Primary measurement standard

The international prototype kilogram is an example of a primary measurement standard its value and uncertainty have been established without relation to another standard of the same kind. When the primary measurement standard is used for calibration, it becomes a primary calibrator. 

A primary measurement standard is expected to have a known quantity value with minimum uncertainty, although this is not specified as such in the definitions. Because the measurement uncertainty is propagated down the calibration hierarchy, you should start with as small an uncertainty as possible and choose methods to establish purity of primary reference materials with this in mind. 

The concepts of calibration and especially of metrological traceability were elaborated by physicists as mentioned above. The reference or top of the calibration hierarchy preferably is the definition of an SI unit, which is realized or embodied as a primary measurement standard. By direct comparison, the quantity value of a secondary measurement standard can be established. Subsequent comparisons may furnish quantity values of reference measurement standard, working measurement standard, and routine measurement standard with which the object carrying the measure and is compared to obtain its measurement result which then retrospectively is metrologically traceable to the SI unit. The primary measurement standard, as the definition of metrological traceability says, is preferably an international or national measurement standard. 

The purpose of the national standards network is to provide the primary measurement standards as references for the measurements carried out on the workshop floor. In order to reach the working level in an efficient way, a dissemination mechanism is required. 

Most of the primary measurement standards are the realization of the SI units, and are under custody of each country s National Metrology Institute (NMI). The link between the realization of the SI units and primary standards is established through primary methods of measurement. These are methods which do not require any reference of the same quantity. Additionally, through a series of comparisons between NMIs, comparability of measurements among traceable measurement systems at international level are recognized by each country. 

Irrespectively of historical aspects, the word standard should be reserved, at present, for cases dealing with the highest metrological quality. For instance, very pure metals and high purity compounds with exactly established stoichiometry can be regarded as primary measurement standards and serve for the preparation of standard solutions. 

Primary measurement standard Standard that is designated or widely acknowledged as having the highest metrological qualities and whose value is accepted without reference to other standards of the same quantity. ... 

The National Physical Laboratory (NPL) is the UK s National Measurement Institute which sits at the heart of the NMS. For more than a century NPL has developed and maintained the UK s primary measurement standards. These standards underpin an infrastructure of traceability throughout the UK and fhe world which ensures accuracy and consistency in measurement The national primary standards, which constitute the basis of measurement in the UK, are maintained in strict accordance with internationally agreed recommendations. They are based on the International System of Units (SI). 

The motivation for establishing accurate values of the various transport properties has been discussed in previous chapters. The current stams of fundamental kinetic and statistical mechanical theory, computer simulation and experimental technique and data acquisition impose severe limits on the accuracies achievable in any description of a transport property surface for a real fluid. For instance, it will not be possible to determine viscosities to one part in 10 for the near future. Thus, the accuracies associated with primary standards for transport properties fall an order of magnitude below those associated with primary measurement standards for equilibrium thermodynamic properties. Fortunately, the technological applications of transport property information do not require extreme accuracies. 

Primary calibration standard A calibration standard based on direct measure of a reference value. 

It is important that a measurement made in one laboratory by a particular analyst can be repeated by other analysts in the same laboratory or in another laboratory, even where the other laboratory may be in a different country. We aim to ensure that measurements made in different laboratories are comparable. We are all confident that if we measure the length of a piece of wire, mass of a chemical or the time in any laboratory, we will get, very nearly, the same answer, no matter where we are. The reason for this is that there are international standards of length, mass and time. In order to obtain comparable results, the measuring devices need to be calibrated. For instance, balances are calibrated by using a standard mass, which can be traced to the primary mass standard (see also Chapter 5). The primary standard in chemistry is the amount of substance, i.e. the mole. It is not usually possible to trace all of our measurements back to the mole. We generally trace measurements to other SI units, e.g. mass as in 40 mg kg-1 or trace back to reference materials which are themselves traceable to SI units. 

Most currently used oxidant and ozone monitors need to be calibrated with a predetermined concentration of ozone in air. Regardless of the principle used to measure ambient ozone or oxidant concentrations, the primary reference standard for calibrating each monitoring device or system should be identical everywhere. This requirement remains to be achieved in practice. Up to June 1975, at least seven calibration procedures were practiced in the United States. These are listed in Table 6-5... 

The control of NO from stationary sources includes techniques of modification of the combustion stage (primary measures) and treatment of the effluent gases (secondary measures). The use oflow-temperature NO,.burners, over fire air (OFA), fiue gas recirculation, fuel reburning, staged combustion and water or steam injection are examples of primary measures they are preliminarily attempted, extensively applied and guarantee NO reduction levels of the order of 50% and more. However, they typically do not fit the most stringent emission standards so that secondary measures or flue gas treatment methods must also be applied. 

Gradient RP-HPLC separation with 0.1% formic acid and methanol as mobile phase components (0.25 mL/min) combined either with ESI-time-of-flight [23] MS detection or with chemiluminescence nitrogen detection (CLND) was used to identify and, respectively, quantify illicit drugs in seized material without primary reference standards [24], The method exploits the accurate mass measurement provided by TOE MS, enabling the unequivocal identification of molecular formula of the unknown analyte, and the CNLD equimolar response to nitrogen. 

