Uncertainty Of Measurement System Research Articles

THE WYOMING SURVEY FOR Hα. II. Hα LUMINOSITY FUNCTIONS AT z ≈ 0.16, 0.24, 0.32, AND 0.40

The Wyoming Survey for H-alpha, or WySH, is a large-area, ground-based imaging survey for H-alpha-emitting galaxies at redshifts of z ~ 0.16, 0.24, 0.32, and 0.40. The survey spans up to four square degrees in a set of fields of low Galactic cirrus emission, using twin narrowband filters at each epoch for improved stellar continuum subtraction. H-alpha luminosity functions are presented for each Delta(z) ~ 0.02 epoch based on a total of nearly 1200 galaxies. These data clearly show an evolution with lookback time in the volume-averaged cosmic star formation rate. Integrals of Schechter fits to the incompleteness- and extinction-corrected H-alpha luminosity functions indicate star formation rates per co-moving volume of 0.010, 0.013, 0.020, 0.022 h_70 M_sun yr^{-1} Mpc^{-3} at z ~ 0.16, 0.24, 0.32, and 0.40, respectively. Statistical and systematic measurement uncertainties combined are on the order of 25% while the effects of cosmic variance are at the 20% level. The bulk of this evolution is driven by changes in the characteristic luminosity L_* of the H-alpha luminosity functions, with L_* for the earlier two epochs being a factor of two larger than L_* at the latter two epochs; it is more difficult with this data set to decipher systematic evolutionary differences in the luminosity function amplitude and faint-end slope. Coupling these results with a comprehensive compilation of results from the literature on emission line surveys, the evolution in the cosmic star formation rate density over 0 < z < 1.5 is measured to be rho_dot_SFR(z) = rho_dot_SFR(0) (1+z)^{3.4+/-0.4}.

LoCuSS: A COMPARISON OF CLUSTER MASS MEASUREMENTS FROMXMM-NEWTONAND SUBARU—TESTING DEVIATION FROM HYDROSTATIC EQUILIBRIUM AND NON-THERMAL PRESSURE SUPPORT

We compare X-ray hydrostatic and weak-lensing mass estimates for a sample of 12 clusters that have been observed with both XMM-Newton and Subaru. At an over-density of \Delta=500, we obtain 1-M^{X}/M^{WL}=0.01+/-0.07 for the whole sample. We also divided the sample into undisturbed and disturbed sub-samples based on quantitative X-ray morphologies using asymmetry and fluctuation parameters, obtaining 1-M^{X}/M^{WL}=0.09+/-0.06 and -0.06+/-0.12 for the undisturbed and disturbed clusters, respectively. In addition to non-thermal pressure support, there may be a competing effect associated with adiabatic compression and/or shock heating which leads to overestimate of X-ray hydrostatic masses for disturbed clusters, for example, in the famous merging cluster A1914. Despite the modest statistical significance of the mass discrepancy, on average, in the undisturbed clusters, we detect a clear trend of improving agreement between M^{X} and M^{WL} as a function of increasing over-density, M^{X}/M^{WL}=(0.908+/-0.004)+(0.187+/-0.010) \log_{10}(\Delta /500). We also examine the gas mass fractions, f_{gas}=M^{gas}/M^{WL}, finding that they are an increasing function of cluster radius, with no dependence on dynamical state, in agreement with predictions from numerical simulations. Overall, our results demonstrate that XMM-Newton and Subaru are a powerful combination for calibrating systematic uncertainties in cluster mass measurements.

Fusion Power Measurement Using a Combined Neutron Spectrometer-Camera System at JET

Fusion power production is the ultimate goal of fusion research, and its determination is crucial in any fusion energy application. In this paper the principles of collimated neutron flux measurements for fusion plasma power determination are described. In this method, a high-resolution neutron spectrometer provides an absolutely calibrated neutron flux, and a neutron profile monitor (“camera”) gives information on the neutron emission profile of the plasma. The total neutron flux seen by the spectrometer is discussed in terms of direct and scattered flux, and a model is set up to evaluate the magnitude of these different components. Particular care is taken to estimate the uncertainties involved, both in the model and the measurements. The method is put to practical use at JET, where a magnetic proton recoil spectrometer and a neutron profile monitor are available. Results from JET’s trace tritium experimental campaign in 2003 are presented and show that the systematic uncertainties in fusion power measurements are reduced in comparison to what has been presented for foil activation systems. A systematic error of 6% is reported here. For ITER these results imply that the fusion power can be redundantly measured and with better accuracies than for traditional methods.

