Geokon 4911, Installation Manual, Page: 24 / 27

18
APPENDIX C. DERIVING THE CALIBRATION FACTOR (C) FROM THE
TEST DATA
Geokon Rebar Strain Meters are calibrated by loading them in a testing machine; hence, the
gauge factor (C) must be determined after converting loads to strains. This is done as follows:
The central section of the rebar strain meter, (the unbonded length) which is 7.5" or 19.05 cm
long, contains a vibrating wire sensor located axially at the midsection. See Figure 9.
Figure 9 - Rebar Strain Meter Schematic
To convert the observed change in readout digits (
∆ R) into a strain (
ε
t) for the entire length of
the unbonded section (7.5 in./19.05 cm), requires the following equation:
ε
t
=
(
ε
1
×
L
1
)+(
ε
2
×
L
2
)+(
ε
3
×
L
3
)
L
1
+L
2
+L
3
Equation 7 - Total Strain Calculation
Where;
L
1
is 2.000" (5.08 cm).
L
2
is 5.000" (12.7 cm).
L
3
is 0.500" (1.27 cm).
ε
t is the total strain of the unbonded section.
ε
1
is the strain in zone one, determined empirically from the equation for the vibrating wire
sensor, i.e.,
ε
1
= ∆ R × 0.359 × 10-6 where ∆ R is the change in readout digits.
ε
2
,
ε
3
are the strains in zones two and three, respectively, dependent on the load and cross-
sectional area, see Equation 8 and Equation 9.