Combined Measurement Uncertainty Research Articles

Estimation of uncertainty in the determination of nitrogen and oxygen in U(Al, Si)3

U(Al, Si)3, an inter-metallic compound, is a proposed fuel in advanced nuclear reactors. Determination of trace amounts of nitrogen and oxygen in this material is an essential requirement for quality assurance. Mass fraction of nitrogen and oxygen were determined in this compound employing inert gas fusion in conjunction with thermal conductivity and infrared detection techniques, respectively. Uncertainty in the measured data has been estimated employing bottom-up approach as per ISO guidelines. Uncertainty in the calibration of the measurement procedure employing standards with own uncertainty of the measurement was estimated following linear regression equation and uncertainty propagation laws. The relative combined uncertainty of measurement has been calculated employing law of uncertainty propagation. Finally, the expanded uncertainty of measurement (coverage factor k = 2) for the determination of nitrogen and oxygen was estimated, and they were found to be 10 and 12 %, respectively.

In situ gas analysis for high pressure applications using property measurements

As the production, distribution, and storage of renewable energy based fuels usually are performed under high pressures and as there is a lack of in situ high pressure gas analysis instruments on the market, the aim of this work was to develop a method for in situ high pressure gas analysis of biogas and hydrogen containing gas mixtures. The analysis is based on in situ measurements of optical, thermo physical, and electromagnetic properties in gas mixtures with newly developed high pressure sensors. This article depicts the calculation of compositions from the measured properties, which is carried out iteratively by using highly accurate equations of state for gas mixtures. The validation of the method consisted of the generation and measurement of several mixtures, of which three are presented herein: a first mixture of 64.9 mol. % methane, 17.1 mol. % carbon dioxide, 9 mol. % helium, and 9 mol. % ethane at 323 K and 423 K in a pressure range from 2.5 MPa to 17 MPa; a second mixture of 93.0 mol. % methane, 4.0 mol. % propane, 2.0 mol. % carbon dioxide, and 1.0 mol. % nitrogen at 303 K, 313 K, and 323 K in a pressure range from 1.2 MPa to 3 MPa; and a third mixture of 64.9 mol. % methane, 30.1 mol. % carbon dioxide, and 5.0 mol. % nitrogen at 303 K, 313 K, and 323 K in a pressure range from 2.5 MPa to 4 MPa. The analysis of the tested gas mixtures showed that with measured density, velocity of sound, and relative permittivity the composition can be determined with deviations below 1.9 mol. %, in most cases even below 1 mol. %. Comparing the calculated compositions with the generated gas mixture, the deviations were in the range of the combined uncertainty of measurement and property models.

The effect of tip size on the measured Ra of surface roughness specimens with rectangular profiles

When measuring rectangular and trapezoidal profile roughness specimens, the stylus tip increases the measured profile peak width and decreases the measured valley width. This can cause either an increase or a decrease in the apparent roughness average Ra, depending on the tip size and the ratio of peak width to valley width. Sometimes the change is larger than the combined measurement uncertainty from other sources. This raises the question as to whether measured surface parameters should be corrected for the effect of tip size.

Calorimetric concept for measurement of power losses up to 2 kW in electric drives

Accurate measurement of power losses in high-efficiency devices is difficult. Measurement standards for industrial converters and complete electric drives, including both motors and converters, will come into effect soon, and measurement methods for these devices should be included. In the calorimetric method, the power losses are measured directly. However, the calorimeters presented previously are mainly tailored systems, and therefore, they typically have very complicated constructions. Hence, their applicability to the evaluation of general electric drives is limited. A functional calorimetric measurement concept is suggested in this study for power losses up to 2 kW. Such a power loss can be applied with present-day power electronic converters up to 110 kW. The construction of the concept is simple and lightweight. It does not require a complex structure or a large area in the measurement site. The concept is scalable and duplicable for different sizes. Different devices with different cablings can be measured without any problems. The combined standard heat loss measurement uncertainty for the calorimeter is ±0.4%.

