Campbell Scientific 4WFB120, 4WFB350, 4WFB1K User Manual
4WFB120, 4WFB350, 4WFB1K
4 Wire Full Bridge Terminal
Input Modules
Revision: 5/07
Copyright © 1996-2007
Campbell Scientific, Inc.
Warranty and Assistance
The 4WFB120, 4WFB350, 4WFB1K 4 WIRE FULL BRIDGE
TERMINAL INPUT MODULES are warranted by CAMPBELL
SCIENTIFIC, INC. to be free from defects in materials and workmanship
under normal use and service for twelve (12) months from date of shipment
unless specified otherwise. Batteries have no warranty. CAMPBELL
SCIENTIFIC, INC.'s obligation under this warranty is limited to repairing or
replacing (at CAMPBELL SCIENTIFIC, INC.'s option) defective products.
The customer shall assume all costs of removing, reinstalling, and shipping
defective products to CAMPBELL SCIENTIFIC, INC. CAMPBELL
SCIENTIFIC, INC. will return such products by surface carrier prepaid. This
warranty shall not apply to any CAMPBELL SCIENTIFIC, INC. products
which have been subjected to modification, misuse, neglect, accidents of
nature, or shipping damage. This warranty is in lieu of all other warranties,
expressed or implied, including warranties of merchantability or fitness for a
particular purpose. CAMPBELL SCIENTIFIC, INC. is not liable for special,
indirect, incidental, or consequential damages.
Products may not be returned without prior authorization. The following
act information is for US and International customers residing in countries
cont
served by Campbell Scientific, Inc. directly. Affiliate companies handle
repairs for customers within their territories. Please visit
www.campbellsci.com to determine which Campbell Scientific company
serves your country. To obtain a Returned Materials Authorization (RMA),
contact CAMPBELL SCIENTIFIC, INC., phone (435) 753-2342. After an
applications engineer determines the nature of the problem, an RMA number
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container. CAMPBELL SCIENTIFIC's shipping address is:
CAMPBELL SCIENTIFIC, INC.
RMA#_____
815 W
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est 1800 North
ah 84321-1784
4WFB120, 4WFB350, 4WFB1K
Table of Contents
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1. Function........................................................................1
2. Specifications ..............................................................1
3. Measurement Concepts..............................................2
4. Wiring............................................................................3
5. Program Examples......................................................3
5.1 Edlog.........................................................................................................4
5.1.1 CR10(X)..........................................................................................4
5.1.2 21X .................................................................................................7
5.1.3 CR7...............................................................................................10
5.2 CRBasic..................................................................................................13
5.2.1 CR9000(X)....................................................................................14
6. Calculation of Strain..................................................15
Figures
1-1 Terminal Input Module ............................................................................1
2-1 Schematic.................................................................................................2
4-1 Wiring for Example Programs .................................................................3
6-1 Strain Gage in Full Bridge......................................................................15
Table
5-1 Input Locations Used in CR10(X), 21X, and CR7 Examples..................4
i
This is a blank page.
4WFB120, 4WFB350, 4WFB1K 4 Wire Full Bridge Terminal Input Modules
1. Function
Terminal input modules connect directly to the datalogger's input terminals to
provide completion resistors for resistive bridge measurements, voltage
dividers, and precision current shunts. The 4WFB120, 4WFB350, and
4WFB1K complete a full bridge for a strain gage or other sensor that acts as a
single variable resistor. The difference between the three models is in the
resistor that matches the nominal resistance of a 120 ohm, 350 ohm, or 1000
ohm quarter bridge strain gage.
H
L
G
H
L
AG
2. Specifications
2:1 Resistive Divider
Resistors
Ratio Tolerance @ 25 °C
Ratio Temperature
coefficient
Power rating 0.25 W
Completion Resistor: 120, 350, or 1000 Ω
Tolerance @ 25 °C
Temperature coefficient
0-60 °C
-55-125 °C
Power rating 0.25 W
H
L
AG
FIGURE 1-1. Terminal Input Module
1 kΩ/1 kΩ
±0.02%
2 ppm/°C
±0.01%
4 ppm/°C
8 ppm/°C
1
4WFB120, 4WFB350, 4WFB1K 4 Wire Full Bridge Terminal Input Modules
ε
µ
=
∆
∆
µ
x
⋅
x
Vx
H
L
or AG
3. Measurement Concepts
Measuring strain is measuring a change in length. Specifically, the unit strain
()
is the change in length divided by the unstrained length
Strain is typically reported in microstrain
length by one millionth of the length.
A metal foil strain gage is a resistive element that changes resistance as it is
stretched or compressed. The strain gage is bonded to the object in which
strain is measured. The gage factor,
resistance for change in strain:
factor of 2 means that if the length changes by one micrometer per meter of
)
(1
ε
length
resistance.
, the resistance will change by two micro-ohms per ohm of
H
1kΩ
1kΩ
FIGURE 2-1. Schematic
H
L
G
()
GF , is the ratio of the relative change in
GF R R l l
120Ω, 350Ω, or 1kΩ
ε
=∆ll/
()
ε
; a microstrain is a change in
//
. For example, a gage
.
Because the actual change in resistance is so small, a full bridge configuration
is used to give the maximum resolution. A "quarter bridge" strain gage is so
named because the strain gage becomes one of the four resistors that make up a
full bridge. The 4WFBxxx module provides the other three resistors (Figure 4-
1). Quarter bridge strain gages are available in nominal unstrained resistances
of 120, 350, and 1000 ohms. The 4WFB model must match the resistance of
the gage (e.g., the 4WFB120 is used with a 120 ohm strain gage).
The resistance of an installed gage will differ from the nominal value. A zero
measurement can be made with the gage installed. This zero measurement can
be incorporated into the datalogger program; subsequent measurements can
report strain relative to the zero.
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
VV
/
voltage
millivolts output per volt of excitation,
measurement,
measurements. Strain is calculated from the change in the bridge
measurement,
outex
. (The actual result of the full bridge instruction is the
1000
VV
/
is stored and used to calculate future strain
0
out e
1000⋅VV
/
out e
) The result of the zero
2
r
x
x
=
−
4. Wiring
4WFB120, 4WFB350, 4WFB1K 4 Wire Full Bridge Terminal Input Modules
VVV VV
(/)( /0)
out e
ε
=
GF V
out e
V
r
−412
()
r
: 3.1.
3.2.
The calculations are covered in more detail in section 6.
Datalogger
Vx
Figure 4-1 illustrates the wiring of the strain gage to the 4WFB module and the
wiring of the module to the datalogger. It is important that the gage be wired
as shown with the wire from H connected at the gage, and that the leads to the
L and G terminals be the same length, diameter, and wire type. With this
configuration, changes in wire resistance due to temperature occur equally in
both arms of the bridge with negligible effect on the output from the bridge.
5. Program Examples
The following examples for the CR10(X), 21X, CR7, and CR9000(X) all have
a subroutine that measures the unstrained "zero" output of the strain gage. The
examples calculate strain using equation 3.2 for a strain gage with a GF=2.
These are just examples. Besides adding additional measurement instructions,
the programs will probably need to have the scan and d ata storage intervals
altered for actual applications. The instructions in the subroutine will also
need to be modified for the actual gage factor.
H
H
L
G
or AG
or G
H
L
Shield
FIGURE 4-1. Wiring for Example Programs
This zeroing subroutine is called automatically when the progr am is first
executed. The user can call the subroutine by setting Flag 1 low using the
datalogger support software or the *6 mode with the keyboard display. The
"zero" reading is then used during normal measurements for the strain
calculations.
3
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