Device rapidly measures tension in pre-stressed wire and cable for concrete and theatre industries.

Prompted by a customer asking for a non-contact method to determine the tension in reinforcing cables and wire for pre-stressed concrete beams, Peter Lewis, the managing director of Applied Measurements, discovered that a vibrating steel cable or wire had an externally applied transverse magnetic field.

After significant development, the company designed and patented the Digiforce device, which is used to make rapid and accurate tension measurements on-site. Following the success of the Digiforce in pre-stressed concrete, other applications have been realised.

Within the theatre industry the product is used to monitor the tension on various cables and wired elements used to support stage scenery. Some of these cables support very large loads and stage engineers need to feel confident that the safe working load of cables is not being exceeded. Prior to this invention, the process of load measurement on cables was time-consuming and difficult to log multiple readings with any great accuracy or confidence. The Digiforce has enabled users to achieve rapid, repeatable, non-contact tension measurement using cutting-edge technology.

By striking the wire or cable under tension and observing its fundamental natural frequency of vibration, it is possible to accurately determining the tensile forces applied to them. Although simple in theory, the low frequencies of up to only a few tens of Hz, and complex sound spectra, have prevented accurate analysis on-site. The Digiforce addresses this issue via on-board software that processes only those frequencies 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 pertinent to that piece of equipment or installation.

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 aluminium striker supplied. After a few moments the tension is displayed on the LCD display of the instrument.

Tensions in stainless steel or other non-ferromagnetic wires can be measured by attaching clips (supplied), made of ferromagnetic materials, provided that 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 range 3Hz to 70Hz and not the vibratory note heard by the ear. Because of the very low frequencies, tuned circuit
Tension Measurement Device

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.

Because the principal business of Applied Measurements is in design and manufacture of precision load cells and force sensors, the measurements are traceable back to NPL calibration standards.

The Digiforce boasts an on-board data-logging facility for collecting results and an RS232 port / USB converter for later downloads to a host PC. It is also supplied with driver software, aluminium striker, rugged carrying case, and full instructions.