Metrological evaluation of skin conductance measurements

Electrodermal activity is a frequently measured physiological response in various applications. It is also being increasingly used in clinical applications. Numerous published papers report results of skin conductance measurements in absolute values, but few are concerned with the quality of results. This paper describes a procedure for metrological evaluation of skin conductance measurement. Three commercial devices for measuring skin conductance were calibrated by comparison with a precision digital ohmmeter used as a reference. Combined measurement uncertainty of skin conductance meters was calculated by means of uncertainty of reference instrument and uncertainties due to measurement repeatability, reproducibility, resolution and environmental condition. Additionally, a procedure for evaluation of the effect of electrode displacement and electrode gel was shown. A model of finger skin conductance profile was build. Measurement uncertainty analysis showed that contributions due to resolution and sensitivity of the measuring device, usually obtained from specifications, are negligible when compared to uncertainty of measuring method. Our results indicate that measurement uncertainty does not meet target uncertainty requirements for certain applications.

A statistical approach to quantify uncertainty in carbon monoxide measurements at the Izaña global GAW station: 2008–2011

Abstract. Atmospheric CO in situ measurements are carried out at the Izaña (Tenerife) global GAW (Global Atmosphere Watch Programme of the World Meteorological Organization – WMO) mountain station using a Reduction Gas Analyser (RGA). In situ measurements at Izaña are representative of the subtropical Northeast Atlantic free troposphere, especially during nighttime. We present the measurement system configuration, the response function, the calibration scheme, the data processing, the Izaña 2008–2011 CO nocturnal time series, and the mean diurnal cycle by months. We have developed a rigorous uncertainty analysis for carbon monoxide measurements carried out at the Izaña station, which could be applied to other GAW stations. We determine the combined standard measurement uncertainty taking into consideration four contributing components: uncertainty of the WMO standard gases interpolated over the range of measurement, the uncertainty that takes into account the agreement between the standard gases and the response function used, the uncertainty due to the repeatability of the injections, and the propagated uncertainty related to the temporal consistency of the response function parameters (which also takes into account the covariance between the parameters). The mean value of the combined standard uncertainty decreased significantly after March 2009, from 2.37 nmol mol−1 to 1.66 nmol mol−1, due to improvements in the measurement system. A fifth type of uncertainty we call representation uncertainty is considered when some of the data necessary to compute the temporal mean are absent. Any computed mean has also a propagated uncertainty arising from the uncertainties of the data used to compute the mean. The law of propagation depends on the type of uncertainty component (random or systematic). In situ hourly means are compared with simultaneous and collocated NOAA flask samples. The uncertainty of the differences is computed and used to determine whether the differences are significant. For 2009–2011, only 24.5% of the differences are significant, and 68% of the differences are between −2.39 and 2.5 nmol mol−1. Total and annual mean differences are computed using conventional expressions but also expressions with weights based on the minimum variance method. The annual mean differences for 2009–2011 are well within the ±2 nmol mol−1 compatibility goal of GAW.

A Validated Quantification of Benzocaine in Lozenges Using TLC and a Flatbed Scanner

We present a video-densitometric quantification method for benzocaine in lozenges. The quantification is based on a derivatisation reaction with 4-dimethylaminobenzaldehyde. Measurements were carried out using a 16-bit flatbed scanner. Benzocaine was separated to a distance of 50 mm in a vertical developing chamber without vapour saturation. We present an RP-18 phase separation on a cyanopropyl plate (Merck, Darmstadt, Germany) using water, CH3CN, dioxane, ethanol, and NH3 (25 %) (8 + 2 + 1 + 1 + 0.05, v/v) as the mobile phase. We also separated benzocaine in a normal phase system on silica gel 60 LiChrospere® plates (Merck, Darmstadt, Germany) with the mobile phase MTBE/cyclohexane (1 + 1, v/v). The calibration functions for benzocaine in both separations were linear in the range from 1 to 1,000 ng per spot. The range of linearity covers two magnitudes of power because the Kubelka–Munk expression was used for data transformation. In the cyanopropyl-system, the benzocaine amount was quantified as 242.5 ± 18.2 ng in a spot or 6.86 ± 0.52 mg in a single lozenge. The amount of 7.0 mg benzocaine per lozenge was labelled. The combined uncertainty of sample and calibration measurements was statistically calculated using a significance level of α = 0.05 to a total relative uncertainty of 7.49 %. The separation method is inexpensive, fast and reliable.

