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CORRECTING   FOR   TEMPERATURE Variation

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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 12-17



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