Browsing by Author "Hodgkinson, Jane"

Now showing 1 - 20 of 58
  • No Thumbnail Available
    ItemOpen Access
    A long-wave infra-red pulsed external-cavity QCL spectrometer using a hollow waveguide gas cell: Data
    (Cranfield University, 2024-04-25 08:45) Francis, Daniel; Hodgkinson, Jane; Tatam, Ralph
    A spectrometer built using an external cavity pulsed quantum cascade laser is described. The spectrometer has a tuning range from 10 €“ 13 um (1,000 €“ 769 cm-1) and is designed to target volatile organic compounds (VOCs) which often exhibit water-free molecular absorption within the region. The spectrometer utilizes a hollow silica waveguide gas cell which has an internal volume of a few millilitres, a fast response time (~1 s), and is advantageous when only low sample volumes, similar to the cell volume, are available.
  • Thumbnail Image
    ItemOpen Access
    All-electronic frequency stabilization of a DFB laser diode
    (Optical Society of America, 2017-05-10) Asmari, A.; Hodgkinson, Jane; Chehura, Edmon; Staines, Stephen E.; Tatam, Ralph P.
    A laser diode’s junction voltage is a sensitive measure of its temperature and can be used in a thermal control feedback loop. To compensate for the temperature dependence of the laser’s internal resistance, we have measured the dynamic resistance, ∂V/∂I, by modulating the injection current and measuring the demodulated voltage. The junction voltage was thus controlled while operating at fixed DC injection current. Over an external temperature range of 15°C to 35°C, this stabilised the centre frequency (wavelength) of a 1651 nm DFB laser diode with a residual mean frequency shift of 60 MHz (0.5pm), less than the uncertainty on the centre frequency of 80 MHz (0.7 pm). Under the same conditions, conventional thermistor control gave a systematic wavelength shift of −8.4 GHz (−76 pm), and control of the uncompensated forward voltage gave a shift of 9.9 GHz (90 pm).
  • No Thumbnail Available
    ItemOpen Access
    Applied Optics: HSW gas cells for mid-IR spectroscopy (2016)
    (Cranfield University, 2016-08-22 13:32) Francis, Daniel; Hodgkinson, Jane; Tatam, Ralph
    Data associated with Applied Optics submission, 'Low volume, fast response-time hollow silica waveguide gas cells for mid-IR spectroscopy'
  • Thumbnail Image
    ItemOpen Access
    Compact methane sensor using an integrating sphere and interband cascade laser at 3313 nm
    (Elsevier, 2023-04-23) Davis, Nicholas M.; Francis, Daniel; Hodgkinson, Jane; Tatam, Ralph P.
    We present a sensor for gas detection utilising an integrating sphere and interband cascade laser for tunable diode laser absorption spectroscopy (TDLAS). Measurements were made of the methane absorption line at 3313 nm using an integrating sphere with an effective pathlength of 70.1 cm. Gas concentrations were estimated from normalised absorption spectra with line fitting to 7 absorption lines within the scan. Testing showed that measurements were linear for methane in synthetic air for concentrations in the range 0-50ppm. With an averaging time of 20 seconds, the noise-equivalent methane concentration was 180ppb (1σ). The system requires minimal alignment, with preliminary measurements indicating the system is insensitive to vibration and misalignment of the laser input. The lack of alignment optics also allows it to be compact (190x170x120mm) and robust.
  • Thumbnail Image
    ItemOpen Access
    Comparison of self-referencing techniques for photothermal detection of trace compounds in water
    (Elsevier Science, 2000-09) Hodgkinson, Jane; Johnson, Mark; Dakin, John P.
    Self-referencing techniques are compared for a closed-cell photothermal detector that uses a water meniscus as a pressure sensor. Deflection of the meniscus was measured using an optical fibre Fabry–Perot interferometer. For long measurement integration times, interference fringe drift was a serious limitation on the detection repeatability for non self-referenced measurements. Two self-referencing techniques were compared for measurements of optical absorption. The first technique used a simultaneous reference absorption signal at a second wavelength, and the second used a simultaneous volumetric modulation within the cell. Both methods have been evaluated with photothermal excitation by a 658-nm LED, a 478-nm LED and a UV discharge lamp. For the detection of absorption in aqueous solutions, the two methods had similar performance. However, the volumetric method could be used for detection of any absorbing compound, regardless of its absorption spectrum and was more convenient to use.