Determination of cyanuric acid using exact matching isotope dilution mass spectrometry

A high-accuracy double isotope dilution mass spectrometric method using an exact matching approach with GC coupled to a mass spectrometer for the analysis of cyanuric acid in fortified milk powder was developed. Various parameters for sample clean-up such as the type of SPE cartridge, GC column and type of derivatizing agent used were investigated. The method was found to be linear in the concentration range of 0.03 to 2 mg/kg of cyanuric acid in milk powder. LOD and LOQ were found to be 0.9 and 3 μg/kg, respectively. Recoveries in the range of 95.7 to 102.2% were obtained for the in-house fortified milk powder samples, with RSD of measurements in the range of 0.2 to 3.0%. A metrological approach was undertaken to examine all possible biases that contributed to the combined measurement uncertainty of the method. This high-accuracy method can serve as a reference method for techniques commonly applied in routine testing laboratories.

Experimental determination of the Boltzmann constant: An undergraduate laboratory exercise for molecular physics or physical chemistry

This article describes an undergraduate laboratory exercise that uses optical spectroscopy to determine the magnitude and the uncertainty of the Boltzmann constant kb. The more accurate approach uses photoacoustic spectroscopy to measure the Doppler-broadened line profile of individual spectral lines of N2O to extract kb. Measurements and estimates of the uncertainties in the quantities needed to calculate kb from the line profiles are then used to estimate the uncertainty in kb. This experiment is unusual in that it uses advanced laser-based spectroscopy techniques to emphasize standard practices of uncertainty analysis. The core instrumentation is modular and relatively affordable; it requires a tunable single-mode laser, photoreceiver, optical cell, and vacuum pump. If this instrumentation is not available, an alternate approach can be performed which uses the intensity of each rotational transition of an infrared band to measure kb. Although there is more uncertainty using the alternate approach, low concentrations of CO2, DCl, or N2O give reasonable results for the magnitude of kb. Student assessment results indicate retention and mastery of the concept of combined measurement uncertainty.

Software support for the Nordtest method of measurement uncertainty evaluation

Software support for the Nordtest method of measurement uncertainty evaluation is described. According to the Nordtest approach, the combined measurement uncertainty is broken down into two main components—the within-laboratory reproducibility (intermediate precision) sRw and the uncertainty due to possible laboratory bias u(bias). Both of these can be conveniently estimated from validation and quality control data, thus significantly reducing the need for performing dedicated experiments for estimating detailed uncertainty contributions and thereby making uncertainty estimation easier for routine laboratories. An additional merit of this uncertainty estimation approach is that it reduces the danger of underestimating the uncertainty, which continues to be a problem at routine laboratories. The described software tool—MUkit (measurement uncertainty kit)—fully reflects the versatility of the Nordtest approach: it enables estimating the uncertainty components from different types of data, and the data can be imported using a variety of means such as different laboratory data systems and a dedicated web service as well as manual input. Prior to the development of the MUkit software, a laboratory survey was carried out to identify the needs of laboratories related to uncertainty estimation and other quality assurance procedures, as well as their needs for a practical tool for the calculation of measurement uncertainty.

