National Physical Laboratory Research Articles

Differential Absorption Lidar (DIAL) Measurements of Landfill Methane Emissions

Methane is one of the most important gaseous hydrocarbon species for both industrial and environmental reasons. Understanding and quantifying methane emissions to atmosphere is therefore an important element of climate change research. Range-resolved infrared differential absorption Lidar (DIAL) measurements provide the means to map and quantify a wide range of different methane sources. This paper describes the DIAL measurement technique and reports the application of an infrared DIAL system to field measurements of methane emissions from active and closed landfill sites. This paper shows how the capability of the DIAL to measure the spatial distribution of methane plumes enables DIAL vertical scans to spatially separate and independently quantify emissions from different sources. It also allows DIAL horizontal scans carried out above the surface to identify emission hot-spots. An overview of the landfill emission surveys carried out over the last decade by the National Physical Laboratory (NPL) DIAL system is presented. These surveys were part of research projects and commercial works aimed to validate the method and to provide reliable information on the methane emissions measuring the total site and area-specific emissions from active areas, capped areas, and gas engine stacks. This work showed that methane emissions are significantly higher for active sites than closed sites due to the methane emitted directly to air from the uncapped active areas. On active sites, the operational tipping areas generally have higher emission levels than the capped areas, although there is considerably variation in the emission from different capped areas. The information obtained with DIAL measurements allow site operators to identify significant fugitive emission sources and validate emissions estimates, and they allow the regulators to revise and update the emission inventories. Operators’ remediation actions driven by DIAL measurements have also been shown to considerably decreased total site methane emission.

Investigation of γ-γ coincidence counting using the National Nuclear Array (NANA) as a primary standard

The National Physical Laboratory has recently been in the process of commissioning a multi-detector γ ray array - the National Nuclear Array (NANA). In this study we have sought to exploit the NANA and the excellent timing characteristics of its intrinsic LaBr3(Ce) scintillation detectors for use as a primary standardisation system. For this initial investigation, the absolute standardisation of 60Co has been performed by the γ-γ coincidence technique using NANA and the result compared to the established 4π(LS)-γ Digital Coincidence Counting (DCC) system. The effect of the angular correlation of the stretched E2 transitions emitted from the 4+→2+→0 states of 60Ni on the activity determined by NANA was observed between the pairs of detectors. Corrections for these angular correlations were derived through Monte Carlo simulations. An activity per unit mass by NANA of 330.8 (10) kBqg−1 for the 60Co solution was determined. There was no significant statistical difference between the results of NANA and the 4π(LS)-γ DCC, with a relative difference of 0.04% observed. This study shows that NANA can be used as a primary standard.

Realization of Four-Terminal-Pair Capacitors as Reference Standards of Impedance at High Frequency Using Impedance-Matrix Method

The four-terminal-pair air capacitors of nominal values 1000 and 100 pF have been realized as reference standards of impedance upto 10 MHz using impedance-matrix method. The reported work has been successfully practically implemented at the National Physical Laboratory India (NPLI) in the direction of establishing metrological traceability of capacitance standards at high frequency (HF). The realization procedure is based on the determination of effective capacitance of each capacitor as a function of frequency upto 10 MHz. It involves single-port-impedance measurements at HF using RF impedance analyzer and capacitance measurements at 1 kHz using ultraprecision capacitance bridge. The least squares data approximation method is thereafter used to extrapolate the resistive and inductive components of the measured HF single-port-impedances to lower frequencies down to 1 kHz. The measurement automation program has been developed to measure the single-port impedances at set of frequencies between 40 and 100 MHz using impedance analyzer. The air capacitors characterized using impedance-matrix will be used as reference standards of impedance at NPLI to calibrate impedance bridges, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCR</i> meters, and impedance analyzers upto 10 MHz.

