1 Application
Model VWD-J Vibrating Wire Crackmeter is located inside or on the surface of the concrete structures or structures of other material to measure the extension of the crack or the deformation of the surrounding crack. At the same time the temperature value of the embedding point can be measured. The Vibrating Wire Crackmeter has the intelligent identification function.
2 Technical Specifications
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VWD-20J
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VWD-50J
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VWD-100J
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Measurement Range (mm)
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0～20
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0～50
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0～100
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Sensitivity k (mm/F)
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≤0.01
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≤0.02
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≤0.04
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Accuracy (F.S)
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±0.1%
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±0.1%
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±0.1%
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Temperature Range (℃)
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-40～+150
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-40～+150
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-40～+150
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Temperature Accuracy (℃)
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±0.5
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±0.5
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±0.5
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Outer Diameter (mm)
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30.5
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30.5
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30.5
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Length (mm)
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300
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340
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400
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Water Pressure Resistance (MPa)
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≥1
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≥1
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≥1
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Insulation Resistance (MΩ)
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≥50
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≥50
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≥50
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Remark: Frequency Modulus F= Hz2×10-3
3 Theory of Operation
3.1 Constitution
The model VWD-J Vibrating Wire Crackmeter consists of the front and back stands, protecting tube, cables, and vibrating and excited magnetic coils.
3.2 Mechanism
The variation of this crackmeter will be brought as there is deformation from the measured structure. Thus, this variation is transferred to the vibrating wire via the front and back stands in order to generate the strain variation. Thereby, the oscillating frequency of the vibrating wire is changed as the consequence. The electromagnetic coils stimulate the vibrating wire and measure the oscillating frequency. As the result, the frequency signal is transmitted via the cable to the readout device and the deformation value of the measured structure is thereby obtained.
3.3 Calculation
a)The deformation value J has a linear relationship with the output frequency modulus △F as the meter is bearing the axial deformation under the outside environmental temperature as constant:
J = k△F
△F = F - F0
Herewith,
k: Sensitivity with the unit of mm/F;
△F: Difference between the real-time measured value and the reference one with the unit of F;
F: Real-time measured value with the unit of F;
F0: Reference value with the unit of F.
b)When the Crackmeter is not affected by external force (gauge length between both ends is unchanged), there is an output value △F´ if the temperature is increased by △T. This output is only casued by the changing of the temperature, thus it should be deducted in calculation.
Experiment shows that △F´ and △T has the following linear relationship:
J´= k△F´+ b△T = 0
k△F´= -b△T
△T = T - T0
Herewith,
b: Temperature correction coefficient with the unit of mm/℃;
△T: Difference between the measured real-time value and the reference one with the unit of ℃;
T: Real-time measured temperature value with the unit of ℃;
T0: Reference temperature value with the unit of F;
c)The Crackmeter settled inside or on the surface of concrete or other material structures is subject to the effects of deformation and temperature. The general calculation formula is:
Jm = k△F + b△T = k (F - F0) + b (T - T0)
Herewith,
Jm: The deformation value of the measured structure with the unit of mm;
Remark:
The Crackmeter 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.
