geokon’s 4100/4150 Series Vibrating Wire Strain Gauges are intended primarily for measuring strain on structural steel members, such as tunnel linings, arches, struts, piles, sheet piling, etc.

■Model 4100 consists of a vibrating wire gage element and removable coil assembly.

■Model 4150 consists of a vibrating wire gage element and integral coil assembly. (Also available is Model 4150-5, which has a range of 10,000 microstrains. See Appendix G for more information.)

■Model 4151 is a modification of the 4150 strain gauge, in which the spot-weldable tabs have been replaced by pins welded to the end blocks. These are designed to be grouted into holes drilled into the material under test.

Versions of Model 4151 are available with extended ranges. These gauges are useful for measurements in high strain regimes such as on plastic pipes or piles and on fiberglass structural members and rebars. Their small size makes them particularly suitable where space and or access is limited.

The primary means of attaching the Model 4100/4150 strain gauges to steel members is spot-welding, but they may also be epoxy bonded to the surface (see Section 4.3).

Strain is measured using the vibrating wire principle. A length of steel wire is tensioned between two end points that are welded to the steel surface being studied. Deformations of the surface will cause the two end points to move in relation to each other, altering the tension in the steel wire. This change in tension is measured as a change in the wire’s resonant frequency of vibration.

Two coils, one with a magnet insert and the other with a pole piece insert, are located close to the vibrating wire.  Applying a pulse of varying frequency (swept frequency) to the coils causes the wire to vibrate primarily at its resonant frequency.

Portable readouts and dataloggers are available from geokon. These devices, when used with vibrating wire strain gauges, provide the necessary voltage pulses to pluck the wire. During vibration, a sinusoidal signal is induced in the coils and transmitted to the readout box where it is conditioned and displayed.

This manual contains installation instructions, readout instructions, recommended maintenance, and troubleshooting procedures. The theory of the gauge is also provided, along with some suggestions for data interpretation.