also involved using multiple print systems in order to combine both ME and digital deposition of conductive material. To demonstrate multi-material 3D Printing (3DP) we used three thermoplastics to print specimens, on top of which a unique Ag nano-particulate ink was printed using a non-contact jetting process, during which drop characteristics such as shape, velocity, and volume were assessed using a bespoke drop watching system. Electrical analysis of printed conductive tracks on polymer surfaces was performed during mechanical testing (static tensile and flexural testing and dynamic fatigue testing) to assess robustness of the printed circuits. Both serpentine and straight line patterns were used in the testing of Ag particle loaded ink and they showed very similar resistance changes during mechanical exposure. Monitored resistance and stress changed as a function of strain exhibiting hysteresis with more prominent residual strain during stretching and compression cycles and 3-point bending flexural tests of PA and CoPA substrates. Bare and encapsulated tracks exhibited low electrical resistivity (1–3*10−6 Ω*m), and its change was more rapid on ABS and minor on PA and CoPA when increasing tensile and flexural strain up to 1.2% and 0.8%, respectively. Resistance of Ag tracks on ABS also increased rapidly during fatigue testing and the tracks easily fractured during repeated stretching-compression cycles at 1% and 1.2% strain. No resistance changes of Ag tracks printed on PA and CoPA were observed at lower strain amplitudes whereas at higher strain amplitudes these changes were the lowest for conductive tracks on CoPA. Thermal analyses were conducted to determine the printed material’s glass transition temperature (Tg), stability and degradation behavior to find the optimum annealing conditions post printing. The novel AM printer has the ability to fabricate fully functional objects in one build, including integrated printed circuitry and embedded electronics. It enables product designers and manufactures to produce functional saleable electronic products. This new technology also gives the opportunity for designers to improve existing products, as well as create new products with the added advantages of geometrically unconstrained 3DP