Length, measurement standards

Calibration grade bars are intended for use in the calibration of length measuring standards and equipment. Both reference and calibration grades should only be used in a standards room controlled at 20°C. Both have completely plain end faces and have UKAS calibration certificates. 

Making a measurement of any kind involves comparing the unknown (i.e. the test sample being measured) with a standard. The standard provides the link to the measurement scale being used (e.g. a ruler to measure length, a standard weight to measure mass, a pure chemical substance to determine the amount of a compound present). This is illustrated in Figure 5.1. 

Fig. 22 Plot of C-OX bond length versus Hammett a for the aromatic substituent Y for triphenylmethyl ethers and esters [108]. The error bars represent two standard deviations in the bond-length measurements. Reprinted with permission from Edwards et al. (1986a). Copyright 1986 American Chemical Society.

Traceability is the relationship of a result of a measurement to a value of a standard through an unbroken chain of comparisons (traceability chain). In the case of the length measurements this chain can be realized in the way presented in the slide. When we want to measure the length of a shark, the result of oirr measurement is dependent on the tape we are using (comparison). 

Now let s see how a traceability chain can be realized for a chemical measurement. Generalizing and in an analogous way to length measurements it can be like the chain in the slide. The amount content of a compound X in a solution is compared with the amount content in a working standard. This in turn was compared with the amount content in a reference standard and after some further comparisons, in an ideal situation we end up with the SI unit, the mole. 

A number of issues influenced the selection of the dose-response model form and the treatment of the data prior to fitting the model. First, shoot weight and shoot length are continuous response measurements therefore, use of a standardized logistic model form is not appropriate. Second, the natural variation in plant growth often resulted in apparent increased shoot weight and shoot length measurements relative to the control at low herbicide application rates. A dose-response model needs to perform well even when some measurements in treatment levels exceed the controls. 

To alleviate the burden of trying to prepare a potassium dichromate solution of exact concentration, the specific absorbance value cm can be used to compare the measured and standard values. The specific absorbance value normalizes the absorbance value obtained from using a solution of approximate 0.006% w/v to the equivalence of 1% concentration and a 1-cm path length measurement ... 

Since 1893, the U.S. basis of length measurement has been derived from metric standards. In 1959, a small refinement was made in the definition of the yard to resolve discrepancies both in this country and abroad which changed its length from 3600/3937 m to 0.9144 m exactly. This resulted in the new value being shorter by two parts in a million. At the same time, it was decided that any data in feet derived from and published as a result of geodetic surveys within the U.S. would remain with the old standard (1 ft = 1200/3937 m) until further decision. This foot is named the U.S. survey foot. As a result, all U.S. land measurements in U.S. customary units will relate to the meter by the old standard. All the conversion factors in this table for units referenced to this footnote are based on the U.S. survey foot rather than on the international foot. 

NBS was created in 1901 to be the nation s central reference laboratory for measurements in the physical sciences and engineering. NBS develops, maintains, and/or disseminates hundreds of measurement standards including the fundamental standards for temperature, time, frequency, mass, length, amount of substance, and electrical measurements. 

Figure 6-6. Simulated Mooney-Rivlin plot, equation (6-82), for C2/C = 2. Solid line is the undistorted response of the material "data" are the results with a 1% (standard deviation) random error incorporated into both the force and length "measurements."...

There exist well-established methods for the measurement of / values. These are discussed at length in standard references [149]. More recent experimental developments have been quite comprehensively reviewed [151]. The present section is not intended as a substitute for this general literature, but concentrates specifically on rather sensitive techniques based on differential refractive index determinations, which have been successfully applied to observations of high Rydberg members in the laboratory. 

Primary length measurements are these days based on optical frequency standards. If one needs a unit of length, for example, a wave-length for interferometric measurement, then one divides the optical frequency by the value of the speed of light (299 792 458 m.s ) as defined in the SI metre. The mise en pratique of the metre [44] lists a number of frequency-stabilised lasers at various wavelengths in the visible and near infrared spectral regions. 

The obvious length measurement is a side-by-side comparison with a standard meter, the obvious volume measurement, the evaluation of the content of a standard vessel. Early length measurements of this type were based on anatomical lengths. Naturally, the variation in human size was a basic problem that was solved either by averaging, or by arbitrary choice. The sixteenth century woodcut of how to produce a right and lawful rood is illustrated in Fig. 4.11. It shows that one should line up sixteen men, tall and short, as they happen to come out of the church after the service. One sixteenth of this rood was the right and lawM foot. It is surprising... 

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