1Application
Model VWSW Vibrating Wire NonStressmeter is applicable to be embedded in hydraulic and concrete geotechnical structures to measure the inner strain value caused by the selfchanging of the concrete volume of the structure. Meanwhile, the temperature of the embedding point can be measured. The NonStressmeter consists of strain gauge and nonstress bucket. And this Strain Gauge has the intelligent identification function.
2Technical Specifications

VWS10W

VWS15W

Size Parameters

Gauge Length L (mm)

100

150

Valid Diameter d (mm)

22

22

End Diameter D (mm)

33

33


Range

Stretching (10^{6})^{}

1500

1500

Performance

Compressing (10^{6})

1500

1500

Sensitivity k (10^{6}/F)

≤0.5

≤0.5

Accuracy (F.S)

±0.1%

±0.1%

Temp.^{ 1} Measure.^{ 2} Range (℃)

40～+150

40～+150

Temp. Measure.^{ }Accuracy(℃)

±0.5

±0.5

Temp. Correct.^{ 3 }Co.^{4} b (10^{6}/℃)

≈13.5

≈13.5

modulus of elasticity Eg (MPa)

300～800

300～800

Water Pressure Resistance (MPa)

≥1

≥1


Insulation Resistance (MΩ)

≥50

≥50

Remark: Frequency Modulus F= Hz^{2}×10^{3}
^{1}: Temperature
^{2}: Measurement
^{3}: Correction
^{4}: Coefficient
3The Theory of Operating
3.1 Constitution
The NonStressmeter consists of strain gauge and nonstress bucket. The bucket is made of two iron sheets and the interlayer is about 8mm10mm. The bucket is with a circular or cone shape. This bucket is used for isolate the concrete from the strain caused by outside load, but this can measure the strain caused by the selfchanging of the concrete volume.
The dimension parameters are:
Small opening: 110 mm;
Larger opening: 220 mm;
Height: 320 mm.
3.2 Mechanism
When the strain is changed from the inner part of the geotechnical structure then the gauge will sense the deformation synchronously. The deformation transfers to the vibrating wire via the front and rear stands and turns out to be the changing of the strain, thus, the vibration frequency of the vibrating wire is changed. Further, the electromagnetic coils excite the vibrating wire and measure the vibration frequency and the frequency signal is transmitted to the readout device via the cable. As the result, the strain value caused by the selfchanging of the concrete volume of the structure can be measured. Meanwhile, the temperature value of the embedding point can be measured at the same time.
3.3 Calculation
a)The strain value ε 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 (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:
ε´= k△F´+ b△T = 0
k△F´= b△T
△T = T  T_{0}
Herewith,
b: Temperature correction coefficient with the unit of 10^{6}/℃;
△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 gauge settled in the hydraulic or other concrete structures is subject to the effects of deformation and temperature. Then the temperature correction coefficient should be the temperature correction coefficient of the gauge subtracts from the liner expansion coefficient. Thus, the general calculation formula is:
ε_{m }= k△F + b′△T = k (F  F_{0})+ (b α)(T  T_{0})
Herewith,
ε_{m}: Strain value of the measured structure with the unit of 10^{6};
α: Liner expansion coefficient of the measured structure with the unit of 10^{6}/℃. 