Calibration

“ Calibrate daily with at least six media spiked calibration standards and a blank. a. Prepare the target analyte spiking solutions. (See SOLUTIONS, 9111-2) b. Prepare calibration standards and media blanks in clean shipping containers (e.g. 50-mL polypropylene centrifuge tubes.) c. Spike a known volume of target analyte spiking solution into each calibration standard by spiking directly onto the media. Use the spiking volumes suggested in Table 3 to cover the desired range. ”

“ This process of calibration is particularly important in laboratory research, where the observer has frequently to make his own apparatus, and cannot afford the time or outlay required to make special tools for fine work, but is already provided with apparatus and methods of accurate testing. For non-scientific purposes it is generally possible to construct instruments to measure with sufficient precision without further correction. The present article will therefore be restricted to the scientific use and application of methods of accurate testing. ”

“ Where there are many in use, it is good practice to have a master pyrometer of a rare metal couple, which is used only for checking up the others. The master pyrometer, after calibrating against the melting points of various substances, will have a calibration chart which should be used in the checking operation. ”

“ The amount of light scattered by a particle into the detector is a complex function of the particle size, shape and refractive index. For spherical particles of known refractive index the instrument response can be calculated. However, in general, calibration must be carried out experimentally. An example of instrument response as a function of particle size for spherical particles of two different refractive indices is shown in Figure 1. It can be seen that there is a peak at approximately 0.6 mm and that there is a drop in instrument response to larger particles. ”

“ The errors and corrections are given in thousandths of 1°.
Complete Calibration.—The simple method of Gay Lussac does very well for short intervals when the number of steps is not excessive, but it would not be satisfactory for a large range owing to the accumulation of small errors of estimation, and the variation of the personal equation. The observer might, for instance, consistently over-estimate the length of the thread in one half of the tube, and under-estimate it in the other. ”

“ A comparatively rough balance, capable of weighing to centigrams, is sufficiently accurate for use in calibrations, for a moment's reflection will show that it would be useless to weigh the water with an accuracy greater than that of the readings taken on the burette. ”

“ To adjust calipers so finely that a piece of work may be turned by caliper measurement to just fit a hole; a working or a driving fit without trying the pieces together, is a refinement of measurement requiring considerable experience and skill, because, as will be readily understood from the remarks made when referring to gauge measurements, there are certain minute allowances to be made in the set of the calipers to obtain the desired degree of fit. ”

“ So accustomed do workmen become in estimating from this pressure of contact how nearly the work is reduced to the required diameter, that they are enabled to estimate, by forcing the calipers over the work, the depth of the cut required to be taken off the work, with great exactitude, whereas with solid gauges, or even caliper gauges of solid proportions, this cannot be done, because they will not spring open. ”

“ Fig. 4 shows a method of setting the calipers by using a small square to abut the scale and to afford a surface against which to place the leg of the caliper. The scale, lying flat on the blade of the square, is always sure to be square edgewise, and is easily held squarely against the stock of the square as shown. This method has also the advantage of being portable, and can be taken to the window or to any place where the light is satisfactory. When using a long scale, the free end may be held against the body to assist in holding it in place. ”

“ CALIPERS, DIVIDERS, AND SURFACE GAGES
In the present chapter we shall deal with the simpler forms of tools used for measuring, such as ordinary calipers, and their use; surface gages; special attachments for scales and squares, facilitating accurate measuring; and vernier and beam calipers. ”

“ When the surfaces of work are to be parallel, they can be measured with calipers. Surfaces to be at a right angle can be tested with a square; those to be in line can be tried with a straight-edge, and in each case the truth or alignment of the surfaces is tested by contact of the testing tool. ”

“ At the present day we designate more peculiarly by the name of metrology that part of the science of measurements which devotes itself specially to the determining of the prototypes representing the fundamental units of dimension and mass, and of the standards of the first order which are derived from them. If all measurable quantities, as was long thought possible, could be reduced to the magnitudes of mechanics, metrology would thus be occupied with the essential elements entering into all phenomena, and might legitimately claim the highest rank in science. ”

“ Consequently, when the vernier is set to the mercurial column, the difference shown by the vernier from the tenth on the scale is the hundredths to be added to the inches and tenths of the scale.
A little practice will accustom a person to set and read any barometer quickly; an important matter where accuracy is required, as the heat of the body, or the hand, is very rapidly communicated to the instrument, and may vitiate, to some extent, the observation. ”

“ In our work our standard is first placed in an ice-box, in front of which is a refractometer. There we measure the fraction and count the whole number. When we have had it in there as long as may be required, we transfer it to our comparer, which is already in an icebox; so there are no changes of temperature suffered by our standard between the time when we measure its length in wave lengths and the time when we stop off its length for comparison with, or verification of some other standards. ”

“ When the standard quantity is greater than that to be measured, we often adopt the first mode, and subdivide the unit until we get a magnitude equal to that measured. The angles observed in surveying, in astronomy, or in goniometry are usually smaller than a whole revolution, and the measuring circle is divided by the use of the screw and microscope, until we obtain an angle undistinguishable from that observed. The dimensions of minute objects are determined by subdividing the inch or centimetre, the screw micrometer being the most accurate means of subdivision. ”

