TECHNICAL INSIGHT

Shortcomings of analogue load cell systems

This Technical Insight considers the technical shortcomings of analogue loadcells.

Weighbridges fitted with analogue load cells are rarely as accurate as digital due to:

• High-Temperature Effects meaning that the output of a load cell is sensitive to temperature changes. A weighbridge fitted with analogue load cells and calibrated to a high accuracy level at say +20C will generally not retain the same accuracy at higher or lower relative temperatures such as say either say -10C or + 40C. 
• Stable Communications meaning analogue load cell communications have a tendency to amplify errors and are prone to electromagnetic interference. 
• Cable lengths are fixed on Analogue load cells, cutting them shorter or extending them invalidates the Type Approval as the gain is altered by the cable length, whilst with Digital load cells cable lengths can be varied reducing excess cabling or allowing for long home runs back to the weigh office.  

Results of laboratory tests

Tests have been carried out by Bilanciai in Italy, in accordance with “OIML R60 Metrological Regulations For Load Cells ED.1991” and “OIML R60 Annex A ed 1993” (“Laboratory Test Conditions”) on various analogue load cells to determine the temperature effect on these load cells at various loading weights.

Set out below are the results for the Avery 8701 load cell, a load cell found on many older weighbridges in Africa. These load cells were evaluated under Laboratory Test Conditions:

• By calibrating the load cell at a given load (Mmax) and given temperature (C))and establishing the error (mpe); and
• Then subjecting this load cell to different loads (Mmax) and temperatures (C) and determining the resulting errors (mpe).

The graph below shows the results:

The cost-effective solution

The most cost-effective solution for the digitisation of weighbridges with analogue instrumentation is not to remove all the existing analogue instrumentation and to install a completely new digital system. Instead it is cheaper to retain the existing load cells and to digitise the output through installing a “digitisation box” and add a modern digital indicator.

The D-Link, a “digitisation box” manufacturer by Bilaniciai and distributed by Sasco in Africa, is a digitisation system that has been used to evaluate under Laboratory Test Conditions the improved results that can be obtained from 8701 load cells when digitised.

The graph below shows the improved results of performing the exact same evaluation tests as set out under Section B above on 8701 load cells digitised using the D-Link to a Bilanciai digital indicator. Please note that the test made using a Bilanciai Terminal with up to three-point linearisation calibration. The mpe results speak for themselves and are profound, especially when one factors in the fact that Africa holds the record for the highest world temperature ever of +57.8C and has also recorded a minimum low of -24C.

Other significant benefits of load cell digitisation

In addition to improved mpe through a range of loadings and temperatures, the digitisation of analogue load cells through the use of “digitisation boxes” makes each individual load cell “visible” to the digital indicators and therefore provides the following added benefits:

• Calibrations are made much easier especially with the deck corner calibrations automated.
• No corner adjustments required via traditional junction box potentiometers or inline resistors which can become unstable over time or temperature.
• Each Loadcell is visualized as an individual channel, allowing for monitoring of each load cells performance.
• Fault finding is made simpler as the faults can be found without accessing the junction box and using a multimeter as the issue is “visible” on the weight terminal.

In the case of the Bilanciai’s D-Link used in conjunction with the DD1010 or higher DD range specification indicator, analogue weighbridges are also capable of providing in addition to enhanced total accuracy, axle weighing as well.