Spider DSA User’s Manual
213
That is, the channel’s sensitivity is GF V
in
/4 volt-per-strain (μV-per-microstrain).
For most practical studies the very slight non-linearity is not a problem. However,
the resistance of the wires leading to the gage can present a problem. Note that
the resistance of both leads actually add to the gage resistance. Further, resistance
changes due to temperature change can be a significant problem. Both the gage
and its connecting wires can increase in resistivity as temperature increases. For
brief dynamic measurements, neither of these effects is a problem. Just re-zero
the bridge before every measurement to eliminate this “DC shift”.
In situations where long-term observation of the strain is required, the Quarter
Bridge can be improved by moving another resistor to the remote location as
shown in the following figure. This configuration is called a “Type 2” Quarter
Bridge, though its connections with the Spider are identical with those of the Half
Bridge subsequently discussed. The Type 2 Quarter Bridge is not unbalanced
by thermal effects on the wires, since wire resistance now adds equally to the gage
and the “dummy” fixed resistor. (The resistance of the wire leading to the IN-
terminal is without consequence because the IN+ and IN- inputs are of very high
impedance. Almost no current flows through this wire, hence no voltage drop
error.) However, unless the thermal properties of the dummy exactly match those
of the gage, an unbalance with temperature can still occur. For this reason, the
Half Bridge (type 1) is strongly preferred to cancel temperature effects.
Figure xx8: Quarter Bridge (Type 2) connections
To Next Page
Loading...