1Application

Model VWS Vibrating Wire Strain Gauge is applicable to be embedded in hydraulic and concrete geotechnical structures to measure the inner strain value of the structure. Meanwhile, the temperature of the embedding point can be measured. With adding accessories, it can be constituted to be the Multiple Direction Strain Gauges Set, Non-Stressmeter, Rock Strain Gauge and so on instruments that measure the strain. The strain gauge with a larger modulus of elasticity is mainly used for the continuously cementing of the concrete from high position, e.g. underground diaphragm wall, impervious wall, cast-in-place pile, etc engineering situations. And this Strain Gauge has the intelligent identification function.

2Technical Specifications

			VWS-10	VWS-15	VWS-10M	VWS-15M
?? Size Parameters	Gauge Length L (mm)		100	150	100	150
	Valid Diameter d (mm)		22	22	22	22
	End Diameter D (mm)		33	33	33	33
??	Range	Stretching (10-6)	1500	1500	1500	1500
Perfor- mance		Compressing (10-6)	1500	1500	1500	1500
	Sensitivity k (10-6/F)		??0.5	??0.5	??0.5	??0.5
	Accuracy (F.S)		??0.1%	??0.1%	??0.1%	??0.1%
	Temp. 1 Measure. 2 Range (??)		-40??+150	-40??+150	-40??+150	-40??+150
	Temp. Measure. Accuracy(??)		??0.5	??0.5	??0.5	??0.5
	Temp. Correct. 3 Co.4 b (10-6/??)		??13.5	??13.5	??13.5	??13.5
	modulus of elasticity Eg (MPa)		300??800	300??800	800??1500	800??1500
	Water Pressure Resistance (MPa)		??1	??1	??2	??2
??	Insulation Resistance (M??)		??50	??50	??50	??50

Remark: Frequency Modulus F= Hz²??10^-3
1: Temperature
²: Measurement

³: Correction

⁴: Coefficient

 

3The Theory of Operating

3.1 Constitution

Model VWS VW Strain Gauge consists of front and rear stands, stainless steel protecting shield, signal transmission cable, vibrating wire and excitation coils.

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 electro-magnetic 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 inner strain value can be measured. Meanwhile, the temperature value of the embedding point can be measured at the same time.

3.3 Calculation

The strain value ?? has a linear relationship with the output frequency modulus ??F as the gauge is bearing the axial deformation under environmental temperature as constant:

                             ??= k??F

                            ??F = F - F₀

Herewith,

k: Sensitivity with the unit of 10^-6/F;

??F: Difference between the measured real-time 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.

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:

                         ??´= k??F´+ b??T = 0

                    k??F´= -b??T

                       ??T = T - T₀

Herewith,

b: Temperature correction coefficient with the unit of 10^-6/??;

??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 ??;

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:

                ??_m = k??F + b????T = k (F - F₀)+ (b -??)(T - T₀)

Herewith,

??_m: Strain value of the measured structure with the unit of 10^-6;

??: Liner expansion coefficient of the measured structure with the unit of 10^-6/??.