Radon problem in an underground low-level laboratory

The low-level gamma-ray spectroscopy as well as the investigation of rare nuclear processes require not only low, but also constant levels of relevant background radiations. In underground laboratories dedicated to this type of measurements, one of the main and hard to control sources of unwanted radiation is the radioactive gas radon. It is well-known that radon concentration varies daily and seasonally, primarily due to the variation of atmospheric parameters. This introduces unwanted and hard to evaluate systematic uncertainties in long-term low-level measurements. In this paper, the system for radon reduction in the underground Low-Background Laboratory for Nuclear Physics at the Institute of Physics in Belgrade is presented in some detail. The laboratory exists for ten years and different measurements of radon concentration were carried out during this period. The indoor radon measurements are performed using nuclear track detectors (type CR-39 and LR-115) for long-term measurements and the commercially available radon monitor for short-term measurements. In this work we present the results of these measurements for the period 2003–2008.

THE TEMPERATURE-DENSITY RELATION OF THE INTERGALACTIC MEDIUM AFTER HYDROGEN REIONIZATION

We use an analytic model to study how inhomogeneous hydrogen reionization affects the temperature distribution of the intergalactic medium (IGM). During this process, the residual energy of each ionizing photon is deposited in the IGM as heat, increasing its temperature to 20,000-30,000 K; subsequent expansion of the Universe then cools the gas. Because reionization most likely proceeds from high to low densities, underdense voids are ionized last, have less time to cool, and are (on average) warmer than mean-density gas immediately after reionization is complete (an "inverted" equation of state). From this initial configuration, the low-density gas cools quickly and eventually returns to a more normal equation of state. The rapidly evolving temperature introduces systematic uncertainties in measurements of the ionizing background at z~6. For example, late reionization implies rapid cooling, so that the ionizing background would have to evolve even more rapidly at z ~5-6 than typically claimed. This degeneracy is difficult to disentangle, because the Lyman-alpha forest probes only a narrow range in densities (over which the gas is nearly isothermal). However, higher Lyman-series transitions probe wider density ranges, sampling different effective temperatures, and offer a new way to measure the IGM equation of state that should work where nearly saturated absorption precludes other methods. This will help to separate evolution in temperature from that in the ionizing background. While more detailed study with hydrodynamic simulations is needed, we show that such measurements could potentially distinguish early and late reionization using only a handful of lines of sight.

Improvements in the NIST Johnson Noise Thermometry System

We have developed a Johnson noise thermometry (JNT) system that is calibrated by precision waveforms synthesized with a quantized voltage noise source (QVNS). Improvements to the QVNS and the cross-correlation measurement electronics have significantly reduced systematic errors and measurement uncertainty. We describe in detail a number of these improvements, such as amplifier performance and frequency response. We also show details of recent measurements that characterize the nonlinearities and distortion properties of the measurement electronics. Finally, we present an uncertainty analysis and discuss the potential for the QVNS-JNT technique to contribute to the redefinition of Boltzmann's constant.

An information theory approach to minimise correlated systematic uncertainty in modelling resonance parameters

Covariance matrix elements depict the statistical and systematic uncertainties in reactor parameter measurements. All the efforts have so far been devoted only to minimise the statistical uncertainty by repeated measurements but the dominant systematic uncertainty has either been neglected or randomized. In recent years efforts has been devoted to simulate the resonance parameter uncertainty information through covariance matrices in code SAMMY. But, the code does not have any provision to check the reliability of the simulated covariance data. We propose a new approach called entropy based information theory to reduce the systematic uncertainty in the correlation matrix element so that resonance parameters with minimum systematic uncertainty can be modelled. We apply our information theory approach in generating the resonance parameters of 156Gd with reduced systematic uncertainty and demonstrate the superiority of our technique over the principal component analysis method.

A spectral simulation approach to evaluate probabilistic measurement precision of a reactor coolant pump torsional vibration shaft crack monitoring system

