In the last 30 days

• Open/close High-precision gravity measurements using atom interferometry

  A Peters et al 2001 Metrologia 38 25 Tag this article

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  We have built an atom interferometer that can measure g, the local acceleration due to gravity, with a resolution of Δ g/ g = 2 × 10 ⁻⁸ after a single 1.3 s measurement cycle, 3 × 10 ⁻⁹ after 1 min and 1 × 10 ⁻¹⁰ after two days of integration time. The difference between our value for g and one obtained by a falling corner-cube optical interferometer is (7 ± 7) × 10 ⁻⁹ g. The atom interferometer uses velocity-selective stimulated Raman transitions and laser-cooled caesium atoms in an atomic fountain. We extend previous methods of analysing the interferometer to include the effects of a gravitational gradient. We also present detailed experimental and theoretical studies of potential systematic errors and noise sources.

• Open/close On the proposed redefinition of the mole

  W T Chyla 2012 Metrologia 49 L11 Tag this article

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  This letter calls for reconsidering the new definition of the mole, which has been proposed in the draft chapter 2 for the SI Brochure, posted on the BIPM website. It is argued that the list of particles should be deleted from the new definition of the mole. The necessary elements of the definition are summarized and another wording is suggested. Proposals to rename ‘amount of substance’ are briefly discussed.

• Open/close About the probability distribution of a quantity with given mean and variance

  Stefano Olivares and Matteo G A Paris 2012 Metrologia 49 L14 Tag this article

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  Supplement 1 to the GUM (GUM-S1) recommends the use of the maximum entropy principle (MaxEnt) in determining the probability distribution of a quantity having specified properties, e.g., specified central moments. When we only know the mean value and the variance of a variable, GUM-S1 prescribes a Gaussian probability distribution for that variable. When further information is available, in the form of a finite interval in which the variable is known to lie, we indicate how the distribution for the variable in this case can be obtained. A Gaussian distribution should only be used in this case when the standard deviation is small compared with the range of variation (the length of the interval). In general, when the interval is finite, the parameters of the distribution should be evaluated numerically, as suggested by Lira (2009 Metrologia 46 L27). Here we note that the knowledge of the range of variation is equivalent to a bias of the distribution towards a flat distribution in that range, and the principle of minimum Kullback entropy (mKE) should be used in the derivation of the probability distribution rather than the MaxEnt, thus leading to an exponential distribution with non-Gaussian features. Furthermore, up to evaluating the distribution negentropy, we quantify the deviation of mKE distributions from MaxEnt ones and, thus, we rigorously justify the use of the GUM-S1 recommendation also if we have further information on the range of variation of a quantity, namely, provided that its standard uncertainty is sufficiently small compared with the range.

• Open/close Simulated workplace neutron fields

  V Lacoste et al 2011 Metrologia 48 S304 Tag this article

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  The use of simulated workplace neutron fields, which aim at replicating radiation fields at practical workplaces, is an alternative solution for the calibration of neutron dosemeters. They offer more appropriate calibration coefficients when the mean fluence-to-dose equivalent conversion coefficients of the simulated and practical fields are comparable. Intensive Monte Carlo modelling work has become quite indispensable for the design and/or the characterization of the produced mixed neutron/photon fields, and the use of Bonner sphere systems and proton recoil spectrometers is also mandatory for a reliable experimental determination of the neutron fluence energy distribution over the whole energy range. The establishment of a calibration capability with a simulated workplace neutron field is not an easy task; to date only few facilities are available as standard calibration fields.

• Open/close On the equivalence of generalized least-squares approaches to the evaluation of measurement comparisons

  A Koo and J F Clare 2012 Metrologia 49 340 Tag this article

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  Analysis of CIPM international comparisons is increasingly being carried out using a model-based approach that leads naturally to a generalized least-squares (GLS) solution. While this method offers the advantages of being easier to audit and having general applicability to any form of comparison protocol, there is a lack of consensus over aspects of its implementation. Two significant results are presented that show the equivalence of three differing approaches discussed by or applied in comparisons run by Consultative Committees of the CIPM. Both results depend on a mathematical condition equivalent to the requirement that any two artefacts in the comparison are linked through a sequence of measurements of overlapping pairs of artefacts. The first result is that a GLS estimator excluding all sources of error common to all measurements of a participant is equal to the GLS estimator incorporating all sources of error, including those associated with any bias in the standards or procedures of the measuring laboratory. The second result identifies the component of uncertainty in the estimate of bias that arises from possible systematic effects in the participants' measurement standards and procedures. The expression so obtained is a generalization of an expression previously published for a one-artefact comparison with no inter-participant correlations, to one for a comparison comprising any number of repeat measurements of multiple artefacts and allowing for inter-laboratory correlations.

