Specifications

The Wagner Ground To increase measurement accuracy, the relay strain gage multiplexers
contain a Wagner Ground circuit similar to that shown in Figure 4-7. The
purpose of the Wagner Ground is to reduce stray leakage currents by
driving the midpoint (-E1) of the Wheatstone Bridge via the guard lead to
the same potential as the specimen. Because of this circuit, the strain gage
accessories are identified as having a "driven guard".
When the strain gage (Rg) is mounted to the specimen, there is generally
thousands of Megohms of isolation resistance between the gage and the
specimen. If a difference in potential exists between the specimen and the
midpoint of the bridge, stray leakage currents are generated (current =
voltage/resistance). These leakage currents in turn, introduce random
voltages along the measurement path which are then interpreted as strain
related signals. By keeping the potential on the specimen and the midpoint
of the bridge the same, leakage current is reduced.
In applications where the specimen is maintained at a specific potential by an
external source, the Wagner Ground will "float" (set) the midpoint of the bridge
to the same potential. For example, if 100V is applied to the specimen and the
bridge excitation voltage is 5.0V, the +E point on the Bridge is floated to
102.5V, -E2 is floated to 97.5V, and the midpoint (-E1) is maintained at 100V.
The Wagner Ground circuit is only in effect when the guard lead is connected
from the specimen to the G terminal on the bridge completion channel. The
guard lead will generally be the shield surrounding the conductors on a
twisted-shielded cable. Note that if several gages are mounted to the specimen,
the guard (shield) must be connected to each channel to maintain the Wagner
Ground. This is because the guard (G) terminal (on the relay multiplexers) is
switched to the multimeter in addition to the HI and LO bridge outputs (see
“The Chevron Bridge” on page 56).
Figure 4-7. The Wagner Ground
Chapter 4 Understanding the Strain Gage Multiplexers 57
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