“ It is the 'Metroscope,' 'an instrument for measuring inaccessible heights and distances, and for levelling.' To be able to measure the width of a river, or the height of a tower to the very top of the weathercock, is a manifest advantage, which is further increased by the instrument being provided with a scale, on which the measure can be read off in feet and inches, or feet and decimals. ”

“ In scribing a circle with a pair of dividers mark the center with your center punch first as this will prevent your dividers from slipping.
(10) Inside calipers are used for measuring the inside diameters of cylinders and the like, and, conversely (11) , outside calipers are used for measuring the outside of anything that is round. In either case you measure the distance between the points of your caliper with your rule to find the diameter of the thing. (12) A center punch is always useful to make [62] a starting point in metal with, for it can’t be rubbed off or lost sight of. ”

“ It must, however, be remembered that in all practical applications of formulae the data have first to be ascertained by direct or indirect measurement; and this measurement involves a certain margin of error.
The two sources of error mentioned under (a) and (b) above are closely related. Suppose, for instance, that we require the area of a circular grass-plot of measured diameter. As a matter of fact, no grass-plot is truly circular; and it might be found that if the breadth in various directions were measured more accurately the want of circularity would reveal itself. ”

“ For measuring the distance of a slot or keyway from a surface, the form of calipers shown in Fig. 1424 is employed; the straight leg has its surface a true plane, and is held flat against the surface b of the slot or keyway, and the outside or curved leg is set to meet the distance of the work surface measuring the distance c. These are termed keyway calipers.
There are in general machine work four kinds of fit, as follow: The working or sliding fit; the driving fit; the hydraulic press fit; and the shrinkage fit. ”

“ Ordinary measurement by a foot rule, a surveyor’s chain, or the excessively careful measurements of the base line of a trigonometrical survey by standard bars, are sufficient instances of this procedure.
In the second case, where p y/x = q, we multiply or divide a magnitude until we get what is equal to the unit, or to some magnitude easily comparable with it. As a general rule the quantities which we desire to measure in physical science are too small rather than too great for easy determination, and the problem consists in multiplying them without introducing error. ”

“ In rough work, such as carpentry, it does not, indeed, matter if measurements are the hundredth of an inch or so out. But when we have to deal with scientific instruments, telescopes, measuring machines, engines for dividing distances on a scale, or even with metal turning, the utmost accuracy becomes needful; and a number of instruments will be much more alike in all dimensions if compared individually with a common standard than if they were only compared with one another. Supposing, for instance, a bar of exact diameter is copied ”

“ Standards of length are preserved by a Bureau of Weights and Measures in most countries. Delambre, a French authority on the decimal system of measures, taught at the beginning of the nineteenth century that magnitudes as small as the hundredth of a millimeter are incapable of observation. The International Bureau of Weights and Measures now guarantees to determine two or three ten-thousandths of a millimeter. So much has the science of measurement progressed in a century. ”

“ Calipers possess one great advantage over more rigid and solid gauges, in that the calipers may be forced over the work when the degree of force necessary to pass them on indicates how much the work is too large, and therefore how much it requires reducing. ”

“ When desiring to measure a number of pieces, a standard size piece or gage is placed between the plunger B and the face L of the micrometer screw, and the instrument is adjusted until the indicator points exactly to zero on the small scale provided on the body of the micrometer. ”

“ In almost every case a measuring instrument serves, and should serve only as a means of comparison between two or more magnitudes. As a general rule, we should not attempt to make the divisions of the measuring scale exact multiples or submultiples of the unit, but, regarding them as arbitrary marks, should determine their values by comparison with the standard itself. ”

“ The state of any point of the medium at the time is found by taking the average of the state of every part of the medium, the weight assigned to each part in taking the average being where is the distance of that part from the point considered. This average, in the case of vector-quantities, is most conveniently taken by considering each component of the vector separately. ”

“ But when a few base lines have been obtained in different places on the earth's surface, the measuring rods are to be laid aside, and the subsequent task of the survey of the earth is to be conducted by the measurement of angles from one station to another and trigonometrical calculations based thereon. Starting from a base line a few miles long, distances of greater length are calculated, until at length stretches 100 miles long, or even more, can be accomplished. ”

“ If the work is not to size they will not determine how much the error or difference is, hence they are gauges rather than measuring tools. It is obvious, however, that if the work is sufficiently near to size, the plug or male gauge may be forced in, or the collar or female gauge may be forced on, and in this case the tightness of the fit would indicate that the work was very near to standard size. ”

“ In many experiments and measurements we know that there is a preponderating tendency to error in one direction. The readings of a thermometer tend to rise as the age of the instrument increases, and no drawing of means will correct this result. Barometers, on the other hand, are likely to read too low instead of too high, owing to the imperfection of the vacuum and the action of capillary attraction. If the mercury be perfectly pure and no appreciable error be due to the measuring apparatus, the best barometer will be that which gives the highest result. ”

“ The answer is that the requirements of scientific measurement are growing more and more rigorous every year. A hundred years ago a measurement made to within one-thousandth of an inch was considered rather phenomenal. Now it is one of the modern requirements in the most accurate machine work. At present a few measurements are relied upon to within one ten-thousandth of an inch. There are cases in which an accuracy of one-millionth of an inch has been attained, and it is even possible to detect differences of one five-millionth of an inch. ”