1Application
Model VWR Vibrating Wire Rebar Meter is applicable to be embedded in hydraulic or other concrete structures to measure the inner rebar strain. Meanwhile, the temperature value of the embedding point can be measured. By adding accessories, it can be constituted to be anchor dynamometer, baserock strain gauge and so on instruments that measure the strain. This rebar meter has the intelligent identification function.
2Technical Specifications

R20

R25

R28

R32

R36

R40

Size

Rebar Diameter d (mm)

20

25

28

32

36

40

Cross Sectional Area
of steel tube A (cm^{2})

3.14

4.90

6.30

8.05

10.2

12.6

Length of steel tube L (mm)

120

Perfor
mance

Range

Stretching (MPa)

300

Compressing(MPa)

200

Sensitivity k (MPa/F)

≤0.10

Accuracy (F.S)

±0.1%

Temp.^{ 1} Measure.^{ 2} Range (℃)

40～+150

Temp. Measure.^{ }Accuracy(℃)

±0.5

Water Pressure Resistance (MPa)

≥1

Insulation Resistance (MΩ)

≥50

Remark: Frequency Modulus F= Hz^{2}×10^{3}
^{1}: Temperature
^{2}: Measurement
3The Theory of Operating
3.1Constitution
Model VWR Vibrating Wire Rebar Meter consists of strain transducer, connecting steel covers (standard parts), signal transmission cable, etc.
3.2Mechanism
The rebar meter suffers the stretching or compressing when the inner rebar of the measured structure has the changing of the strain. Then the steel cover will be deformed synchronously. The deformation will let the rebar meter sense the stretching or compressing deformation. And then this deformation is transferred to the vibrating wire to change to the changing of the strain. Thus the vibration frequency of the vibrating wire will be changed. The electromagnetic coils excite the vibrating wire and measure the vibration frequency. The frequency signal is transferred to the readout device via cable. As the result, the suffered strain of the rebar inside the measured structure will be obtained. Meanwhile, the temperature value of the embedding point can be measured.
3.3Calculation
a)The strain value σ has a linear relationship with the output frequency modulus △F as the gauge is bearing the axial deformation under environmental temperature as constant:
ε= k△F
△F = F  F_{0}
Herewith,
k: Sensitivity with the unit of 10^{6}/F;
△F: Difference between the measured realtime value and the reference one with the unit of F;
F: Realtime measured value with the unit of F;
F_{0}: Reference value with the unit of F.
b)When the gauge is not affected by external force, there is an output value △F´ if the temperature is increased by △T. This output is only caused by the changing of the temperature, thus it should be deducted in calculation.
Experiment shows that △F´ and △T has the following linear relationship:
σ´= k△F´+ b△T = 0
k△F´= b△T
△T = T  T_{0}
Herewith,
b: Temperature correction coefficient with the unit of MPa/℃;
△T: Difference between the measured realtime value and the reference one with the unit of ℃;
T: Realtime measured temeprature value with the unit of ℃;
T_{0}: Reference temperature value with the unit of ℃;
c)The gauge setteled in the hydraulic or other concrete structures is subject to the effects of deformation and temperature. Thus, the general calculation formular is:
σ_{m }= k△F + b′△T = k (F  F_{0})+ b(T  T_{0})
Herewith,
σ_{m}: Strain value of the measured structure with the unit of MPa;
Remark:
The Rebar Meter has the automatic temperature compensation. Experiment shows that the temperature correction coefficient is smaller than the minimal reading value, and the calculation formula a) is applicable for general situation.