Line Graduated Measuring Instruments (Metrology)

2.2.
Line graduated measuring instruments incorporate graduation spacings representing
known distances. These are used for direct measurements of specific distances within their capacity
range. The sensitivity of the measurement is dependent primarily on the instrument’s basic design
(the least distance between the individual graduations). The measuring accuracy is affected by the
original accuracy of the graduation, the level of resolution of the graduation lines and of the read-out
members, excessive thickness or poor definition of the graduation lines, the instrument’s design
and the general workmanship exercised in its manufacture, the geometric deficiencies resulting
from flatness and parallelism errors or caused by deflection. The precision of measurements made
with line graduated instruments will also be a function of how truly are the actual distances to be
measured associated with the corresponding instrument graduations. Observational errors may
occur due to alignment deficiencies (due to improper coincidence of the distance boundaries with
the selected scale graduations), and parallax errors when observing the scale and the object in a
direction not perfectly normal to the surface being measured. However the observational errors can
be reduced by means of auxiliary devices which improve the alignment of viewing and by keeping
the measuring operation under control. But the instrument errors are inherent inaccuracies of the
process and these must commensurate with the requirements of the particular measuring process.
Line graduated elements are used in a great variety of measuring instruments. Differences
exist in various essential respects, such as the intended use, the designed sensivity, the accuracy
of execution, the level of sophistication with regard to read-out and many others.
Various types of line graduated measuring instruments are :
(i) Line graduated rules and tapes—These are used for direct length comparison and they
have no auxiliary devices. These are available in widely different degrees of accuracy to suit diverse
requirements for plain length measuring tools.
(ii) Line graduated bar standards—These are usually made in modified “H”, “U” or “X”
cross-sectional forms, and the graduation lines are applied to that portion of the surface which lies
in the neutral plane of the cross-section. It represents a very high level of accuracy. The essential
characteristic is the sustained accuracy of the graduation, and not the least distance between them.
It requires an auxiliary device like microscope tube. Internals on bar may be subdivided by other
equipment used in conjunction with the reference standard. Special precautions are required in
supporting these reference bars in order to avoid the effect of sag.
Other devices include vernier caliper gauges, micrometers, line graduated angle measuring
instruments, line graduated master scale (incorporating extra fine graduation lines ; increased
resolution of the scale graduation for the purpose of fine incremental sub-division being ac-
complished through optical magnification of the master scale and the projection of the image on a
screen which is equipped with a subdividing device), diffraction gratings, etc.
Ideally, all measurements or inspections should yield a result that is understood quantita-
tively through the use of numbers. A measurement is described more accurately if it includes a
number that is referenced against a standard unit of measure. Quantitative measurements are
described as variable data (in numbers) and offer more tangible information than qualitative
assessment-(attributes data-pass or fail).