Development of a Ground Based Remote Sensing Approach for Direct Evaluation of Aerosol-Cloud Interaction

The possible interaction and modification of cloud properties due to aerosols is one of the most poorly understood mechanisms within climate studies, resulting in the most significant uncertainty as regards radiation budgeting. In this study, we explore direct ground based remote sensing methods to assess the Aerosol-Cloud Indirect Effect directly, as space-borne retrievals are not directly suitable for simultaneous aerosol/cloud retrievals. To illustrate some of these difficulties, a statistical assessment of existing multispectral imagers on geostationary (e.g., GOES)/Moderate Resolution Imaging Spectroradiometer (MODIS) satellite retrievals of the Cloud Droplet Effective Radius (Reff) showed significant biases especially at larger solar zenith angles, further motivating the use of ground based remote sensing approaches. In particular, we discuss the potential of using a combined Microwave Radiometer (MWR)—Multi-Filter Rotating Shadowband Radiometer (MFRSR) system for real-time monitoring of Cloud Optical Depth (COD) and Cloud Droplet Effective Radius (Reff), which are combined with aerosol vertical properties from an aerosol lidar. An iterative approach combining the simultaneous observations from MFRSR and MWR are used to retrieve the COD and Reff for thick cloud cases and are extensively validated using the DoE Southern Great Plains (SGP) retrievals as well as regression based parameterized model retrievals. In addition, we account for uncertainties in background aerosol, surface albedo and the combined measurement uncertainties from the MWR and MFRSR in order to provide realistic uncertainty estimates, which is found to be ~10% for the parameter range of interest in Aerosol-Cloud Interactions. Finally, we analyze a particular case of possible aerosol-cloud interaction described in the literature at the SGP site and demonstrate that aerosol properties obtained at the surface can lead to inconclusive results in comparison to lidar-derived aerosol properties near the cloud base.

Airborne intercomparison of HO<sub>x</sub> measurements using laser-induced fluorescence and chemical ionization mass spectrometry during ARCTAS

Abstract. The hydroxyl (OH) and hydroperoxyl (HO2) radicals, collectively called HOx, play central roles in tropospheric chemistry. Accurate measurements of OH and HO2 are critical to examine our understanding of atmospheric chemistry. Intercomparisons of different techniques for detecting OH and HO2 are vital to evaluate their measurement capabilities. Three instruments that measured OH and/or HO2 radicals were deployed on the NASA DC-8 aircraft throughout Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) in the spring and summer of 2008. One instrument was the Penn State Airborne Tropospheric Hydrogen Oxides Sensor (ATHOS) for OH and HO2 measurements based on Laser-Induced Fluorescence (LIF) spectroscopy. A second instrument was the NCAR Selected-Ion Chemical Ionization Mass Spectrometer (SI-CIMS) for OH measurement. A third instrument was the NCAR Peroxy Radical Chemical Ionization Mass Spectrometer (PeRCIMS) for HO2 measurement. Formal intercomparison of LIF and CIMS was conducted for the first time on a same aircraft platform. The three instruments were calibrated by quantitative photolysis of water vapor by ultraviolet (UV) light at 184.9 nm with three different calibration systems. The absolute accuracies were ±32% (2σ) for the LIF instrument, ±65% (2σ) for the SI-CIMS instrument, and ±50% (2σ) for the PeRCIMS instrument. In general, good agreement was obtained between the CIMS and LIF measurements of both OH and HO2 measurements. Linear regression of the entire data set yields [OH]CIMS = 0.89 × [OH]LIF + 2.8 × 104 cm−3 with a correlation coefficient r2 = 0.72 for OH, and [HO2]CIMS = 0.86 × [HO2]LIF + 3.9 parts per trillion by volume (pptv, equivalent to pmol mol−1) with a correlation coefficient r2 = 0.72 for HO2. In general, the difference between CIMS and LIF instruments for OH and HO2 measurements can be explained by their combined measurement uncertainties. Comparison with box model results shows some similarities for both the CIMS and LIF measurements. First, the observed-to-modeled HO2 ratio increases greatly for higher NO mixing ratios, indicating that the model may not properly account for HOx sources that correlate with NO. Second, the observed-to-modeled OH ratio increases with increasing isoprene mixing ratios, suggesting either incomplete understanding of isoprene chemistry in the model or interferences in the measurements in environments where biogenic emissions dominate ambient volatile organic compounds.