Viscosity and Structure of CaO-SiO2-P2O5-FetO System with Varying P2O5 and FeO Content

A rotary viscosimeter and Raman spectrum were employed to measure the viscosity and structural information of the CaO-SiO2-P2O5-FetO system at 1673 K. The experimental data have been compared with the calculated results using different viscosity models. It shows that the National Physical Laboratory (NPL) and Pal models fit the CaO-SiO2-P2O5-FeOt system better. With the P2O5 content increasing from 5% to 14%, the viscosity increases from 0.12 Pa s to 0.27 Pa s. With the FeO content increasing from 30% to 40%, the viscosity decreases from 0.21 Pa s to 0.12 Pa s. Increasing FeO content makes the complicated molten melts become simple, and increasing P2O5 content will complicate the molten melts. The linear relation between viscosity and structure parameter Q(Si + P) was obtained by regression analysis. The calculated viscosity by using the optimized NPL and Pal model are almost identical with the fitted values.

3D thermography for improving temperature measurements in thermal vacuum testing

The application of thermography to thermal vacuum (TV) testing of spacecrafts is becoming a vital additional tool in the mapping of structures during thermal cycles and thermal balance (TB) testing. Many of the customers at the European Space Agency (ESA) test centre, European Space Research and Technology Centre (ESTEC), The Netherlands, now make use of a thermal camera during TB-TV campaigns. This complements the use of embedded thermocouples on the structure, providing the prospect of monitoring temperatures at high resolution and high frequency. For simple flat structures with a well-defined emissivity, it is possible to determine the surface temperatures with reasonable confidence. However, for most real spacecraft and sub-systems, the complexity of the structure’s shape and its test environment creates inter-reflections from external structures. This and the additional complication of angular and spectral variations of the spacecraft surface emissivity make the interpretation of the radiation detected by a thermal camera more difficult in terms of determining a validated temperature with high confidence and well-defined uncertainty. One solution to this problem is: to map the geometry of the test specimen and thermal test environment; to model the surface temperatures and emissivity variations of the structures and materials; and to use this model to correct the apparent temperatures recorded by the thermal camera. This approach has been used by a team from NPL (National Physical Laboratory), Psi-tran, and PhotoCore, working with ESA, to develop a 3D thermography system to provide a means to validate thermal camera temperatures, based on a combination of thermal imaging photogrammetry and ray-tracing scene modeling. The system has been tested at ESTEC in ambient conditions with a dummy spacecraft structure containing a representative set of surface temperatures, shapes, and spacecraft materials, and with hot external sources and a high power lamp as a sun simulator. The results are presented here with estimated temperature measurement uncertainties and defined confidence levels according to the internationally accepted Guide to Uncertainty of Measurement as used in the IEC/ISO17025 test and measurement standard. This work is understood to represent the first application of well-understood thermal imaging theory, commercial photogrammetry software, and open-source ray-tracing software (adapted to realize the Planck function for thermal wavebands and target emission), and to produce from these elements a complete system for determining true surface temperatures for complex spacecraft-testing applications.

Radiotherapy reference dose audit in the United Kingdom by the National Physical Laboratory: 20 years of consistency and improvements

Background and Purpose Audit is imperative in delivering consistent and safe radiotherapy and the UK has a strong history of radiotherapy audit. The National Physical Laboratory (NPL) has undertaken audit measurements since 1994 and this work examines results from these audits. Materials and Methods This paper reviews audit results from 209 separate beams from 82 on-site visits to National Health Service (NHS) radiotherapy departments conducted between June 1994 and February 2015. Measurements were undertaken following the relevant UK code of practice. The accuracy of the implementation of absorbed dose calibration across the UK is quantified for MV photon, MeV electron and kV x-ray radiotherapy beams. Results Over the measurement period the standard deviation of MV photon beam output has reduced from 0.8 % to 0.4 %. The switch from air kerma- to absorbed dose-based electron code of practice contributed to a reduction in the difference of electron beam output of 0.6 % (p < 0.01). The mean difference in NPL to local measurement for radiation output calibration was less than 0.25 % for all beam modalities. Conclusions The introduction of the 2003 electron code of practice based on absorbed dose to water decreased the difference between absolute dose measurements by the centre and NPL. The use of a single photon code of practice over the period of measurements has contributed to a reduction in measurement variation. Within the clinical setting, on-site audit visits have been shown to identify areas of improvement for determining and implementing absolute dose calibrations.

Identifying measurement challenges in the hydrogen industry

The National Physical Laboratory (NPL) in the UK has published a report outlining the role that hydrogen could play in the decarbonisation of energy and the measurement challenges facing its growth.