There are a number of useful standardized scales to monitor severity and treatment outcomes, (reviewed by Conners [1998] and Barkley [1998]) Because of the overlap with other disorders, an ADHD-specific scale is strongly recommended (such as the Conners, SNAP, Dupaul scales) in which symptom items are based on the DSM criteria and do not include items of other disorders (such as anxiety or mood) or nonspecific functional items. Some ADHD scales provide separate ratings of oppositionality or aggression (SNAP, Conners). It may be helpful to monitor symptoms from non-ADHD conditions as well as functional deficits, and thus a broad-spectrum scale may also be employed but should not be used as the primary measure of ADHD severity or anti-ADHD treatment. Normed rating scales provide comparative information on severity based on age and gender however, such tests are not diagnostic and are not a substitute for the clinical interview. 

Water is the primary viscosity standard with an accepted viscosity at 20°C of 0.010019 poise. Hydrocarbon liquids such as hexane are less viscous. Molasses may have a viscosity of several hundred centistokes, while for a very heavy lubrication oil the viscosity may be 100 centistokes. There are many empirical methods for measuring viscosity. 

In power quality studies, we are mainly concerned with the propagation of EMI in space (or air) and as such we are only concerned with the properties applicable to free space. Electrical field intensity is the primary measure of electrical fields applicable to power quality. Most field measuring devices indicate electric fields in the units of volts/meter, and standards and specifications for susceptibility criteria for electrical fields also define the field intensity in volts/meter, which is the unit used in this book. 

Measures of Response for Substances Causing Deterministic Responses. For purposes of health protection in routine exposure situations, incidence has been the primary measure of deterministic response for both radionuclides and hazardous chemicals. Fatalities also are of concern for substances that cause deterministic responses, but only at doses substantially above the thresholds for nonfatal responses. Given that the objective of standards for health protection is to prevent the occurrence of deterministic responses, incidence is not modified by any subjective factors that take into account, for example, the relative severity of different nonfatal responses with respect to a diminished quality of life. Judgments about the importance of deterministic responses are applied only in deciding which responses are sufficiently adverse to warrant consideration in setting protection standards. 

In the field of chemical measurements, the question as to which are the national measurement standards is far from being completely answered. There is no doubt, however, that primary reference materials, e.g. high-purity substances which are ultimately necessary as reference points, will play a role as national (or even better international) measurement standards. But these alone will not be sufficient. As the task of chemical analysis is usually the determination of chemical composition, national reference points closer to this task, namely standard reference mixtures, are also required, and if the preparation of these is not feasible, e.g. due to instability problems, devices and procedures furnishing well-known compositions with small uncertainties must also be included as national measurement standards. All these kinds of national references or standards are currently in use or under development. 

It consists of three levels. At the top of the structure a network of national laboratories provides the primary chemical measurement standards and ensures that these are linked up with the international reference framework for chemical measurements. Via primary reference materials and reference measurements, a secondary level consisting of accredited chemical calibration laboratories, including verification authorities in the regulated area, is connected to the national standards level. 

The national standard for electrolytic conductivity measurement is a primary measuring set-up developed and maintained at PTB. Its central element is a measuring cell of exactly known geometry in which the distance of the electrodes can be changed and exactly measured. Resistance measurements are carried out with at least two different electrode spacings with exactly known shift, with all other conditions kept constant. The measured electrode shift, the cross section of the cell and the two resistance values allow the electrolytic conductivity... 

The users of pH meters thus need calibration solutions of long-time stability which are traceable to primary pH standards pH(PS) related as closely as possible to the definition of pH. Although pH measurements are carried out on a large scale, the problems posed by the traceability of pH have not yet been adequately solved. 

The secondary gas standards are blended using the same apparatus as that employed to prepare the primary gravimetric standards. Precise measurements of the gas pressure and gas mass are carried out at each stage in the process and these enable the concentrations of the mixtures to be produced to within 1% of the concentration of the appropriate NPL primary standards. Up to three mixtures with the same nominal concentrations can be produced together. The mixtures are subjected to stability checks before their concentrations are certified with respect to NPL primary standards. 

The primary element standards especially the primary pure elements are used for the production of other reference materials (Element-solutions for PTB/Merck and EMPA/Fluka as well as isotopic standards for IRMM [Joint European Project for Primary Isotopic Measurements=JEPPIM]). 

Even if the concept of primary element standards has already proved a success it covers presently only a few elements. Only an international division of labor can build up this part of measurement infrastructure. In addition to the initiative of CIPM and the national metrology institutes the leading producers of reference materials should cooperate to establish a worldwide system... 

The following table lists the 15N chemical shifts (in ppm) for common standards. The estimated precision is better than 0.1 ppm. Nitromethane, according to Levy and Lichter,1 is the most suitable primary measurement reference, but has the disadvantage of lying in the low-field end of the spectrum. Thus, ammonia (which lies in the most upfield region) is the most suitable for routine experimental use.1-6... 

Manometers consisting of liquid columns of, commonly, mercury or a fluid such as silicone oil, have been used extensively in the past to measure gas mixtures in, for example, experimental, static investigations of the overall kinetics of gas-phase reactions. They continue to be used in many applications, including the establishment of primary pressure standards in several countries. 

---