A prototype torsional vibration monitoring system has been installed on two reactor coolant pumps at Tennessee Valley Authority Sequoyah Power Plant Unit 1 nuclear reactor for shaft crack monitoring. The system uses a change in torsional natural frequencies as a diagnostic feature since as a crack grows the shaft line torsional dynamics change commensurately. For effective diagnostic monitoring, it is critical to know the inherent feature variation resulting from the instrumentation and data processing. The integrated nature of the prototype hardware/software system and the inaccessibility of the equipment inside the reactor containment building make it impossible to evaluate the measurement capability in a traditional instrumentation chain approach. Hence, a simulation-based evaluation was performed to determine the cumulative effect of the measurement system on the uncertainty in the torsional natural frequency estimation. The identification algorithm operates on the measured torsional spectrum to estimate a natural frequency. All of the inherent measurement and data processing uncertainties ultimately affect the spectrum in some manner. Therefore, if the characteristics of the reactor coolant pump torsional spectrum can be replicated via a model; it can be used to assess the overall prototype system measurement uncertainty. The underlying premise is that the line shaft dynamics remain constant, in the absence of a fault, and hence any natural frequency variation is due strictly to the measurement system uncertainty. To this end, a single degree of freedom simulation model was first developed which matched the natural frequency and damping of a crack sensitive torsional mode in the reactor coolant pump. The model input was carefully selected such that the averaged spectral amplitude and variance from the ensembles matched the measured reactor coolant pump torsional vibration. The model was then used to create one hundred sample responses that replicated pump spectral response in a statistical sense. The same data processing routines used in the prototype system were applied to the simulation data thus creating sample populations of the natural frequency and damping. Statistical tolerance limits containing 99.9% of the sample population with a 99.9% probability were used to quantify the system's measurement uncertainty. The limits for the crack sensitive mode 3 of the reactor coolant pump were (1) natural frequency: ±0.04740 Hz and (2) damping ratio: ±0.00036.

Uncertainty of annual net ecosystem productivity estimated using eddy covariance flux measurements

Eddy‐covariance‐based estimates of net ecosystem exchange are subject to various sources of systematic bias and random measurement uncertainty. Here we concentrate on the cumulative effect of random uncertainty on annual estimates of net ecosystem productivity of carbon (NEP). A 8‐year data set of eddy covariance measurements over a mixed deciduous forest at the Morgan‐Monroe State Forest (MMSF, Indiana, USA) was used, in conjunction with a 6‐day period of paired observations with the AmeriFlux portable system, to evaluate two different approaches to estimate measurement system uncertainty, and an analogous method to estimate the uncertainty in a standard parametric model used to fill data gaps in the annual time series. The cumulative annual uncertainty was obtained by Monte Carlo simulation, separately for the observations and the model estimates. Our results indicate that the overall uncertainty of annual NEP is dominated by the contribution of the gap‐filling model, even at relatively small gap fractions of 20%. The magnitude of random uncertainty in NEP varied between ±10–12 gC m−2 yr−1 (i.e., 3–4% of annual NEP at MMSF for years 1999–2006). Thus it must be expected that random uncertainty of eddy‐covariance‐based NEP is small compared to other potential sources of systematic bias, but we note that very little is known about the long‐term cumulative covariate effects of systematic bias in the measured flux.

A study of systematic biases and measurement uncertainties in ozone mole fraction measurements with the NIST Standard Reference Photometer

Sources of bias in the National Institute of Standards and Technology ozone Standard Reference Photometer (SRP) maintained by the Bureau International des Poids et Mesures have been investigated. A relative bias of −0.4% in the ozone mole fraction measurement caused by a temperature gradient in the gas cells of the instrument was characterized and corrected for in a modified version of the instrument. A second relative bias of +0.5% due to the multiple reflections of light within the gas cells was also corrected. The Guide to the Expression of Uncertainty in Measurement approach was used to develop an uncertainty budget for the modified SRP, including a relative value for the ozone absorption cross-section uncertainty of 2.1% (k = 2). The measurement uncertainty for the bias-corrected SRPs is enlarged compared with earlier studies, but their comparability improved.

Reverse engineering: a selective sampling acquisition approach

Creation of a CAD model from a hard model is something necessary for design modification, part replication or rapid prototyping and surface inspection. This is reverse engineering. Scientific literature presents many different approaches, even if, actually all the systems, mechanical (contact devices) and optical (non contact devices), work with constant acquisition pitches. This became a great deal in relation to the different object morphology combinations that the same surface could show. Working in fact with a constant pitch on a free-form surface it is possible to struggle with an insufficient points cloud density, when the acquisition pitch would be a compromise between the complex and elementary features that describe the object to acquire, or with an excessive points cloud density, when the acquisition pitch represents the highest scanner resolution. Referring to this situation, this paper proposes, starting from a first raw acquisition, an automatic methodology, directly implemented on the acquisition device, for the selective individuation of surface zones which present sensible curvature. In this approach the curvature of the measured surfaces is analyzed by defining a threshold over which it is necessary to perform a deeper scansion of the surface. In the present paper a methodology for the definition of the threshold value based on the measurement system uncertainty is described. In the current description the method is applied to an algorithm for curvature analysis, but it could be extended to any other approaches. Furthermore, it will be demonstrated that this new methodology is simple to apply and can be easily automated directly in the control scanner software. In the end of the paper a practical example is described in order to give an experimental validation of the method.

