A demountable mechanical strain gauge has advantages of
accuracy, reliability and cost over other methods of strain
measurement. However, the desired accuracy can only be obtained via
precision manufacture and care in the use of the instrument. 200
strains/hr can be measured at an accuracy of ±5 x 10^(-6) ɛ under most
laboratory test conditions. Great accuracy is achieved by using the
gauge in its ideal horizontal position with readings lying within a
small range on the dial. The DEMEC strain gauge uses a lever and
dial/digital indicating gauge arrangement mounted on an Invar Main Beam,
and is easy to use. The choice of material for the main beam and the
reference bar is due to the requirement for the metal with low
coefficient of thermal expansion. Care is required in mounting the
reference/locating discs onto the measuring surfaces. Should concrete
under temperature change during the testing, a concrete dummy bar is
required in order to eliminate the temperature effect on the instrument.