tapes

Taapes come in a variety of lengths and materials . for engineering work the lengths are generally 10 m , 30 m , 50 m and 100 m .

Linen or glass fibre tapes may be used for general use , where precision is not a prime consideration the linen tapes are made from high quality linen , combined with metal fibres to increase their strength .

They are sometimes encased in plastic boxes with recessed handles . These tapes are often graduated in 5-mm intervals only .

 More precise versions of the above tapes are made of steel and graduated in millimeters .

For high-accuracy work , steel bands mounted in an open frame are used . They are standardized so that they measure their nominal length at a designated temperature usually 20ً C and at a designated applied  tension usually between 50 N to 80 N .

This information is clearly printed on the zero end of the tape .shows a sample of the equipment .

For the most precise work , invar tapes made from 35% nickel and 65 % steel are available . The singular advantage of such tapes is that they have a negligible coefficient of expansion compared with steel ,

and hence temperature variation are not critical , Their disadvantages are that the metal is soft and weak , whilst the price is more than ten times that of steel tapes . An alternative tape , called a lovar tape , is roughly , midway between steel and invar .

Much ancillary equipment is necessary in the actual taping 

process , e.g

(1) Ranging rods are made of wood or steel , 2 m long and 25 mm in diameter painted alternately red and white and have pointed metal shoes to allow them to be thrust into the ground . They are generally used to align a straight line between two points .

(2) Chaining arrows made from No. 12 steel wire are used to mark the tape lengths .

(3) Spring balances are generally used with roller-grips or tapeclamps to grip the tape firmly when the standard tension is applied .

As it is quite difficult to maintain the exact tension required with a spring balance , it may be replaced by a tension handle , which ensures the application of correct tension .

(4) Field thermometers are also necessary to record the tape temperature at the time of measurement , thereby permitting the computation of tape corrections when the temperature varies from standard these thermometers are metal cased and can be clipped onto the tape if necessary , or simply laid on the ground alongside the tape but must be shaded from the direct rays of the sun .

(5) Hand levels may be used to ensure that the tape is horizontal . This is basically a hand-held tube incorporation a spirit bubble to ensure a horizontal line of sight .Alternatively .an Abney level may be used to measure the slope of the ground .
(6) plumb-bobs may be necessary if stepped taping is used.

(7) Measuring plates are necessary in rough ground , to afford a mark against which the tape may be read. shows the tensioned tape being read against the edge of such a plate. The corners of the triangular plate are turned down to form grips .

when the plate is pressed into the earth and thereby prevent its movement .

In addition to the above , light oil and cleaning rags should always be available to clean and oil the tape after use .