Campbell Scientific 4WFBS1K 4WFBS120, 4WFBS350, 4WFBS1K 4 Wire Full Bridge Ter - Page 10

Quarter Bridge Strain

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4WFBS120, 4WFBS350, 4WFBS1K 4 Wire Full Bridge Terminal Input Modules (TIM) function used in CRBasic uses this raw output as its input to calculate µstrain. See Section 4.5 Calculation of Strain for ¼ Bridge Circuits for a detailed derivation of the equations used. 4. Quarter Bridge Strain A "quarter bridge strain circuit" is so named because an active strain gage is used as one of the four resistive elements that make up a full Wheatstone bridge. The other three arms of the bridge are composed of inactive elements. There are various circuits that use a single active element, including 2-Wire gauges, 3-Wire gauges, as well as a few circuits that utilize a dummy gauge for the arm opposite the arm holding the active gage instead of a resistor, RD in Figure 4.1.-1 (See Figures 4.3-1, 4.3-2, and 4.3-3). The 4WFBS TIM modules can support all types of these ¼ Bridge Strain circuits. 4.1 Quarter Bridge Strain with 3 Wire Strain Element A 3-wire quarter bridge strain circuit is shown in figure 4.1-1. Strain gages are available in nominal resistances of 120, 350, and 1000 ohms. The 4WFBSXXX model must match the nominal resistance of the gage when using the 3-Wire circuit (e.g., the 4WFBS120 is used with a 120 ohm strain gage). In Figure 4.1-1, R1 and R2 are 1000 ohm resistors making up the back plane of the Wheatstone bridge, as is done in the TIM design. RD, the third resistive element, is the complementary resistor that has a nominal resistance of the unstrained gage. The 4th resistive element is the active strain gage. R2=1 KΩ RD Excite V -+ L3 R1=1 KΩ L2 L1 R4 = Gauge FIGURE 4.1-1. Three wire quarter bridge strain circuit The 3-Wire gage alleviates many of the issues of the 2-Wire gage. As can be seen in Figure 4.1-1, lead wire L3 is in the arm of the Wheatstone bridge that has the completion resistor while lead wire L1 is in the arm that has the active gage. L2 is tied back to the input channel of the datalogger that has an input resistance greater than 1 Gohm, thus the current flow is negligible, negating effects of L2's resistance. This circuit nulls temperature induced resistance changes in the leads as well as reduces the sensitivity effect that the wires have on the gauge. See Section 4.4 for more on Lead resistance effects and methods to compensate for them. 4

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4WFBS120, 4WFBS350, 4WFBS1K
4 Wire Full Bridge Terminal Input Modules (TIM)
function used in CRBasic uses this raw output as its input to calculate μstrain.
See
Section 4.5
Calculation of Strain for ¼ Bridge Circuits
for a detailed
derivation of the equations used.
4.
Quarter Bridge Strain
A "quarter bridge strain circuit" is so named because an active strain gage is
used as one of the four resistive elements that make up a full Wheatstone
bridge. The other three arms of the bridge are composed of inactive elements.
There are various circuits that use a single active element, including 2-Wire
gauges, 3-Wire gauges, as well as a few circuits that utilize a dummy gauge for
the arm opposite the arm holding the active gage instead of a resistor, R
D
in
Figure 4.1.-1 (See Figures 4.3-1, 4.3-2, and 4.3-3). The 4WFBS TIM modules
can support all types of these ¼ Bridge Strain circuits.
4.1
Quarter Bridge Strain with 3 Wire Strain Element
A 3-wire quarter bridge strain circuit is shown in figure 4.1-1. Strain gages are
available in nominal resistances of 120, 350, and 1000 ohms.
The
4WFBSXXX model must match the nominal resistance of the gage when using
the 3-Wire circuit (e.g., the 4WFBS120 is used with a 120 ohm strain gage).
In Figure 4.1-1, R
1
and R
2
are 1000 ohm resistors making up the back plane of
the Wheatstone bridge, as is done in the TIM design. R
D
, the third resistive
element, is the complementary resistor that has a nominal resistance of the un-
strained gage. The 4
th
resistive element is the active strain gage.
Excite V
+
-
R
2
=1 K
Ω
R
1
=1 K
Ω
R
D
R
4
= Gauge
L
3
L
2
L
1
FIGURE 4.1-1.
Three wire quarter bridge strain circuit
The 3-Wire gage alleviates many of the issues of the 2-Wire gage. As can be
seen in Figure 4.1-1, lead wire L
3
is in the arm of the Wheatstone bridge that
has the completion resistor while lead wire L
1
is in the arm that has the active
gage.
L
2
is tied back to the input channel of the datalogger that has an input
resistance greater than 1 Gohm, thus the current flow is negligible, negating
effects of L
2
’s resistance.
This circuit nulls temperature induced resistance
changes in the leads as well as reduces the sensitivity effect that the wires have
on the gauge.
See Section 4.4 for more on Lead resistance effects and methods
to compensate for them.
4