Calibration of Line Standards of Length and Measuring Tapes at the National Bureau of Standards Lewis V. Judson The methods used at the National Bureau of Standards in calibrating line standards of length and measuring tapes submitted for standardization are outlined. The equipment used is described briefly. There is a discussion of some considerations that should be given as to whether or not a standard should be submitted to the Bureau. Instructions are given for submitting items to the Bureau for calibration. The appendix contains useful information on the use of steel tapes. 1. Introduction The Bureau receives many requests from intists, industry, and governmental agencies to brate line standard of length and measuring es. The Circular is issued as a guide for anycontemplating the submission of such items the Bureau for standardization. It replaces cular 332, Testing of line standards of length, | Circular 328, Testing of measuring tapes at National Bureau of Standards. Line Standards of Length at NBS The primary standard of length in the United tes is the National Prototype Meter 27 (fig. 1), ich is identical in form and material with the ernational Prototype Meter deposited at the ernational Bureau of Weights and Measures at res, near Paris, and also with the other national totype meters distributed in 1889 in accordance h the treaty known as the Convention of the ter, dated May 20, 1875. In the United States, De 1893, the yard has been defined in terms of meter by the relation Proposals have been made to adopt, as the definition of the meter, a specified number of wavelengths of a suitable isotope of an element such as mercury (for example, the 5461-A wavelength of mercury 198). Interferometric calibrations of end standards of length have been made for many years. Such calibrations of line standards of length have been made in Germany, and the method is being followed with considerable interest at the Bureau. 3. Facilities for Calibrating Line For the calibration of line standards of length the most precise measurements now being made at the Bureau are on its 1-m longitudinal comparator (fig. 2). This has been briefly described by Page. It is especially suited for the calibration of the subintervals of a bar, using any one of the several methods that have been developed. Although this comparator is nominally a 1-m comparator, it is readily adaptable for standardizing 48-in. bars, and can be used for calibrating even longer bars. For comparing two bars with an accuracy not better than 1 micron (0.001 mm or 0.00004 in.) a simple transverse comparator with no thermal insulation is commonly used. In this instrument the bars are mounted parallel to each other, and the carriage supporting the two bars moves back FIGURE 1. U. S. National Prototype Meter 27. nd forth in a direction perpendicular to a line arallel to the axis of each of the two bars. The micrometer microscopes are not moved during a omparison with this transverse comparator. Short bars and small scales are often more coneniently calibrated by means of a linear dividing ngine having two microscopes conveniently mounted so that they can be focused on the bars mounted on the longitudinally moving carriage. The instrument is then, in effect, a longitudinal omparator. Micrometer microscopes having a pair of parlel "cross hairs" moving in the focus of a posive-type ocular are probably the most important arts of any instrument for comparing line standrds of length. The movement of the cross wires measured by a precision screw and a graduated rum. In the comparators in the length section the Bureau the axes of the microscopes are ertical. - Facilities for Calibrating Measuring Tapes At the Bureau there is a temperature-controlled boratory about 212 ft long (fig. 3). Two distinct stallations have been made in this laboratory. me is a 200-ft steel bench for calibrating steel pes used in general surveying and engineering ractice, as shown at the right in figure 3. The her is a geodetic comparator designed especially rcalibrating the 50-m invar base-line tapes, such are used by the U. S. Coast and Geodetic Sury. Its use is not limited, however, to a 50-m ngth. It is shown at the left in figure 3. 4.1. Steel-Tape Bench The steel-tape bench has an over-all length of 0 ft 1 in., a width of 21⁄2 in., and a thickness of in., and is constructed of stainless-steel bars proximately 12 ft long, with individually lapped d fitted ends. Stainless-steel conical dowel pins ving threaded lower ends for locking nuts murely hold adjoining bars in the correct position. ne supports for the bench are attached firmly to e wall, but can be adjusted whenever necessary. ne bench is graduated at intervals ordinarily quired for testing tapes graduated in the metric in the United States customary system. The uipment is furnished with the necessary supports the tape when it is to be supported at specified ints, with apparatus for applying the tension, ermometers for observing the temperature, and e necessary clamps and other auxiliary equip ent. Comparisons of a tape with the bench standard ordinarily made with a low-power microscope d a precision steel scale graduated either to in. or to % mm. When unusually high accuracy required and the character of the graduation es warrants, comparisons with the bench can be ade with a micrometer microscope. 4.2 Geodetic Tape Comparator A 5-m bar, packed in melting ice when measurements are being taken, is the working standard used as the basis of measurements in the geodetic tape comparator. Piers bearing microscopes are spaced 5 m apart for a total length of 50 m. Auxiliary piers 1 m apart, also bearing microscopes, are placed between the 20-m and the 25-m piers. These provide intervals so that the 5-m bar can be standardized by means of a calibrated 1-m bar. By use of a special bracket at the 15-m pier, a double pier at the 30-m location, and an additional pier near the 45-m pier, it is possible to mount microscopes at the 50-, 100-, and 150-ft points. In figure 4 are shown the 15-m and 50-ft microscopes at the extreme left, then the covered trough and carriages for the 5-m bar, immediately beyond that the six piers with their microscopes spaced 1-m apart, and other piers beyond them. In addition to the basic equipment of this comparator, there is the necessary auxiliary equipment, such as tape clamps, thermometers, etc. Most of the work done with this comparator is with the 50-m interval, and the use of this interval will be assumed for the remainder of this section. After the 50-m interval is established by moving the 5-m bar to measure the distance between each of the 10 possible 5-m intervals between microscopes, and the positions of the 0- and 50-m points are transferred from the focuses of the microscopes to centers of two hemispheres in piers at floor level, the 5-m bar is moved outside the 50-m interval. An invar tape to be calibrated is then mounted in the comparator, supported on ballbearing wheels, and proper tension is applied to the tape by means of a calibrated weight. The difference between the interval on the tape and the established 50-m interval is obtained by the micrometer microscopes. 5. Calibration of Line Standards of Length The measurement of line standards of length undertaken by the Bureau is classified under the several headings that follow. A complete calibration of this type of length standard includes the determination of the length at a known temperature and also of the expansivity usually expressed by the average coefficient of linear thermal expansion over a small range of temperature that embraces the temperatures at which the standard is likely to be used. For most work with any standard except one of the highest grade it is sufficient to assume a coefficient of expansion derived from a knowledge of the composition of the materials of which it is made. If the standard is subdivided, a calibration of the subdivisions may also be necessary. The number of intervals that it is advisable to compare will depend on the character of the standard and on the use to which it is to be put. In many cases |