Weigh Forum


  1. What types of load cell can be found in the marketplace?

    Weighing Community would offer its registred members a comprehensive range of load cells, consisting of all the following types:

    • Single point/off-center/platform – load cells that are placed underneath the center of a plane surface, they are capable of correctly measuring the force applied at any point on the surface. They are usually employed in all single-cell platforms and small weighing scales.
    • Bending beams – load cells that are commonly used in small weighing systems, dosing machines, bagging and filling machines, big-bag (FIBC) filling and emptying systems, small multi-cell platform scales
    • Shear beams – load cells that are commonly used in larger, multi-cell platforms. They are suitable for weighing small and medium-sized hoppers, tanks or silos and transport (conveyor) systems with belts, rollers, etc.
    • Double shear beams &ndas
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  2. What regulations must be followed during the verification of weighing instrumentation used by a company that operates with a Quality System certified according to ISO 9001?

    A company operating with a Quality System certified according to ISO 9001 must verify all weighing instruments at their disposal on a periodic basis, the frequency of which is to be determined at the discretion of the company in combination with their Quality Inspector and depends on the use for which the instrument is intended.

    If the weighing instrument is crucial to the major activities of the company and is used with high frequency, the required verification period may even be monthly or half-yearly, but if the weighing instrument is used rarely, e.g. for minor activities within the company, inspections should be less frequent, for example every 2-3 years.

    The verification can be carried out using certified weights traceable to national standards, following a properly documented procedure.

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  3. What is the difference between 4-wire and 6-wire load cells?

    Some load cells have a cable with 4 wires and a screen; others have a cable with 6 wires and a screen. Those with 6 wires, in addition to the +input, -input, +signal and -signal terminations, have 2 wires called +Sense and -Sense. These are sometimes called +Reference (or +Ref) and -Reference (or -Ref).

    The main difference in function of these 2 types is that load cells with a 6 wire cable can compensate for variations in the actual excitation voltage they receive from the amplifier/indicator. The resistance of an electrical cable (conductor) varies according to its length and any temperature changes, resulting in variations in the excitation voltage at the load cells. With long cables, there will be a drop in voltage from the original value supplied by the amplifier/indicator and the advantage of a 6 wire load cell is that this drop in voltage can be quickly and effectively compensated without it affecting the weight measurement.

    4 wire load cells

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  4. What is the precision that i may expect from a weighing system?

    The precision that we may expect from a weighing system is the highest value obtained between

    the two following calculations (a) and (b):

     

    (a) Limit by minimum division of the load cell (related to repeatability):

    emin(rep) = vmin *  N (a)

    Being,

    emin(rep) =minimum error that can be obtained by the minimum division of the cell

    vmin = the minimum load cell verification interval

    N = number of load cells

     

    (b) Limit by range of use of the cell (related to linearity):

    emin(lin) = Max / nlc (b)

    being,

    emin(lin) =minimum error that can be obtained by range of use of cell

    nlc = number of load cell verification intervals

    Result: emin = the highest of emin(rep) o emin(lin)

     

    Recommendation:

    The precis

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  5. What is a load cell?

    load cell is an electronic component (or transducer) to which a weight/force is applied.

    It converts the weight/force applied to the load cell into an electrical signal (in units of millivolt or mV) that is proportional to the mechanical deformation (stresses and strains) caused by that weight/force.

    A high quality load cell is able to deform under that weight/force in an extremely repeatable manner, just like a good mechanical spring.

    Typical applications:

    • Scales
    • Packaging machinery
    • Dosing and filling machinery
    • Level/Inventory Control on Tanks and Silos
    • Test Machinery
    • Quality Control

    Typical materials used to manufacture load cells are special alloys with high strength, high fatigue life, high repeatability, high linearity and low hysteresis. For example steel alloys, stainless steels, aluminium alloys, beryllium-copper and others.

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