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1. INTRODUCTION
The Model 3900 Embedment Strain Gauge is designed for the measurement of dynamic strains
in concrete structures and soils. It comprises a full-bridge strain gauge proving ring element
coupled in series with a tension spring which is stretched between two end flanges. An outer
PVC tube sealed with O-rings provides a waterproof housing. The end flanges are embedded and
move in accordance with the surrounding material. The voltage signals from the strain gauge are
transmitted via cable to the readout location.
The strain gauge can be read out by means of the Geokon Model 502 Readout Box or by a
applying a 2 to 12 Volt excitation to the input leads and reading the corresponding millivolt
output. The output at full range is approximately 3 mV/V. The standard wiring for use with the
GK-502 readout box is configured to compensate for the use of long cables by the remote
sensing of the input voltage at the sensor. A Thermistor is included inside the sensor to measure
temperatures. This requires a cable with four shielded pairs of conductors.
2. INSTALLATION
The embedment gauge is delivered with the sensor set at an approximately midrange position
ready for installment. The standard range is ± 2500 microstrains. (Other ranges are available).
An initial test can be conducted by connecting the strain gauge to the GK-502 Readout Box or to
a regulated voltage supply. Movement of the end flanges should produce a corresponding change
in the gauge output. At no time should the end flanges be twisted or pulled beyond the range of
the sensor as this could permanently damage the gauge.
The gauge should be installed directly in the concrete or soil by hand. Large aggregate should be
removed from the area immediately surrounding the sensor. The use of vibrators immediately
next to the gauge should be avoided. The standard cable has a thick PVC jacket and can be
placed directly in the concrete.
3. TAKING READINGS
Connect the ten-pin connector to the GK-502 readout box or connect the bare wires to a voltage
supply and millivoltmeter as follows:
Bendix Pin Circuit Label
A
B
C
D
E
F
G
H
J
K
Description
S-
Bridge Output -
P+
Bridge Excitation +
P-
Bridge Excitation -
S+
Bridge Output +
NC
No Connection
G
Ground for shield
T
Thermistor
T
Thermistor
RS+
Remote Sense +
RS-
Remote Sense -
Table 1 - Input Connections
Geokon Purple Cable
White's Black
Red
Red's Black
White
NC
Shield
Blue
Blue's Black
Green
Green's Black
1
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