An analysis of type F2 software measurement standards for profile surface texture parameters

This paper reports on an in-depth analysis of ISO 5436 part 2 type F2 reference software for the calculation of profile surface texture parameters that has been performed on the input, implementation and output results of the reference software developed by the National Physical Laboratory (NPL), the National Institute of Standards and Technology (NIST) and Physikalisch-Technische Bundesanstalt (PTB). Surface texture parameters have been calculated for a selection of 17 test data files obtained from the type F1 reference data sets on offer from NPL and NIST. The surface texture parameter calculation results show some disagreements between the software methods of the National Metrology Institutes. These disagreements have been investigated further, and some potential explanations are given.

Structural health monitoring of infrastructure with sensors: from detection to prevention

UK researchers have found a way of using infrastructure sensor data to predict – rather than simply to detect – structural damage. Valerie Livina at the National Physical Laboratory and Marcus Perry at the University of Strathclyde say it works with concrete and is now being tested on steel.

Tests of the Stability of Chinese RhFe Resistance Thermometers at Low Temperatures

Rhodium–iron resistance thermometers are recommended as precise thermometers at temperatures below 25 K. The thermometers were developed at the National Physical Laboratory and produced by H. Tinsley and Co Ltd almost 50 years ago. Later, they were made by other companies and institutes as well, but despite this, the availability of the thermometers decreased and a new source of supply was needed. Several years ago, the Technical Institute of Physics and Chemistry (TIPC), Chinese Academy of Sciences, developed its own technology for making wire of Rh-0.5 at% Fe alloy which was used in the production of new thermometers. These devices have been tested previously at INRIM (Italy) and later at INTiBS (Poland). INTiBS has carried out an investigation focused on the thermometers’ stability after thermal cycling treatment. This paper presents the results of stability tests of about 30 thermometers produced by TIPC in two batches. The resistance of each thermometer was measured at temperatures of about 4.6 K and 7.2 K before and after 10, 30 and 50 thermal cycles from room temperature. The methods of measurement and the design of the cryostat used for the research are also presented.

Doped silica fibre thermoluminescence measurements of radiation dose in the use of 223Ra

Using tailor-made sub-mm dimension doped-silica fibres, thermoluminescent dosimetric studies have been performed for α-emitting sources of 223RaCl2 (the basis of the Bayer Healthcare product Xofigo®). The use of 223RaCl2 in the palliative treatment of bone metastases resulting from late-stage castration-resistant prostate cancer focuses on its favourable uptake in metabolically active bone metastases. Such treatment benefits from the high linear energy transfer (LET) and associated short path length (<100µm) of the α-particles emitted by 223Ra and its decay progeny. In seeking to provide for in vitro dosimetry of the α-particles originating from the 223Ra decay series, investigation has been made of the TL yield of various forms of Ge-doped SiO2 fibres, including photonic crystal fibre (PCF) collapsed, PCF uncollapsed, flat and single-mode fibres. Irradiations of the fibres were performed at the UK National Physical Laboratory (NPL). Notable features are the considerable sensitivity of the dosimeters and an effective atomic number Zeff approaching that of bone, the glass fibres offering the added advantage of being able to be placed directly into liquid. The outcome of present research is expected to inform development of doped fibre dosimeters of versatile utility, including for applications as detailed herein.

Study of intra-city urban heat island intensity and its influence on atmospheric chemistry and energy consumption in Delhi

Urban heat island intensity (UHII) has been determined as difference in air temperatures between two locations of Delhi namely Safdarjung (SAFD) and National Physical Laboratory (NPL) regions which represent urban and sub-urban areas respectively. High UHI has been obtained in morning time as compared to day and night times, with the highest magnitude ranging from 2.8 to 3°C in spring (MAM), autumn (SON) and winter (DJF) seasons in morning hours (0700–0900h). Monsoon season (JJA) shows less UHII values compared to other seasons. Wind speed reduced the UHII. Larger impervious area (i.e. 67.2% in SAFD as compared to 33.4% in NPL), lesser vegetated area (i.e. 32.8% in SAFD as compared to 66.6% in NPL) and low Normalized Difference Vegetation Index (NDVI) value (0.15 in SAFD as compared to 0.24 in NPL) in SAFD area supports the existence of UHI there. Ambient O3 concentrations show maximum values during April and May for both regions and remain insignificantly influenced by UHI. Intra city temperature difference of 0.2–3°C is capable of raising electricity demand by 37.87–1856 GWh over the base electricity requirement of the city with corresponding increase in CO2 emissions by 0.031–1.52 million ton.

