Your correction here will be applied in theopposite direction. Since the tape reads short, thelaid tape distance of 362.73 ft is LONGER than362.73 ft by the amount of the total correctionfor standard error (0.01 ft). Therefore, you mustSUBTRACT the total tape error. To lay off adistance of 362.73 ft with this tape, you wouldactually measure off a distance of 362.72 ft.Suppose now that the Bureau of Standardscalibration certificate states that when a tapeindicates 100.00 ft under standard conditions, itis actually measuring only 99.997 ft. Again, thestandard error is 0.003 ft per 100 ft, but this tape“reads long”; that is, the interval it indicatesis LONGER than the interval it is actuallymeasuring. Suppose you measure the distancebetween two given points with the tape and findthat the tape indicates 362.73 ft. The totalstandard error is again 0.01 ft. Because the tapereads long, however, the distance it indicated waslonger than the distance it actually measured.Therefore, the total standard error should besubtracted, and the distance between the givenpoints should be finally recorded as 362.72 ft.Suppose you are using this same tape to seta point 362.73 ft away from another point. Again,the total standard error is 0.01 ft. Because the tapereads long, however, a measurement of 362.73 ftby the tape will actually be LESS than 362.73 ft.Therefore, the total correction for standard errorshould be added, and you should measure off362.74 ft by the tape.CORRECTING FOR TEMPERATUREVARIATION.— Take again a 100-ft steel tapethat has been calibrated at a standard temperatureof 68°F. The coefficient of thermal expansion ofsteel is about 0.0000065 unit per 1°F. The steeltape becomes longer when its temperature ishigher than the standard and shortens the sameamount when it’s colder. The general formula forvariation in temperature correction is as follows:From the above formula, you can deduce thatthe correction for a 100-ft tape is about 0.00065ft per 1°F, which is about 0.01 ft for every 15°Fchange in temperature above or below thestandard temperature of 68°F.The temperature correction is applied in thesame manner and direction as the standard tapeerror. If the tape measurement is taken at a highertemperature than standard, the tape will expandand will read short; naturally the correctionshould be added.The error caused by variation in temperatureis greatly reduced when an Invar tape is used.CORRECTING FOR SAG.— Even understandard tension, a tape supported or held onlyat the ends will sag in the center, based on itsweight per unit length. This sag will cause therecorded distance to be greater than the lengthbeing measured. When the tape is supported atits midpoint, the effect of sag in the two sectionsis considerably less than when the tape issupported only at its ends. As the number ofequally spaced intermediate supports is increased,the distance between the end graduations willapproach the length of the tape when supportedthroughout its length. The correction for the errorcaused by the sag between the two supports forany section can be determined by the followingequation:For full tape-length measurements, thecorrection for sag is usually taken care of byhaving the tape calibrated. The tape must becalibrated regardless of how it is supported andunder standard temperatures and tension. Toreduce the value of the horizontal correction forsag, the Bureau of Standards suggests standard12-17

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