Your correction here will be applied in the
opposite direction. Since the tape reads short, the
laid tape distance of 362.73 ft is LONGER than
362.73 ft by the amount of the total correction
for standard error (0.01 ft). Therefore, you must
SUBTRACT the total tape error. To lay off a
distance of 362.73 ft with this tape, you would
actually measure off a distance of 362.72 ft.
Suppose now that the Bureau of Standards
calibration certificate states that when a tape
indicates 100.00 ft under standard conditions, it
is actually measuring only 99.997 ft. Again, the
standard error is 0.003 ft per 100 ft, but this tape
“reads long”; that is, the interval it indicates
is LONGER than the interval it is actually
measuring. Suppose you measure the distance
between two given points with the tape and find
that the tape indicates 362.73 ft. The total
standard error is again 0.01 ft. Because the tape
reads long, however, the distance it indicated was
longer than the distance it actually measured.
Therefore, the total standard error should be
subtracted, and the distance between the given
points should be finally recorded as 362.72 ft.
Suppose you are using this same tape to set
a point 362.73 ft away from another point. Again,
the total standard error is 0.01 ft. Because the tape
reads long, however, a measurement of 362.73 ft
by the tape will actually be LESS than 362.73 ft.
Therefore, the total correction for standard error
should be added, and you should measure off
362.74 ft by the tape.
CORRECTING FOR TEMPERATURE
VARIATION.— Take again a 100-ft steel tape
that has been calibrated at a standard temperature
of 68°F. The coefficient of thermal expansion of
steel is about 0.0000065 unit per 1°F. The steel
tape becomes longer when its temperature is
higher than the standard and shortens the same
amount when it’s colder. The general formula for
variation in temperature correction is as follows:
From the above formula, you can deduce that
the correction for a 100-ft tape is about 0.00065
ft per 1°F, which is about 0.01 ft for every 15°F
change in temperature above or below the
standard temperature of 68°F.
The temperature correction is applied in the
same manner and direction as the standard tape
error. If the tape measurement is taken at a higher
temperature than standard, the tape will expand
and will read short; naturally the correction
should be added.
The error caused by variation in temperature
is greatly reduced when an Invar tape is used.
CORRECTING FOR SAG.— Even under
standard tension, a tape supported or held only
at the ends will sag in the center, based on its
weight per unit length. This sag will cause the
recorded distance to be greater than the length
being measured. When the tape is supported at
its midpoint, the effect of sag in the two sections
is considerably less than when the tape is
supported only at its ends. As the number of
equally spaced intermediate supports is increased,
the distance between the end graduations will
approach the length of the tape when supported
throughout its length. The correction for the error
caused by the sag between the two supports for
any section can be determined by the following
equation:
For full tape-length measurements, the
correction for sag is usually taken care of by
having the tape calibrated. The tape must be
calibrated regardless of how it is supported and
under standard temperatures and tension. To
reduce the value of the horizontal correction for
sag, the Bureau of Standards suggests standard
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