Uncertainty assessment in gamma spectrometric measurements of plutonium isotope ratios and age

A method for the assessment of the combined uncertainty in gamma spectrometric measurements of plutonium composition and age was evaluated. Two materials were measured. Isotope dilution inductively coupled plasma sector field mass spectrometry (ID-ICP-SFMS) was used as a reference method for comparing the results obtained with the gamma spectrometric method for one of the materials. For this material (weapons grade plutonium) the measurement results were in agreement between the two methods for all measurands. Moreover, the combined uncertainty in all isotope ratios considered in this material (RPu238/Pu239, RPu240/Pu239, RPu241/Pu239, and RAm241/Pu241 for age determination) were limited by counting statistics. However, the combined uncertainty for the other material (fuel grade plutonium) were limited by the response fit, which shows that the uncertainty in the response function is important to include in the combined measurement uncertainty of gamma spectrometric measurements of plutonium.

Determination of combined measurement uncertainty via Monte Carlo analysis for the imaging spectrometer ROSIS

To enable traceability of imaging spectrometer data, the associated measurement uncertainties have to be provided reliably. Here a new tool for a Monte-Carlo-type measurement uncertainty propagation for the uncertainties that originate from the spectrometer itself is described. For this, an instrument model of the imaging spectrometer ROSIS is used. Combined uncertainties are then derived for radiometrically and spectrally calibrated data using a synthetic at-sensor radiance spectrum as input. By coupling this new software tool with an inverse modeling program, the measurement uncertainties are propagated for an exemplary water data product.

Parametric uncertainty in nanoscale optical dimensional measurements

Image modeling establishes the relation between an object and its image when an optical microscope is used to measure the dimensions of an object of size comparable to the illumination wavelength. It accounts for the influence of all of the parameters that can affect the image and relates the apparent feature width (FW) in the image to the true FW of the object. The values of these parameters, however, have uncertainties, and these uncertainties propagate through the model and lead to parametric uncertainty in the FW measurement, a key component of the combined measurement uncertainty. The combined uncertainty is required in order to decide if the result is adequate for its intended purpose and to ascertain if it is consistent with other results. The parametric uncertainty for optical photomask measurements derived using an edge intensity threshold approach has been described previously; this paper describes an image library approach to this issue and shows results for optical photomask metrology over a FW range of 10 nm to 8 µm using light of wavelength 365 nm. The principles will be described; a one-dimensional image library will be used; the method of comparing images, along with a simple interpolation method, will be explained; and results will be presented. This method is easily extended to any kind of imaging microscope and to more dimensions in parameter space. It is more general than the edge threshold method and leads to markedly different uncertainties for features smaller than the wavelength.

Determination of standard sample purity using the high-precision 1H-NMR process

This paper discusses the technique for high-precision quantification using (1)H-NMR to determine the purity of analytical standard samples. The procedure described is based on the use of internal reference samples in an (1)H NMR experiment in our laboratories. The sample preparation and all relevant NMR parameters were optimized for minimum uncertainty. The validation of accuracy and precision was performed by comparing different certified reference materials. It was shown that the high-precision measurement is applicable even for relatively small sample amounts down to 2.5 mg. The relative combined uncertainty of measurement was found to be 0.15%. Two different approaches for uncertainty calculation were compared; a complete uncertainty budget was calculated.

Optimisation and validation of a quantitative and confirmatory method for residues of macrolide antibiotics and lincomycin in kidney by liquid chromatography coupled to mass spectrometry

