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CORRECTING FOR TEMPERATURE
VARIATION. Take again a
100-ft steel tape that has been calibrated at a standard temperature of 68F. The coefficient of thermal expansion of
steel is about 0.0000065 unit per 1F. The steel tape becomes longer when its temperature is
higher than the standard and shortens the same amount when its 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 1F, which is about 0.01 ft for every 15F change in temperature above or below the standard temperature of 68F. 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 tensions for tapes supported at only the ends as follows: For 100-ft tapes, from 20 to 30 lb For 150-ft tapes, from 25 to 30 lb For 200-ft tapes, from 30 to 40 lb Generally, for a heavy 100-ft tape weighing about 3 lb that was standardized, whether supported throughout or at the ends only, the systematic error per tape length caused by sag is as follows: 10-lb tension = 0.37 ft 20-lb tension = 0.09 ft 30-lb tension = 0.04 ft For the Engineering Aids survey work, measurements are normally in the lower order of precision. The correction for sag varies with the cube of the unsupported length; for short spans, it is often negligible. |
 
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