  • No Thumbnail Available
    ItemOpen Access
    Data supporting 'Fibre-Coupled, Multiplexed Methane Detection Using Range Resolved Interferometry'
    (Cranfield University, 2023-02-10 15:51) Bremner, James; Kissinger, Thomas; Hodgkinson, Jane; Tatam, Ralph
    Dataset for paper: Fig 2 & Fig3: Time of measurement, The OPD being interrogated, The amplitude of the signal produced by the RRI system, The applied window used, The raw interferrometric signal from the photdiode Note: The Time measurement only applies to the measurement of the interferometric signal. The RRI signal amplitude is instantaneous Fig 4: The extracted light intensity passing through each tube as a function of time over the period of one ramp. (arbitary units) Fig 5 Absorption Heights: The height of the normalised absorption curves in each test Fig 5 Recorded Gas Flows: The gas flow measurements in each test that were used to calculate the gas concentrations in each test Fig 6: Allan Deviation: The value for the Allan Deviation at each integration period Fig 7: Cross Talk Raw Data: The height of the normalised absorption curve measured for Tube 1 in each test. Fig 7: Cross Talk Protocol: The gas flow supplied to each tube during each test.
  • No Thumbnail Available
    ItemOpen Access
    Data supporting the publication 'Compact methane sensor using an integrating sphere and Interband Cascade Laser at 3313nm'
    (Cranfield University, 2023-04-28 16:45) Davis, Nicholas; Francis, Daniel; Hodgkinson, Jane; Tatam, Ralph
    Data associated with Sensors and Actuators B submission "Compact methane sensor using an integrating sphere and Interband Cascade Laser at 3313nm"
  • Thumbnail Image
    ItemOpen Access
    Detection of a simulated gas leak in a wind tunnel
    (Iop Publishing Ltd, 2006-01-01T00:00:00Z) Hodgkinson, Jane; Shan, Q.; Pride, Russ D.
    This paper brings together considerations of gas leak behaviour and leak detector design and use, with a view to improving the detection of low-pressure natural gas leaks. An atmospheric boundary layer wind tunnel has been used to study ground-based releases of methane at full scale over distances of up to 3 m, under controlled conditions. These scales are relevant to the detection of natural gas leaks from mains and services using hand-portable gas detectors. The mean spatial distribution of the leaking gas plume was determined and used to test and fit a Gaussian dispersion model. This was used for subsequent analysis with respect to the ability of gas leak detectors to confirm and locate a leak. For ground-based leaks, gas concentrations drop rapidly with height such that instruments should ideally sample the air from within 100 mm of ground level. The rapid dilution of gas with distance from the source means that instruments with lower limits of detection, ideally of a few parts per million, have much improved ability to detect a leak from greater distances downwind. Finally, observations showed the variable temporal nature of the gas and the potential for confusion when sampling gas at a single point in time and space.
  • Thumbnail Image
    ItemOpen Access
    Fibre optic hydrogen sensing for long term use in explosive environments
    (Cranfield University, 2015-09) Chowdhury, S. A.; Hodgkinson, Jane; Tatam, Ralph P.
