1 Application
Model VWD Vibrating Wire Displacement Meter is applicable to measure the extension/deformation of hydraulic structures or other concrete structures. It also can be used to measure the displacement, settlement, strain, sliding of structures such as the soil dams, soil embankment, side slopes and so on. Meanwhile, it also measures the temperature of the embedding point synchronously. By adding accessories, it can be formed to be the Baserock Displacement Meter, Multiple point Extensometer, Earth Strain Gauge and so on instruments which measure the deformation. And this gauge has the intelligent identification function.
2 Technical Specifications

VWD20

VWD50

VWD100

Measurement Range (mm)

0～20

0～50

0～100

Sensitivity k (mm/F)

≤0.01

≤0.02

≤0.04

Accuracy (F.S)

±0.1%

±0.1%

±0.1%

Temperature Range (℃)

40～+150

40～+150

40～+150

Temperature Accuracy (℃)

±0.5

±0.5

±0.5

Outer Diameter (mm)

30.5

30.5

30.5

Length (mm)

300

340

400

Water Pressure Resistance (MPa)

≥1

≥1

≥1

Insulation Resistance (MΩ)

≥50

≥50

≥50

Remark: Frequency Modulus F= Hz^{2}×10^{3}
3 Theory of Operation
3.1 Constitution
The model VWD Displacement Meter consists of Cardan joints, stainless steel protecting tube, twolevel mechanical negative amplification mechanism, signal transmission cable, vibrating wire and excited electromagnetic coils,
3.2 Mechanism
The deformation of the measured structure will cause the displacing of the displacement meter. Then it is transferred to the twolevel mechanical negative amplification mechanism. The negative amplified displacement value is transferred to vibrating wire and it turns to be the changing of the strain. Thus the vibration frequency is changed. And the electromagnetic coils excite the vibrating wire and measure the vibration frequency. Afterwards, the frequency signal is transferred to the readout device via cable. As the result, the displacement value of the measured structure is thereby obtained. Meanwhile, temperature of the embedding point can also be measured synchronously.
3.3 Calculation
a) The displacement value L has the following linear relationship with the output frequency modulus △F when the displacement meter is only bearing the axial deformation under the outside environmental temperature as constant.
L = k△F
△F = F  F_{0}
Herewith,
k: Sensitivity with the unit of mm/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 displacement meter 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 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:
L´= k△F´+ b△T = 0
k△F´= b△T
△T = T  T_{0}
Herewith,
b: Temperature correction coefficient with the unit of mm/℃;
△T: Difference between the measured realtime value and the reference one with the unit of ℃;
T: Realtime measured temperature value with the unit of ℃;
T_{0}: Reference temperature value with the unit of ℃;
c)The displacement meter settled in the hydraulic or other concrete structures is subject to the effects of deformation and temperature. Thus, the general calculation formula is:
L_{m }= k△F + b△T = k (F  F_{0}) + b(T  T_{0})
Herewith,
L_{m}: Deformation value of the measured structure with the unit of mm;
Remark:
The material linear expansion coefficient of the sensible measurement component is close to the fixed machine framework. Experiment shows the temperature correction coefficient is very small and thus generally the calculation formula a) can be used. 