Reverse engineering of free-form surfaces: A methodology for threshold definition in selective sampling

Reverse engineering is a technique used during the project phase of a new product, which makes it possible to trace, in terms of mathematical expressions, the geometrical features of a given physical model. Scientific literature presents many different approaches to reverse engineering. Great part of those are based on the analysis of point clouds acquired through coordinate measuring devices, such as, for example, Coordinate Measuring Machines (CMMs), Optical Scanners or Interferometric Systems. Referring to this kind of approach, a common problem is to individuate the surface zones, which present sensible variations of curvature. Many algorithms, commonly implemented on commercial software through semi-automatic procedures, are already based on this method. In these cases the local curvature variation of the measured surfaces is analyzed by defining a threshold over which it is necessary to perform a deeper scansion of the surface zones. However, most of the problems are related to the definition of an appropriate value for the threshold level. In the present paper a methodology for the definition of the threshold value based on the measurement system uncertainty is described. In the current description the method is applied to an elementary algorithm for curvature definition, but it could be extended to any other kind of more complicated approach. Furthermore, it will be demonstrated that this new methodology is simple to apply and can be easily automated in commercial software for points selective sampling in industrial reverse engineering applications. In the end a practical example is described in order to give an experimental validation of the method.

Monitoring coordinate measuring machines by calibrated parts

Coordinate measuring machines (CMM) are essential for quality assurance and production control in modern manufacturing. Due to the necessity of assuring traceability during the use of CMM, interim checks with calibrated objects carried out periodically. For this purpose usually special artefacts like standardized ball plates, hole plates, ball bars or step gages are measured. Measuring calibrated series parts would be more advantageous. Applying the substitution method of ISO 15530-3: 2000 such parts can be used. It is less cost intensive and less time consuming than measuring expensive special standardized objects in special programmed measurement routines. Moreover, the measurement results can directly compare with the calibration values; thus, direct information on systematic measurement deviations and uncertainty of the measured features are available. The paper describes a procedure for monitoring horizontal-arm CMMs with calibrated sheet metal series parts.

A new vacuum interferometric comparator for calibrating the fine linear encoders and scales

A new one-dimensional laser interferometric comparator has been developed for the calibration of the fine linear encoders and scales up to 1600 mm. In the comparator, the interferometer is fully arranged in vacuum and the calibration objects are mounted under atmospheric conditions. The Abbe’s principle on the alignment of workpiece with the measuring beam is satisfied in the structure of a long measuring range. A travelling slide table, on which the calibration objects are mounted, is supported on guide rails by the air bearing and is driven through a recirculating ballscrew. The exhaust of the air bearing is guided to the exterior of the booth in which the comparator is placed. The travel of the table is measured by a reference interferometer with a beam path in vacuum shielded by an evacuated metal bellow, so that the effect of refractive index is eliminated. The laser beam is led by a polarization plane maintaining glass fiber from a self-designed stabilised He–Ne laser, which is placed in an adjacency room, to the beam inlet of the main unit. The measurement system can input the interferometer signal by the encoder signal or the scale signal, and input the encoder or scale data by the interferometer signal. The system resolution is approximately 0.8 nm and maximum travelling measurement speed is 20 mm/s at continuous measurement. The uncertainly ( k=2) of measurement is approximately 30 nm in linear encoders of 500 mm length and, approximately 40 nm in scales of 500 mm, although it depends on the length and the characteristics of encoders and scales. It is successful such a high accuracy that the uncertainty of measurement system is smaller than 40 nm in encoders of 1 m length.

High precision measurement of the polarization in solid state polarized targets using NMR

The factors are reviewed that can affect the systematic uncertainties in CW-NMR measurements of the polarization in solid state polarized targets. The main problem with large signals, typically obtained with protons, is non-linearity arising from failure of the constant RF drive current assumption. The non-linearity can be reduced by operation at lower than normal signal levels. With the much smaller size deuteron signals, a limiting factor may be the instability of the background signal generated by the resonant length cable, used in the standard system. A new system is demonstrated, using non-resonant length cables, which is expected to reduce these uncertainties to a low level.

Uncertainty estimation for DSP‐based power quality measurements

This paper deals with the uncertainty in digital measurement systems designed for power quality applications. The main goal of this work is to evaluate such uncertainty by means of a Monte Carlo method recently proposed in the literature. The accuracy of the measurement result obtained with a DSP‐based instrument for power quality metering depends on the behavior of the devices located in both the conditioning block and A/D conversion stage: it is thus necessary to consider the uncertainties introduced by each component of the system and the propagation of their effects through the measurement chain. Here, the uncertainty is estimated starting from the technical specifications provided by the manufacturers of these devices. Experimental results are reported to show the importance of some concerns about the practical implementation of the proposed methodology in a real instrument.

