Reduce Measurement Uncertainty Research Articles

Thermopile detectors: spatial non-uniformity measurements and correction methods

The spatial non-uniformity of a newly developed electrically calibrated radiometer system is characterized. The detector is a thin-film thermopile with an integral electrical heater. This paper shows that the local responsivities of these detectors have a rotational symmetry and a quadratic dependence on the distance to the centre of the detector. Typical measurement errors, which can be caused by applying different beam sizes, different power distributions of the beams, and different beam positions impinging on the detector, are investigated. A significant reduction of measurement uncertainties is achieved using specific corrections. The influence of spatial non-uniformity on responsivity measurements is experimentally checked with various types of beam.

An intercomparison of heat flow meter apparatus within the United Kingdom and Eire

The heat flow meter (HFM) technique is widely used for rapid and reliable measurement of apparent thermal conductivity of thermal insulation materials. In the British Isles there is a large number of HFM apparatus of different sizes and forms, mainly in the quality assurance and research laboratories of thermal insulation manufacturers. With the advent of European legislation requiring manufacturers to declare thermal performance values of their products, the National Physical Laboratory organised an intercomparison to assess the comparability and accuracy of industrial HFM measurements in the UK and Eire with the aim of helping to establish consistent measurements. Seventeen HFM apparatus featured in the intercomparison, with NPL using a guarded hot-plate apparatus to provide baseline thermal conductivity values. Measurements were made at 10 °C and 23 °C, on two thicknesses of samples of expanded polystyrene, extruded polystyrene, and high density rock fibre. With few exceptions, the measurements agreed to within ±5%. In a total of 154 data points 69% were within ±3%, and 50% were within ±2%. Measurement differences due to calibration issues and equilibrium time variations are discussed, together with proposals for further reduction of measurement uncertainties.

Scanning X-ray interferometry and the silicon lattice parameter: towards relative uncertainty?

In order to reduce measurement uncertainty of the (220) lattice spacing of silicon to a few parts per 109, a combined X-ray and optical interferometer capable of millimeter scans is being tested. A new series of measurements confirmed the value obtained with our previous set-up, and the bounds of measurement uncertainty were investigated. The article supplements the analysis of the error budget and provides a safer footing for the monocrystalline silicon lattice parameter value.

New apparatus for measuring surface areas as low as 0.1 cm2

An apparatus is described which permits accurate determination of gas adsorption of solids having lower adsorptive capacity. Conventional measurement of gas uptake by solids having surface areas less than 1 m2 is not generally feasible at subatmospheric pressure. The small pressure drop due to adsorption is easily lost in the noise created by ambient temperature fluctuations. Possible experimental errors in the volumetric measurement of gas uptake due to temperature fluctuations were greatly reduced by using an apparatus whose reference and sample adsorption cells are disposed in a lateral symmetry. The apparatus consists of a differential micromanometer whose two arms are connected in a lateral symmetry to a pair of sample and reference burets of nearly equal volume for dosing gas into sample and reference cells also of nearly equal volume. When the two cells are immersed in the same temperature bath this design greatly reduces measurement uncertainties due to fluctuations in the temperature. Surface areas as small as 300, 1 and 0.1 cm2 were measured through volumetric gas uptakes of nitrogen at 78 K, krypton at 78 K and xenon at 90 K, respectively.

Uncertainty Analysis for Angle Calibrations Using Circle Closure.

We analyze two types of full-circle angle calibrations: a simple closure in which a single set of unknown angular segments is sequentially compared with an unknown reference angle, and a dual closure in which two divided circles are simultaneously calibrated by intercomparison. In each case, the constraint of circle closure provides auxiliary information that (1) enables a complete calibration process without reference to separately calibrated reference artifacts, and (2) serves to reduce measurement uncertainty. We derive closed-form expressions for the combined standard uncertainties of angle calibrations, following guidelines published by the International Organization for Standardization (ISO) and NIST. The analysis includes methods for the quantitative evaluation of the standard uncertainty of small angle measurement using electronic autocollimators, including the effects of calibration uncertainty and air turbulence.