• Open/close Precision comparison of the quantum Hall effect in graphene and gallium arsenide

  T J B M Janssen et al 2012 Metrologia 49 294 Tag this article

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  The half-integer quantum Hall effect in epitaxial graphene is compared with high precision to the well-known integer effect in a GaAs/AlGaAs heterostructure. We find no difference between the quantized resistance values within the relative standard uncertainty of our measurement of 8.7 × 10 ⁻¹¹. The result places new tighter limits on any possible correction terms to the simple relation R _K =  h/ e ², and also demonstrates that epitaxial graphene samples are suitable for application as electrical resistance standards of the highest metrological quality. We discuss the characterization of the graphene sample used in this experiment and present the details of the cryogenic current comparator bridge and associated uncertainty budget.

• Open/close The Refractive Index of Air

  Bengt Edlén 1966 Metrologia 2 71 Tag this article

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  Present knowledge of the refractive index of air is reviewed. Regarding the absolute values there are as yet no definite indications that the standard adopted in 1953 on the basis of Barrell and Sears' measurements should be changed, but new experiments aiming at reducing the present uncertainty of about ± 5 × 10 ^-8 would be desirable.

  Several recent investigations have contributed important new information on the dispersion of air, which has made it possible to derive an improved dispersion formula for standard air, ( n − 1) _s × 10 ⁸ = 8342.13 + 2406030 (130 − σ ²) ^-1 + 15997 (38.9 − σ ²) ⁻¹, where σ is the vacuum wave-number in μm ^-1. The deviations from the 1953 formula are small and practically negligible in most spectroscopic work.

  An equation for the dependence of refractivity on temperature and pressure based on theoretical considerations has been derived. For the range of atmospheric conditions normally found in a laboratory the equation can be approximated by the formula ( n − 1) _tp = ( n − 1) _s × 0.00138823 p/(1 + 0.003671 t), with p in torr, t in °C, and ( n − 1) _s given by the dispersion formula for standard air.

  The effect of carbon dioxide and water vapour is discussed. From Erickson's dispersion data for water vapour, combined with Barrell and Sears' absolute measurements, one obtains the equation n _tpf – n _tp = −f (5.722 − 0.0457 σ ²) × 10 ^-8 for the difference in refractive index of moist air, containing f torr of water vapour, and dry air at equal temperature and total pressure. The equation is valid for visible radiations and normal atmospheric conditions.

• Open/close Simulations of a predictable quantum efficient detector with PC1D

  Jarle Gran et al 2012 Metrologia 49 S130 Tag this article

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  The spectral responsivity of a predictable quantum efficient detector (PQED) is calculated based on the responsivity of an ideal quantum detector and taking into account reflection losses from the surface of the photodiode and internal charge-carrier gains/losses inside the diode. The internal quantum deficiency (IQD) is obtained from simulations with the PC1D software using the material data of the produced PQED photodiodes. The results indicate that at room temperature the predicted IQD of the PQED is close to zero with an uncertainty of about 100 ppm over the visible range. It is further concluded that a primary standard of visible optical power with an uncertainty of approximately 1 ppm is achievable using the PQED at low temperatures.

• Open/close Optical properties of semiconductor photodiodes/solar cells

  Terubumi Saito 2012 Metrologia 49 S118 Tag this article

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  Among possible losses in the efficiency of semiconductor photodiodes and solar cells, optical losses are dominant in many cases and, therefore, are important. The optical losses, mainly consisting of reflection from the surface and absorption in a dead layer, are evaluated based on a theoretical model whose validity was confirmed by experiments. The effect of the surface layer on the optical properties of the whole system is analysed, using a complex plane representation of reflectance and transmittance, to be quite important, and is sensitive to its layer thickness and the angle of incidence. The results are presented as a function of wavelength, surface film thickness and angle of incidence. The results show that the optical properties of the layered system are completely different from those of a single boundary system and change steeply in the UV range due to the change in optical indices of both the surface layer and the substrate.

• Open/close Measurement of the volume of weights using an acoustic volumeter and the reliability of such measurement

  T Kobata et al 2004 Metrologia 41 S75 Tag this article

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  For the precise measurement of the mass of a weight, buoyancy correction of air for the volume of a weight is necessary. Although hydrostatic weighing is the most accurate method currently used in determining volume, it is a relatively complicated process requiring the weight to be wetted in a reference fluid. Since more weights with high accuracy of mass are being used, a more sophisticated method of determining volume that satisfies uncertainty requirements has been demanded to improve the efficiency of calibration. In this paper, an acoustic volumeter is proposed for the measurement of the volume of weights. An acoustic volumeter can measure volume in atmosphere in a simple manner. Consequently, it is not necessary to consider the contamination and mass change due to the liquid used in the hydrostatic weighing method. In practical applications, a procedure for measuring the volume of weights ranging from 100 g to 10 kg using an acoustic volumeter is proposed. The volumes obtained using an acoustic volumeter were compared with those obtained by the hydrostatic weighing method, and the reliability of the measurement was evaluated with a relative uncertainty below the order of 1 × 10 ⁻³. From the measurement results, it was shown that the use of an acoustic volumeter is effective for the measurement of the volume of weights.