Common Tab Strip Gages

062MD, 062MW and 125MW Pattern Gages

062MD , 062MW and 125MW pattern gages were designed for use as frequency detectors and strain-gradient shift detectors on structures such as turbine blades. When used to measure strain gradients, sizable errors can occur.

The 120-ohm resistance of each grid circuit is based on use of connections to the individual tab and to a point adjacent to the grid on the common tab. The common connecting tab has a resistance of about 3 ohms. As a result, if a single connection to the center of the tab is used, the outer grids will appear to have resistances of about 121.5 ohms. This will cause an apparent gage factor desensitization of more than 1% in these grids, resulting in an error in the indicated strains of about the same magnitude. The error will decrease to nearly zero for the center grids. A single connection to one end of the common tab will result in the gage factor error increasing to the opposite end, with the grid farthest from the connection having an error about twice that stated above. Correction of data is not usually practical since the common tab is strain sensitive to some degree. Instrument balance problems are obviously also a possibility.

An effective method of largely eliminating the strain measurement problems is to tin the entire common tab (being careful to protect the grids). Doing so should also eliminate most of the instrument balance difficulties. However, for best accuracy (i.e., better than 95%), common tab strip gages should not be used.

See also the use of 020MT and 020PF pattern gages .