In situ remote hot erosion scar measurement

When particles of grit, oxide or other contaminants get entrained in high speed gas or steam sections of power or aero turbines they can damage components through repeated impact of particles with the surface. This erosion in turn produces additional debris which can damage other surfaces further down-stream. An instrument to evaluate and quantify high temperature solid particulate erosion (HTSPE) is being designed and built at the National Physical Laboratory (UK). In order to speed up the measurement of the erosion scars, a system is required that can measure the shape and depth of scars on test coupons of materials without the need to remove them from the HTSPE system to perform the measurement. This paper deals with the optical technique that has been developed and validates its performance against a specially prepared test artefact containing ‘erosion pits’ of several different depths as well as on actual erosion scars produced with a particulate erosion system.

Compressed Sensing Current Mapping Spatial Characterization of Photovoltaic Devices

A new photovoltaic (PV) device current mapping method has been developed, combining the recently introduced compressed sensing (CS) sampling theory with light beam induced current (LBIC) measurements. Instead of a raster scan, compressive sampling is applied using a digital micromirror device. The aim is to significantly reduce the time required to produce a current map, compared to conventional LBIC measurements. This is achieved by acquiring fewer measurements than a full raster scan and by utilizing the fast response of the micromirror device to modulate measurement conditions. The method has been implemented on an optical current mapping setup built at the National Physical Laboratory, U.K. Measurements with two different PV cells are presented in this paper and an analytical description for realization of an optimized CS current mapping system is provided. The experimental results illustrate the feasibility of the method and its potential to significantly reduce measurement time of current mapping of PV devices.

Printing: the future of THz

Researchers from Imperial College London, the UK's National Physical Laboratory (NPL), and the University of Tokyo, have demonstrated a 3D-printed metal-pipe rectangular waveguide operating at a record-breaking frequency of 1.1 THz. Stepan Lucyszyn and William Otter, two of the researchers behind the record-breaking waveguides. 3D printing has found uses in a huge number of industries and applications, and its strengths over traditional manufacturing techniques were quickly exploited by the communications community. William Otter and Stepan Lucyszyn, two of the researchers on the THz project, told us that at Imperial College London, ‘the initial research into 3D printing of radio frequency metal-pipe rectangular waveguides was for the microwave frequency range of 8 to 12 GHz (X-band), and the success of this work gave our team the confidence to move up into the millimetre-wave frequency band, of 75–110 GHz (W-band), where the small dimensions require a tighter manufacturing tolerance.’ 3D printing, also known as additive manufacturing, is the layer-by-layer addition of material to create a structure. It dramatically reduces the time and the cost of producing components, while allowing more design freedom. In comparison with traditional manufacturing, ever more complex geometries can be realised with 3-D printing as fewer mechanical constraints are imposed on the process. With large complex structures, this means reducing the number of individual components needed to create a subsystem, thereby reducing the number of tests required during the manufacturing process. This design freedom helped produce excellent initial results in the W-band, comparable with off-the-shelf commercial waveguides. However, as Otter and Lucyszyn explained, they did not stop there, ‘This high level of performance was also demonstrated at W-band with a 6th-order Chebyshev filter, which are notoriously sensitive to manufacturing tolerances.’ The next challenge was to demonstrate a mechanically tuneable component that was fully 3D-printed. This was achieved using an X-band dielectric-flap phase shifter. After these early successes the team kept increasing the frequencies of their components. ‘We were pushing the mechanical limits of all 3D printing processes, but, we set our sights specifically on breaking the record for metal-pipe rectangular waveguides,’ said Otter and Lucyszyn, ‘and this was achieved with an I-Q vector modulator operating up to 500 GHz.’ Overcoming the limitations of 3D printing, specifically in regards to THz technology, to achieve record frequencies is where the collaboration between the UK and Japanese began: ‘The main limitation for THz waveguides is the ability to 3D print with relatively large structures, having small feature sizes and high-quality surface finishes.’ For this reason, the prototype for a new class of 3D-printers called RECILS, and developed at the University of Tokyo was used. The new RECILS technology, through its innovative method of controlling the photocurable resin level and detachment of cured resin, provides the required finish without further post-processing.’ Now that the team have achieved inexpensive THz technology, what doors do they hope to have opened? The work is expected to find future applications in high-performance imaging systems for security monitoring, satellite payloads and medical diagnostic equipment. Thanks to the team's techniques, according to Otter and Lucyszyn, ‘these systems can be developed in a shorter time and at a significantly lower cost, and, as such, [the method] represents a paradigm shift in the manufacture of precision components using mass marketable 3D printers.’ From left to right: X-Band phase shifter, W-Band thru line, W-Band 6th-order Chebyshev filter, 500 GHz I-Q vector modulator. In the background is an X-band twist and the foreground shows the 1.1 THz waveguide. To move ever closer to such a reality, the group have been developing minimal-part subsystems that break conventional design rules by exploiting the multiple degrees-of-freedom offered by additive manufacturing. And this is, as Otter and Lucyszyn explained, the really exciting thing about the technology. ‘Within a decade, ultra-high resolution low-cost 3D printing will be ubiquitous. It will become the norm to replace parts by downloading CAD files and printing and spray coating metal onsite.’ This is particularly important for remote and inaccessible locations. Moreover, a printer offering multiple materials, perhaps even having exotic properties, will be capable of printing whole systems, the only limit being our imaginations.’

