Campbell Scientific 4WFBS120, 4WFBS350, 4WFBS1K 4 Wire Full Bridge Terminal Input Modules - Manual (Page 9)

4WFBS120, 4WFBS350, 4WFBS1K 4 Wire Full Bridge Terminal Input Modules (TIM)
gage factor of 2 means that if the length changes by one micrometer per meter
of length
( ) 1 με
, the resistance will change by two micro-ohms per ohm of
resistance. A more common method of portraying this equation is:
G
G
R GF
R
•
Δ
= ε
3.2
Or in terms of micro-strain:
( )
G
R GF •
= με
V
G
R Δ × 6 10 1
3.3
Because the actual change in resistance is small, a full Wheatstone bridge
configuration is used to give the maximum resolution. The Wheatstone bridge
can be set up with 1 active gage (Quarter bridge strain circuit), two active
gages (Half bridge strain circuit), or 4 active gages (Full bridge strain circuit).
For each of these Wheatstone bridge circuits there are multiple configurations.
The 4WFBS module provides three resistors that can be used for three of the
arms of the Wheatstone Bridge (Figure 4-1). There are two 1000 ohm
precision resistors for the back plane of the Wheatstone bridge, and a resistor
matching the strain gage's resistance for the bridge arm opposite the gage. The
inputs of the 4WFBS are configured so that this matching resistor can be
bypassed if it is desired to utilize a dummy gauge, or to use two active gauges
(Half Bridge Strain circuit).
For Full Bridge Strain circuits, as all four arms of the Wheatstone bridge are
active gages, there is no need for completion resistors, and thus a 4WFBS
module is not required.
The resistance of an installed gage will differ from the nominal value. In
addition, lead resistance imbalances can result in further unbalancing of the
bridge. A zero measurement can be made with the gage installed. This zero
measurement can be incorporated into the datalogger program such that
subsequent measurements can report strain relative to this zero basis point.
This removes the apparent strain resulting from the initial bridge imbalance.
Strain is calculated in terms of the result of the full bridge measurement. This
result is the measured bridge output voltage divided by the bridge excitation
voltage:
V
out ex
/ .
All of the various equations that are used to calculate strain use V
r
, the change
in the bridge measurement from the zero state:
Zero ex out Strained ex out r
V V V V V ) / ( ) / ( − =
3.4
( )
The result of the zero measurement,
Zero ex out
V V / ⋅ , can be stored and used in
the calculation of future strain measurements. Alternatively, the zero reading
value can be left at 0 (zero measurement is neither recorded nor used).
It should be noted the actual result of the full bridge instruction (BrFull) is the
millivolts output per volt of excitation (
1000 V V
out ex
/ ). The StrainCalc ⋅
3