    Hydrogen is an explosive and flammable gas with a lower explosive limit of just 4% volume in air. It is important to monitor the concentration of hydrogen in a potentially hazardous environment where hydrogen may be released as a by-product in a reaction or used as a principal gas/liquid. A fibre optic based hydrogen sensor offers an intrinsically safe method of monitoring hydrogen concentration. Previous research studies have demonstrated a variety of fibre optic based techniques for hydrogen detection. However the long-term stability of the hydrogen sensor and interrogation system has not yet been assessed and is the focus of this study. In the case of sensor heads being permanently installed in-situ, they cannot be removed for regular replacement, making long-term stability and reliability of results an important feature of the hydrogen sensor. This thesis describes the investigation and characterisation of palladium coated fibre optic sensor heads using two designs of self-referenced refractometer systems with the aim of finding a system that is stable in the long term (~6 months). Palladium was the chosen sensing material owing to its selective affinity for absorbing hydrogen. Upon hydrogen absorption, palladium forms a palladium- hydride compound that has a lower refractive index and lower reflectivity than pure palladium. The refractometers measured the changes in the reflectivity to enable calculation of the concentration of hydrogen present. A low detection limit of 10ppm H2 in air was demonstrated, with a response time of 40s for 1000ppm H2 in air. A further aspect to sensor stability was investigated in the form of sensor heads that had a larger area for palladium coverage. Hydrogen induced cracking in palladium is a common failure mechanism. A hypothesis is presented that a larger sensor area can reduce the probability of catastrophic failure resulting from cracks, which may improve the predictability of the sensor’s performance. Two sensor head designs have been proposed – fibre with a ball lens at the tip and fibre with a GRIN lens at the tip, both of which potentially offer a larger area than the core of a singlemode optical fibre. The limit of detection and response times of the sensor heads were characterised in hydrogen. For long term stability assessment of the sensor head and the interrogation unit, the system was left running for a period of 1 to 4 weeks and the noise and drift in the system was quantified using an Allan deviation plot.
  • Thumbnail Image
    ItemOpen Access
    Fibre-coupled, multiplexed methane detection using range-resolved interferometry
    (Institute of Physics, 2021-02-08) Bremner, James; Kissinger, Thomas; Hodgkinson, Jane; Tatam, Ralph P.
    We describe the first use of range-resolved interferometric signal processing for measurement of spectral transmission. This was applied to gas sensing using tunable diode laser spectroscopy, allowing the simultaneous and independent measurement of methane concentrations in multiple gas cells. The system uses a single injection-current modulated diode laser and a single photodetector. For three gas cells, we show the ability of the system to measure methane at noise equivalent concentrations of less than 200 ppm for a 0.5 s measurement period and a potential noise equivalent concentration (1σ) of <20 ppm with 150 s averaging time. We further show that cross-talk between cells is below the experimental uncertainty for the system
  • Thumbnail Image
    ItemOpen Access
    Field evaluation of a multi-point fibre optic sensor array for methane detection (“OMEGA”)
    (Society of Photo-optical Instrumentation Engineers (SPIE), 2000) Hodgkinson, Jane; Pride, Russ D.; Tandy, Chris; Moodie, Dave; Stewart, George
    A multi-point fibre optic sensor array for methane detection (“OMEGA”) has been developed and tested under semiquantitative field conditions. The new system employed wavelength modulation spectroscopy using a DFB laser source scanned across the Q6 methane line at 1.665•m. A branched fibre network connected the single source to up to 64 sensor heads. Controlled releases of natural gas were provided for test purposes within an array of four optical sensors and four pellistor reference sensors. An automated system delivered standard gases to each sensor, to enable routine calibration checks to be carried out. Agreement between the conventional and optical systems was excellent in the range 0-100% LEL (lower explosion limit). The optical system offers a simple, intrinsically safe design with a low cost of ownership per sensor head.
  • Thumbnail Image
    ItemOpen Access
    Formaldehyde sensor using non-dispersive UV spectroscopy at 340nm
    (International Society for Optical Engineering; 1999, 2014-05-22T00:00:00Z) Davenport, John; Hodgkinson, Jane; Saffell, John R.; Tatam, Ralph P.; Berghmans, F.; Mignani, A. G.; De Moor, P.