Portable System for Practical Permittivity Measurements Improved by Homomorphic Deconvolution

This paper presents a versatile low-cost procedure for complex permittivity measurements of liquids from 400 MHz to 5 GHz. Our procedure uses handheld instrumentation and custom software for data acquisition and postprocessing. The purpose is to replace the benchtop vector network analyzer (VNA) generally used in such applications with a portable and cheaper handheld spectrum analyzer used in the VNA mode. Postprocessing software based on homomorphic deconvolution is used to remove possible inaccuracies in the permittivity spectra coming from the reduced performance of the handheld VNA with respect to benchtop models. Our measurements are evaluated by comparison with those of the National Physical Laboratory (NPL). The results of the real and imaginary parts of the permittivity spectra of two well characterized organic compounds, ethanediol and 2-propanol, are in very good agreement with the NPL reference standards, showing a relative root-mean-square error (RRMSE) always less than 5%. Low values of the RRMSE, together with an expanded uncertainty less than 3%, ensure that our permittivity measurements are repeatable and accurate. Thanks to its versatility, portability, and at least half of the cost of commercial models, our system is suitable for on-site measurements in different applications, including food quality monitoring and control of medical treatments and biological procedures.

Wet and Dry Transom Stern Treatment for Unsteady and Nonlinear Potential Flow Model for Naval Hydrodynamics Simulations

We present a model for the fast evaluation of the total drag of ship hulls operating in both wet and dry transom stern conditions, in calm or wavy water, based on the combination of an unsteady semi-Lagrangian potential flow formulation with fully nonlinear free-surface treatment, experimental correlations, and simplified viscous drag modeling. The implementation is entirely based on open source libraries. The spatial discretization is solved using a streamline upwind Petrov-Galerkin stabilization of an iso-parametric, collocation based, boundary element method, implemented using the open source library deal. II. The resulting nonlinear differential-algebraic system is integrated in time using implicit backward differentiation formulas, implemented in the open source library SUNDIALS. The Open CASCADE library is used to interface the model directly with computer-aided design data structures. The model accounts automatically for hulls with a transom stern, both in wet and dry regimes, by using a specific treatment of the free-surface nodes on the stern edge that automatically detects when the hull advances at low speeds. In this case, the transom stern is partially immersed, and a pressure patch is applied on the water surface detaching from the transom stern, to recover the gravity effect of the recirculating water on the underlying irrotational flow domain. The parameters of the model used to impose the pressure patch are approximated from experimental relations found in the literature. The test cases considered are those of the U.S. Navy Combatant DTMB-5415 and the National Physical Laboratory hull. Comparisons with experimental data on quasi-steady test cases for both water elevation and total hull drag are presented and discussed. The quality of the results obtained on quasi-steady simulations suggests that this model can represent a promising alternative to current unsteady solvers for simulations with Froude numbers below 0.35.

