Since the beginning of measurement of pressure in the 17th century, the unit of pressure has been defined by the relationship of force per unit area. The present state of optical technology now offers the possibility of using a thermodynamic definition-specifically the ideal gas law-for the realization of the pressure unit, in the vacuum regime and slightly above, with an accuracy comparable to or better than the traditional methods of force per area. The changes planned for the SI in 2018 support the application of this thermodynamic definition that is based on the ideal gas law with the necessary corrections for real-gas effects. The paper reviews the theoretical and experimental foundations of those optical methods that are considered to be most promising to realize the unit of pressure at the highest level of metrology.
By measuring the refractivity and the temperature of a gas, its pressure can be assessed from fundamental principles. The highest performing instruments are based on Fabry-Perot cavities (FPC). Gas modulation refractometry (GAMOR)is a methodology that has the ability to reduce the influence of disturbances to such an extent that high-precision (sub-parts-per-million) assessments of pressure can be made by the use of FPCs of Invar. To allow for high accuracy assessments, it is of importance to assess the uncertainty contribution from the thermodynamic effects that are associated with the gas filling and emptying of the cavity (pV work). This paper presents a detailed scrutiny of the influence of the gas exchange process on the assessment of gas temperature on an Invar-based dual-FPC (DFPC) instrumentation. It is shown that by virtue of a combination of a number of carefully selected design entities (a small cavity volume with a bore radius of 3 mm, a spacer material with high heat capacitance, large thermal conductivity, and no regions that are connected with low thermal conductance, i.e. no heat islands, and a continuous assessment of temperature of the cavity spacer) the system is not significantly affected by pV work. Simulations show that 10 s after the filling all temperature gradients in the system are well into the sub-mK range. Experiments support that refractivity assessments initiated after 40 s are not significantly affected by the pV work. The analysis given in this work indicates that an upper limit for the influence of pV work on the Invar-based DFPC system using 100 s long gas modulation cycles is 0.5 mK/100 kPa (or 1.8ppm/100 kPa). Consequently, thermodynamic effects will not be a limiting factor when the Invar-based DFPC GAMOR system is used for assessments of pressure or as a primary pressure standard up to atmospheric pressures.
The maximal liver function capacity (LiMAx) test, a novel C-methacetin breath test, has proven clinical validity in determining hepatic metabolic capacity. In contrast to priorC-methacetin breath test protocols, the LiMAx test is performed by intravenous body-weight-adjusted substrate administration. Furthermore, the DOB kinetics (delta over baseline of the time-dependent exhaled CO/CO ratio) are measured online at the bedside with a high time resolution in order to determine the maximum DOB. The aim of this study was to analyze the recorded DOB kinetics in a large population for further refinement of the test protocol. Two new methods of kinetic analysis are proposed in this article: the time dependency of the DOB kinetics and the time interval until half of the DOB maximum. A total of 10 100 LiMAx tests on 8483 patients performed during routine clinics at eight centers were available. The kinetic analysis revealed a specific pattern of DOB kinetics depending upon LiMAx result. In addition, potential co-factors for DOB kinetics, such as weight, height, gender and age, were analyzed, yielding a potential influence of gender and smoking behavior. Both the specific patterns and the proposed kinetic analysis have the potential to further improve the sensitivity and specificity of the test and its clinical applicability by shortening its duration.
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