A solid phase extraction followed by a liquid chromatography (LC)‐tandem mass spectrometry (MS/MS) detection method for the confirmatory analysis of lincomycin (LIN), clindamycin (CLI), tilmicosin (TIM), erythromycin (ERI) and tylosin (TYL) residues in kidney were optimised and validated for monitoring and controlling the use of these antibiotics in food producing‐animals. The method optimisation was carried out by testing changes in the extraction buffer pH and in the ammonium/acetonitrile concentrations on SPE eluent solutions. The optimised extraction procedure involved the extraction of the analytes with a pH 8 phosphate buffer, clean‐up on a reversed‐phase mixed‐cation exchange cartridge, followed by the elution of the analytes in a 98:2 acetonitrile/ammonia solution, concentration in air flow and re‐dissolved with an 1:1 methanol/water solution. The analytes were detected in an LC‐MS/MS system in electrospray positive ionisation mode. The validation was performed according to the European Commission Decision 2002/657/EC. Linearity was established for all analytes using the method of least weighted squares and CCα values ranged from 5.3% to 21.1% higher than the minimum residue limit (MRL) values. The addition levels varied from 0.5 to 1.50 MRL for all analytes, with recoveries exceeding 92.5%. The relative standard deviations (RSD%) in terms of repeatability (n = 54) and reproducibility (n = 108) for all analytes were less than 21.6% and 21.4%, respectively. The uncertainties were calculated by simplified methods using the calibration curve uncertainty and the intermediate precision to obtain the combined measurement uncertainty. The results of the validation process demonstrated that this method is suitable for the quantification and confirmation of antibiotic residues for the Brazilian Residue and Contaminant Control Plan (PNCR).

Stabilization of gas-filled surge arrester’s characteristics by use of ionizing radiation

This paper investigates the stabilization of electrical discharges in gases by means of external ionizing radiation. Discharges in a gas-filled surge arrester model were studied in both passive and active regimes of the device. An originally developed model of the gas-filled surge arrester was used. Gas pressure and the interelectrode gap were the variable parameters in our measurements. Applied radiation types included ?-particles, ?-rays, X-rays, and neutrons. Measurements were performed under highly controlled laboratory conditions. The combined measurement uncertainty of the applied procedure was estimated as being under the 5% level. The results obtained are followed by a theoretical explanation. The crucial result is the conclusion that ionizing radiation does not necessarily degrade the gas-filled surge arrester?s functionality but that it, rather, improves it under certain conditions.

Final report on COOMET key comparison of national pressure standards in the range 100 Pa to 5 kPa of gauge pressure (COOMET.M.P-K14)

This report describes a COOMET key comparison of pneumatic gauge pressure standards of four national metrology institutes (listed in the chronological order of their measurements) that was carried out in the period from November 2008 to March 2010 in order to determine their degrees of equivalence in the range of 100 Pa to 5 kPa of gauge pressure. The pilot laboratory was PTB. The reference pressure standards of the participants were of different design. The transfer standard was a piston gauge model V1600 of the company Pressurements. The quantity under comparison was the effective area of the transfer standard at different pressure values reported together with uncertainty contributions and the conclusive combined uncertainty of measurement. All participants' results agree with the key comparison reference values within the expanded uncertainties calculated with a coverage factor 2, all but one results even within the standard uncertainties. For the participants' results compared in pairs, all of a total of 48 pairs show agreement within the expanded uncertainties and 46 pairs within the standard uncertainties. The results of the comparison demonstrate equivalence of the laboratory standards and support their measurement capabilities stated in the KCDB of BIPM.Main text.To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.The final report has been peer-reviewed and approved for publication by the CCM, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

Evaluation of measurement uncertainty components associated with results of radiochemical neutron activation analysis for determination of uranium traces

Being aware of the importance to consider every step in the evaluation of the combined measurement uncertainty of the result, the purpose of this work was to evaluate the contribution of the radial thermal neutron flux gradient to the uncertainty budget for trace level uranium determination in biological materials by a radiochemical neutron activation analysis (RNAA). Determination of uranium via the short-lived nuclide 239U was based on solvent extraction with TBP and measurement of the chemical yield from the gamma-ray spectrum of the isolated fraction via 235U. It has been shown previously, that radial neutron flux gradient, could have a relevant effect on the final result obtained by RNAA. In the present work, radial neutron flux gradient within the irradiation assembly generally accepted in our lab (standards tapped beside the sample), varied between 93 and 108% around the mean value and contributes approximately 20% to the combined measurement uncertainty of the result.