Calibration Strategies for FT-IR and Other Isotope Ratio Infrared Spectrometer Instruments for Accurate δ13C and δ18O Measurements of CO2 in Air.

This paper describes calibration strategies in laboratory conditions that can be applied to ensure accurate measurements of the isotopic composition of the CO2 in ultradry air, expressed as δ13C and δ18O on the VPDB scale, with either FT-IR (in this case a Vertex 70 V (Bruker)) or an isotope ratio infrared spectrometer (IRIS) (in this case a Delta Ray (Thermo Fisher Scientific)). In the case of FT-IR a novel methodology using only two standards of CO2 in air with different mole fractions but identical isotopic composition was demonstrated to be highly accurate for measurements of δ13C and δ18O with standard uncertainties of 0.09‰ and 1.03‰, respectively, at a nominal CO2 mole fraction of 400 μmol mol-1 in air. In the case of the IRIS system, we demonstrate that the use of two standards of CO2 in air of known but differing δ13C and δ18O isotopic composition allows standard uncertainties of 0.18‰ and 0.48‰ to be achieved for δ13C and δ18O measurements, respectively. The calibration strategies were validated using a set of five traceable primary reference gas mixtures. These standards, produced with whole air or synthetic air covered the mole fraction range of (378-420) μmol mol-1 and were prepared and/or value assigned either by the National Institute of Standards and Technology (NIST) or the National Physical Laboratory (NPL). The standards were prepared using pure CO2 obtained from different sources, namely, combustion; Northern Continental and Southern Oceanic Air and a gas well source, with δ13C values ranging between -35‰ and -1‰. The isotopic composition of all standards was value assigned at the Max Planck Institute for Biogeochemistry Jena (MPI-Jena).

Absolute cross section measurements of neutron-induced fission ofPu242from 1 to 2.5 MeV

The absolute neutron-induced fission cross section of 242Pu was measured at five energies between 1 and 2.5 MeV at the low-scatter neutron measurement facility of the National Physical Laboratory, UK. The measurements are part of an e ort to reduce uncertainties of nuclear data related to fast spectrum reactors. The neutron-induced fission results are in good agreement with the Evaluated Nuclear Data File/B-VII.1 but disagree with several recent measurements near the resonance-like structure around 1.1 MeV. Within the same experimental campaign, the spontaneous fission half-life of 242Pu was measured and it is in good agreement with previous results.

Feasibility study of an innovative active neutron dosemeter

Prototypes of a novel pocket-sized active neutron dosemeter, based on a sensor made from the scintillator material Cs2LiYCl6:Ce (CLYC) coupled to a silicon photomultiplier (SiPM), were exposed to several well-characterised neutron fields produced at the National Physical Laboratory (NPL), UK.6Li-enriched CLYC is extremely interesting as a dosemeter sensor because it can detect not only gamma rays but also thermal and fast neutrons, gammas being distinguishable from neutrons by pulse shape discrimination. Thermal and low energy neutrons are detected by the 6Li(n, α)3H reaction, which gives rise to a well-defined peak in the pulse height spectrum. Fast neutrons typically interact via 35Cl(n, p) or (n, α) reactions, and the kinetic energy of the reaction product gives rise to a pulse height spectrum that relates fairly straightforwardly to the original neutron energy spectrum. Because of this spectrometry capability, such a dosemeter has the potential to retain its accuracy to a much greater extent, compared with conventional devices, when used in radiation fields that differ from the one used for calibration.SiPMs are low-power and compact, allowing the prototype dosemeters produced for testing to fit entirely within a standard existing personal dosemeter housing. Tests were carried out in the low-scatter neutron facility at NPL, with measurements made both on-phantom and free-in-air. The former were done to evaluate the device's performance as a personal dosemeter, and the latter to explore its potential as a very light neutron area survey meter. In this paper the experimental results are presented, performance issues encountered during the trials are discussed, and preferred application scenarios are proposed.