CGEO-SG5
Rebar VW Strain Gauge 9
CGEO INTERNATIONAL LIMITED
Installation Manual
4. DATA REDUCTION
4.1. Strain Calculation
The basic units utilized by Soil for measurement and reduction of data from Vibrating Wire
Rebar Strain Gauges are "digits". Calculation of digits is based on the following equation;
Digits = Hz²
/1000
Equation 1 - Digits Calculation
To convert digits to strain the following equation applies;
F= (R
1
- R
0
) G
Equation 2 – Strain Caculation
Where:
R
0
is the initial reading in digits, usually obtained at installation or at the
commencement of a test.
R
1
is the current reading in digits.
G is the calibration factor from the supplied calibration sheet (see Figure 6).
For example, assume an initial reading, R
0
, of 8000 digits, a current reading, R
1
, of 7700, and a
calibration factor, G, of 0.718439 microstrain per digit.
F= (7700 - 8000)* 0.718439 = - 215.5217 (compression)
Symbol rule:
“
-
”：
compression, “+”: tension
4.2. Temperature Correction
Rebar Strain Gauges are usually embedded in concrete and strained by the concrete, the
assumption being that
the strain in the meter is equal to the strain in the concrete
. When the
temperature changes, the concrete expands and contracts at a rate slightly less than the rate of the
steel of the vibrating wire. The coefficients of expansion are:
Steel (K
steel
):
12.2 ppm/°C
6.7 ppm/°F
Concrete (K
concrete
):
10 ppm/°C
5.5 ppm/°F
Difference (K):
2.2 ppm/°C
1.2 ppm/°F
Table 1 - Thermal Coefficients
Hence a correction is required to the apparent strains equal to the difference of these two
coefficients. See Equation 3.
F= G (R
1
- R
0
)+ K (T
1
- T
0
)
Equation 3 – Load Related Strain
Where:
T
0
is the initial temperature recorded at the time of installation.
T
1
is the current temperature.