    Formaldehyde is a volatile organic compound that exists as a gas at room temperature. It is hazardous to human health causing irritation of the eyes, nose and throat, headaches, limited pulmonary function and is a potential human carcinogen. Sources include incomplete combustion, numerous modern building materials and vehicle fumes. Here we describe a simple method for detecting formaldehyde using low resolution non-dispersive UV absorption spectroscopy for the first time. A two channel system has been developed, making use of a strong absorption peak at 339nm and a neighbouring region of negligible absorption at 336nm as a reference. Using a modulated UV LED as a light source and narrowband filters to select the desired spectral bands, a simple detection system was constructed that was specifically targeted at formaldehyde. A minimum detectable absorbance of 4.5 × 10-5 AU was estimated (as ΔI/I0), corresponding to a limit of detection of approximately 6.6 ppm for a 195mm gas cell, with a response time of 20s. However, thermally-induced drift in the LED spectral output caused this to deteriorate over longer time periods to around 30 ppm or 2 × 10-4 AU
  • Thumbnail Image
    ItemOpen Access
    Gas cells for tunable diode laser absorption spectroscopy employing optical diffusers. Part 1: single and dual pass cells
    (Springer Science Business Media, 2010-08-31T00:00:00Z) Hodgkinson, Jane; Masiyano, Dackson; Tatam, Ralph P.
    New designs for gas cells are presented that incorporate transmissive or reflective optical diffusers. These components offer simple alignment and can disrupt the formation of optical etalons. We analyse the performance-limiting effects in these cells of random laser speckle (both objective and subjective speckle), interferometric speckle and self-mixing interference, and show how designs can be optimised. A simple, single pass transmissive gas cell has been studied using wavelength modulation spectroscopy to measure methane at 1651 nm. We have demonstrated a short-term noise equivalent absorbance (NEA, 1 sigma) of 2x10(-5), but longer term drift of up to 3x10(-4) over 22 hours.
  • Thumbnail Image
    ItemOpen Access
    Gas cells for tunable diode laser absorption spectroscopy employing optical diffusers. Part 2: Integrating spheres
    (Springer Science Business Media, 2010-08-31T00:00:00Z) Masiyano, Dackson; Hodgkinson, Jane; Tatam, Ralph P.
    We have studied the effects of random laser speckle and self-mixing interference on TDLS based gas measurements made using integrating spheres. Details of the theory and TDLS apparatus are given in Part 1 of this paper and applied here to integrating spheres. Experiments have been performed using two commercial integrating spheres with diameters of 50 mm and 100 mm for the detection of methane at 1651 nm. We have calculated the expected levels of laser speckle related uncertainty, considered to be the fundamental limiting noise, and imaged subjective laser speckle in a sphere using different sized apertures. For wavelength modulation spectroscopy, noise equivalent absorbances (NEAs) of around 5x10(-5) were demonstrated in both cases, corresponding to limits of detection of 1.2 ppm methane and 0.4 ppm methane respectively. Longer-term drift was found to be at an NEA of 4x10(-4). This lies within our broad range of expectations. For a direct spectral scan with no wavelength dither, a limit of detection of 75 ppm or fractional measured power uncertainty of 3x10(-3) corresponded well with our prediction for the objective speckle uncertainty.
  • Thumbnail Image
    ItemOpen Access
    Imaging of methane gas using a scanning, open-path laser system
    (IOP Publishing Ltd, 2006-02) Gibson, Graham; van Well, Ben; Hodgkinson, Jane; Pride, Russ D.; Strzoda, Rainer; Murray, Stuart; Bishton, Steve; Padgett, Miles
    We have developed an imaging system for the detection and visualization of methane gas leaks. The system is based on a distributed feedback InGaAs laser diode emitting at 1.65μm, the beam from which is directed at neighbouring objects. The backscattered light is collected by a Fresnel lens and the gas concentration is deduced from the reduction in collected intensity as measured using a second derivative wavelength modulation technique. The incident laser and the collected beam are both scanned over an area to form an image of the gas emission. To ease the task of locating the source of the emission, we combine the resulting low-resolution image of the gas emission with a high-resolution colour image of the scene. Our results show that the system can image a gas cloud of 1mm effective thickness at a range of several metres, sufficient to detect a gas leak of 1 litre min−1 in light to moderate winds.
