Model VWM Vibrating Wire Multiple Point Extensometer

1Application

Model VWM Vibrating Wire Multiple Point Extensometer is applicable to be embedded in hydraulic structures or other structures like soil dams, soil embankment, side slopes and so on. At the same time the extensometer is also able to measure the temperature value of the embedding point. The displacement meter is equipped with Model VWD Vibrating Wire Displacement Transducer and accessories. 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 the temperature correction is not needed in use. And, this Displacement Meter has the intelligent identification function.

2Technical Specifications

Remark: Frequency Modulus F= Hz²??10^-3

3Theory of Operation

3.1 Constitution

Model VWM Vibrating Wire Multiple Point Extensometer consists of displacement transducer, stainless steel measurement rod, PVC protecting tube, installation support base, anchor, protecting shield and signal transmission cable, etc.

3.2 Mechanism

The measurement rod via the anchor moves when there is displacement and deformation of the measured structure. Then the pull rod of the displacement meter will be moved by the measurement rod so that the displacement and deformation is generated. The displacement and deformation of the pull rod will be transferred to the vibrating wire to change to the changing of the strain. As a result, the vibration frequency of the vibrating wire is changed. The electromagnetic coils excite the vibrating wire and measure the vibration frequency. The frequency signal is transmitted to the readout device via cable. Thus the deformation value of the measured structure can be obtained. Meanwhile, the temperature value of the embedding point can be measured at the same time.

3.3 Calculation

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

L = K??F

??F = F- F₀

Herewith,

k: Sensitivity with the unit of mm/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.

  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:

                       L´= k??F´+ (b - h??)??T = 0

                    k??F´= - (b - h??)??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 temeprature value with the unit of ??;

T₀: Reference temperature value with the unit of ??;

H: Rod Length with the unit of mm;

??: Liner expansion coefficient of the rod with unit of 10^-6/??;

Remark:

The material of the rod is stainless steel, generally the expansion coefficient is taken as 16.5??10^-6/??.

  c)The gauge 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- h??)??T = k (F - F₀) + (b - h??)(T - T₀)

Herewith,

L_m: Displacement 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 this calculation formula can be used:

              L_m = k??F - h????T = k (F - F₀) - h??(T -T₀)