Advances in measurement methods and reduction of measurement uncertainties associated with antenna calibration

A major source of uncertainty in electromagnetic-compatibility conformance testing is the antenna factor. Until recently, broadband antennas in the frequency range 30 MHz-1 GHz were calibrated typically to an uncertainty of ±2 dB. Antennas can now be calibrated to an uncertainty of less than ±0.5 dB. This has made it possible to quantify a second category of uncertainties which arise when the antenna is used for emission testing on an open-field site. Ideally, the most useful calibration is of free-space antenna factors, but when antennas are used above a ground plane mutual-coupling effects can alter the antenna factor. The directivity of the antenna affects the magnitude of the received signal; balun imbalance affects the received signal; and the phase centre of log-periodic antennas changes with frequency. On top of this there are site effects which include reflections from surrounding objects, reflections from antenna-support structures and input cables and edge diffraction from the ground plane. These effects can be quantified to uncertainties as low as ±0.1 dB using antennas with calculable antennas factors.

The direct introduction of powders for plasma emission with fluidized-bed and slurry nebulization methods: particle flow rate and analyte heterogeneity

The magnitude of measurement uncertainty caused by particle sampling statistics in the direct ICP-AES analysis of powders was investigated using silica-immobilized 8-hydroxyquinoline with analyte elements spiked homogeneously and at varying degrees of heterogeneity. At panicle flow rates above 10 000 particles (25 μm diameter) per measurement, the statistical uncertainty in the number of particles (Poisson statistics) was less than uncertainty introduced from other sources. Uncertainty originating from sample heterogeneity (analyte elements distributed on a fraction of the sample particles) was over 10% when analyte was distributed on fewer than 1 in 100 particles (flow rates about 25 000 particles per measurement). The measured uncertainty in the emission of Cu agreed well with theory, other elements (Cd, Co) showed considerably higher than expected relative standard deviation. Internal standard compensation for signal noise originating from variation in the particle flow became ineffective as the sample heterogeneity increased. Measurement of emission signals of homogeneous powder samples showed an apparent level of “effective heterogeneity” which set the lower limit to which internal standardization effectively compensated for noise. Increasing the numbers of sample particles by grinding the sample to the smallest possible size is shown to be the most effective practical approach to decrease contribution of particle sampling statistics to the measurement uncertainty. Instrumental improvements which permit simultaneous measurement of analyte lines and off-peak background offer promise for reduction of measurement uncertainty.

Accounting for the effects of gas species on piston gauge calculated area: absolute mode

A model has been developed, and tested experimentally, to account for the apparent dependence of the effective cross-sectional area of pistons in gas-piston-gauge pressure standards on the particular gas with which the gauge is operated, in the absolute mode. The model treats a pressure drop or loss that depends on the pressure difference across the gauge (P1-P2), the fall rate of the piston, the density of the gas and the viscosity of the gas. The model was tested using helium, neon, argon, nitrogen and krypton, and several values of (P1-P2). An algorithm was developed for calculating the piston cross-sectional area. The experimental results confirmed the model and resulted in a reduction in measurement uncertainty arising from this effect by approximately one order of magnitude.

Autonomous navigation for high altitude satellites

The determination of the orbit of high altitude satellites with an accurate horizontal charge coupled device (CCD) sensor is considered using the extended Kalman filter. The measurement nonlinearity is removed by using a coordinate transform, and the corresponding steady state error is less than the steady state error in the Cartesian coordinate system. The performance of both of the navigational filters is evaluated for a reference geosynchronous orbit as a function of measurement error. The reduction of measurement uncertainty decreased steady state errors in position and velocity.

Temperature-compensated, differential tensimeter for measuring gas adsorption by low surface area solids

An apparatus is described which permits convenient determination of gas adsorption of solids having lower adsorption capacity at subatmospheric pressures. The apparatus consists of a differential micromanometer whose two arms are connected to a pair of sample and reference burets of nearly equal volume for dosing gas into sample and reference cells also of nearly equal volume. When the two cells are immersed in the same temperature bath this design greatly reduces measurement uncertainties due to fluctuations in temperature. Surface areas as small as 1000 cm2 can be measured through volumetric measurements of nitrogen uptake at overall pressures between 3 and 30 KPa at liquid nitrogen temperature.