Investigation of Systematic Uncertainties in Brewer–Mast Ozone Soundings Using Observations from a Ground-Based Microwave Radiometer

Abstract The series of balloon-borne Brewer–Mast ozone soundings is one of the few datasets that allow the reconstruction of the history of stratospheric ozone decline over the last three decades. The reanalysis of these data goes along with the revision of the soundings operation procedure, which was first defined in the 1970s. The present study focuses on the normalization of the recorded profiles to total ozone. Independent microwave remote sensing data are used to investigate the influence of systematic ozonesonde measurement uncertainties on the computed normalization factors. The analysis shows that a significant underestimation of the ozonesonde readings compared to microwave data exists above 20 hPa (∼27 km). This suggests that the loss of the sonde pump efficiency with decreasing atmospheric pressure is only partially compensated by the standard correction applied for this effect. The shape of the constant volume mixing ratio extrapolation commonly used for residual ozone is, moreover, inconsiste...

Random and systematic measurement uncertainties considered in the evaluation of parameters derived from the course of the tabletting process

The uncertainty of some characteristic parameters describing the course of the tabletting process, namely the area quotient according to Emschermann and Müller, the apparent net work, the slope of the Heckel-plot, and the parameters of the modified Weibull function, was calculated according to the German norm DIN 1319-4 1999. The method allows to consider random and systematic uncertainties in a consistent way as variances of normal and rectangular probability distributions, respectively, or other suitable probability distributions based on Bayesian statistics and the principle of maximum entropy. So, random and systematic uncertainties known from a calibration and validation study of the measurement of force and displacement, and the uncertainty of the true density were included meaningfully into the uncertainty of the resulting tabletting parameters using the propagation of uncertainties according to the Gauss method. The standard uncertainty for the results calculated this way seems to be suitable for a critical evaluation of the tabletting data.

Herriott Cell augmentation of a quadrature heterodyne interferometer

A quadrature heterodyne interferometer is augmented with a Herriott Cell multipass reflector to increase instrument resolution, enable a separation of the phase shift due to neutral density from room vibrations, and optimize the diagnostic for small scale length measurements or for planar measurements. Analysis is used to illustrate retro-reflective planar measurement geometries attainable with the instrument and to show that phase front degradation and loss of scene beam intensity does not introduce a systematic measurement uncertainty. The diagnostic capability is demonstrated with measurements of the electron and neutral densities in the plasma exhaust from electric propulsion thrusters. Experimental data with up to 18 passes through plasma demonstrates that the instrument resolution to electron and neutral density increases almost linearly with number of passes, and that the Herriott Cell can be used to increase the signal of neutral density phase shifts relative to the noise of the phase shifts due to room vibrations. For the Pulsed Plasma Thruster plasma exhaust used to validate the instrument, the addition of the Herriott Cell reduces the density measurement uncertainty to equal or less than the uncertainty due to discharge irreproducibility.

Precision luminosity for Z $^0$ lineshape measurements with a silicon-tungsten calorimeter

The measurement of small-angle Bhabha scattering is used to determine the luminosity at the OPAL interaction point for the LEP I data recorded between 1993 and 1995. The measurement is based on the OPAL Silicon-Tungsten Luminometer which is composed of two calorimeters encircling the LEP beam pipe, on opposite sides of the interaction point. The luminometer detects electrons from small-angle Bhabha scattering at angles between 25 and 58mrad. At LEP center-of-mass energies around the Z0, about half of all Bhabha electrons entering the detector fall within a 79nb fiducial acceptance region. The electromagnetic showers generated in the stack of 1 radiation length tungsten absorber plates are sampled by 608 silicon detectors with 38,912 radial pads of 2.5mm width.The fine segmentation of the detector, combined with the precise knowledge of its physical dimensions, allows the trajectories of incoming 45GeV electrons or photons to be determined with a total systematic error of less than 7microns. We have quantified all significant sources of systematic experimental error in the luminosity determination by direct physical measurement. All measured properites of the luminosity event sample are found to be in agreement with current theoretical expectations. The total systematic measurement uncertainty is 3.4x10^-4, significantly below the theoretical error of 5.4x10^-4 currently assigned to the QED calculation of the Bhabha acceptance, and contributes negligibly to the total uncertainty in the OPAL measurement of Gamma{invisible}/Gamma{l+l-}, a quantity of basic physical interest which depends crucially on the luminosity measurement.