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Applied Measurements has developed a low-cost device to rapidly and accurately measure tension in pre-stressed wire and cable.

The patented Digiforce device has been developed as a non-contact method for determining the tension in reinforcing cables and bars for pre-stressed concrete beams.

The equipment used for this tensioning process must be tested on a regular basis, but current methods are time consuming and accuracy is, in part, dependent on the skill of the operator, according to the company.

Integral to the durability and structural performance of these products is the correct tensioning of the pre-stressing steel tendons.

There are approximately 30 tendons in the average 1,200mm-wide hollow-core floor slab, which may be up to 160m in length.

By striking the wire or cable under tension and observing its fundamental natural frequency of vibration, it is possible to develop a battery-powered handheld instrument capable of accurately determining the tensile forces applied to them.

The low frequencies and complex sound spectra have prevented accurate analysis on site.

The Digiforce addresses this issue via the integral mathematical software in order to process only those frequencies of interest that contribute to the measurement.

This means that the user no longer has to refer to look-up charts or return to the office to run complex calculations relevant to that piece of equipment or installation.

In terms of operation, the instrument is first programmed with the length of the wire or cable whose tension is to be measured.

Also inputted is its weight per unit length.

The magnetic sensor at the end of the instrument is then placed adjacent to the part to be measured.

The measurement process is triggered and the part is struck lightly with the mallet supplied.

After a few moments, the tension appears on the LCD display on the instrument.

Tensions in stainless-steel or other non-ferromagnetic wires can be measured by attaching clips (also supplied) made of ferromagnetic materials, as long as their masses are insignificant compared to that of the cable to be measured.

The vibrations being analysed are complex and the fundamental frequencies that the method uses are in the 3-70Hz range and not the metallic note heard by the ear.

The tension is determined from the formula T = 4 x length2 x Hz2 x weight per unit length.

As a result of the very low frequencies, tuned circuit filter methods are not an option.

Instead, the device makes an initial 20 measurements of each oscillation and discards these.

The next five measurements from 10 oscillations are then used to establish a tolerance band to establish which measurements are reasonable to contribute to the calculation and which should be rejected as spurious.

Subsequent measurements are then made and those that fall within the tolerance band are used to calculate the final measurement value.

The Digiforce device features an onboard data-logging facility for collecting results and an RS232 port/USB converter for later downloads to a host PC, driver software, a mallet, a rugged carrying case and full instructions.

The device is supplied with a three-year warranty.

Applied Measurements

Applied Measurements offers a comprehensive range of loadcells, force sensors, weighing systems, pressure sensors, torque transducers, LVDTs and displacement transducers along with associated instrumentation.

Applied Measurements offers a comprehensive range of loadcells, force sensors, weighing systems, pressure sensors, torque transducers, LVDTs and displacement transducers along with associated instrumentation.

UKAS-traceable calibration can also be offered on all of our sensors and systems. We specialise in the manufacture of non-standard products and if one of our standard sensors is not exactly right for your application then we can design and build a customised product, tailored precisely to fit your requirements.

All products that we supply are backed up by our extensive in-house expertise and our standard three-year warranty.

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