Electrical Resistance Strain Gages: Their Zero Shift When Bonded to Wood

The purpose of this paper is to discuss the common problem of zero shift of electrical resistance strain gages used on wood. This phenomenon refers to the change in resistance of a gage that occurs where there has been no change in the forces applied to the test sample. Gages mounted on various substrates displayed different magnitudes of drift than did unmounted gages. Therefore, the physical properties of the substrate are very important regarding the stability of the gages. Rigidity and thermal conductivity are two of the important properties. The moisture content of the substrate (wood) had a significant effect on the zero drift. The higher the moisture content, the greater was the negative drift of the gage. Different gage drifts were exhibited when gages mounted parallel and perpendicular to the grain were compared. The higher the energizing current, the greater was the drift in the gage. However, by adding a foil wrap to the gage, about a one-third reduction in drift occurred after a ten minute period. Desirable techniques for the use of bonded electrical resistance strain gages on wood are as follows: 1) Gages should be constructed to cause as little heat concentration as possible when the energizing current is passed through them. 2) A compensating gage should be used in the Wheatstone bridge circuit. 3) Low energizing currents should be used so that the amount and rate of warming drift will be kept to a minimum. 4) Wood with a low moisture content should be used in the testing.

You must be logged in to download any documents. Please login (login accounts are free) or learn how to Become a Member