  • No Thumbnail Available
    ItemOpen Access
    Instrumentation for quantitative analysis of volatile compounds emission at elevated temperatures. Part 1: Design and implementation
    (Cranfield University, 2020-05-27 20:00) Lourenco, Celia; Bergin, Sarah; Hodgkinson, Jane; Francis, Daniel; E. Staines, Stephen; R Saffell, John; Walton, Christopher; Tatam, Ralph
    Data to support manuscript Part 1.
  • Thumbnail Image
    ItemOpen Access
    Instrumentation for quantitative analysis of volatile compounds emission at elevated temperatures. Part 1: Design and implementation
    (Nature Publishing Group, 2020-05-26) Lourenço, Célia; Bergin, Sarah; Hodgkinson, Jane; Francis, Daniel; Staines, Stephen E.; Saffell, John R.; Walton, Christopher; Tatam, Ralph P.
    A novel suite of instrumentation for the characterisation of materials held inside an air-tight tube furnace operated up to 250 °C has been developed. Real-time detection of released gases (volatile organic compounds (VOCs), CO2, NO, NO2, SO2, CO and O2) was achieved combining commercial off-the-shelf (COTS) gas sensors and sorbent tubes for further qualitative and semi-quantitative analysis by gas chromatography-mass spectrometry coupled to thermal desorption (TD-GC-MS). The test system was designed to provide a controlled flow (1000 cm3 min−1) of hydrocarbon free air through the furnace. The furnace temperature ramp was set at a rate of 5 °C min−1 with 10 min dwell points at 70 °C, 150 °C, 200 °C and 250 °C to allow time for stabilisation and further headspace sampling onto sorbent tubes. Experimental design of the instrumentation is described here and an example data set upon exposure to a gas sample is presented.
  • No Thumbnail Available
    ItemOpen Access
    Instrumentation for quantitative analysis of volatile compounds emission at elevated temperatures. Part 2: Analysis of carbon fibre reinforced epoxy composite
    (Cranfield University, 2020-05-27 20:01) Lourenco, Celia; Francis, Daniel; Fowler, Dawn; E. Staines, Stephen; Hodgkinson, Jane; Walton, Christopher; Bergin, Sarah; Tatam, Ralph
    Data to support manuscript Part 2.
  • Thumbnail Image
    ItemOpen Access
    Instrumentation for quantitative analysis of volatile compounds emission at elevated temperatures. Part 2: Analysis of carbon fibre reinforced epoxy composite
    (Nature Publishing Group, 2020-05-26) Lourenço, Cecilia; Bergin, Sarah; Hodgkinson, Jane; Francis, Daniel; Staines, Stephen E.; Saffell, John R.; Walton, Christopher; Tatam, Ralph P.
    We have investigated the release of gases and volatile organic compounds (VOCs) from a carbon fibre reinforced epoxy composite matrix used in aircraft structural components. Analysis was performed at several temperatures both up to and above the recommended operating temperature (121 °C) for the material, to a maximum of 250 °C. Gas chromatography-mass spectrometry (GC-MS) combined with thermal desorption (TD-GC-MS) was used to identify and quantify VOCs, and in parallel real-time gas detection with commercial off-the-shelf (COTS) gas sensors. Under hydrocarbon free air, CO, SO2, NO, NO2 and VOCs (mainly aldehydes, ketones and a carboxylic acid) were detected as the gaseous products released during the thermal exposure of the material up to 250 °C, accompanied by increased relative humidity (4%). At temperatures up to 150 °C, gas and volatile emission was limited.
  • No Thumbnail Available
    ItemOpen Access
    Integrating cavity based gas cells: a multibeam compensation scheme for pathlength variation
    (Cranfield University, 2016-11-28 13:52) Bergin, Sarah; Hodgkinson, Jane; Francis, Daniel; Tatam, Ralph
    Data to support the following paper: Integrating cavity based gas cells: a multibeam compensation scheme for pathlength variation S Bergin, J Hodgkinson, D Francis and R P Tatam Optics Express 24 (12), 13647 - 13664, 2016 doi:10.1364/OE.24.013647