Understanding how to minimise disrupting influences on weighing results

A matter of position: measuring location and temperature

Whether a measurement result actually indicates the exact mass of a product depends on more than just the quality of the scales used. Even if they are correctly installed and adjusted, numerous factors can influence the result despite not being immediately apparent. For example, it is usually assumed that gravity is always the same everywhere on earth. However, this is not the case because the earth is not a perfect sphere and therefore does not have the same density in every location. The distance from the equator and the distance to the centre of the earth also have an impact. Even changing the location of a scale to a higher or lower floor in a building changes the measurement result in the range of 1 ppm, i.e. for a load of 1000 g the weighing result deviates by 1 mg. To reliably avoid such deviations, Minebea Intec offers its customers extensive expertise and numerous engineering services. The company's offerings include mobile, verifiable weighing solutions with an EMFC weighing system (such as the Signum or IS platform models) that also have a built-in calibration weight. Deviations caused by a change of location can thus always be corrected. These systems are extremely flexible and deliver precise measurement results even over several degrees of latitude.

The temperature also has a influence on the measurement results of scales. In particular, heat can cause issues in systems which may manifest themselves, among other ways, as changes in the mechanical dimensions, the generation of forces due to the expansion of the materials used, and changes in the performance of individual weighing components. In extremely cold areas, incorrect weighing results can be caused by ice and snow loads, e.g. on silos, or by temperature errors in the weighing electronics. Industrial scales have to deliver the same results in freezing cold environments as in hot desert regions, the temperatures of which can be more than 100 degrees above those of the Siberian winter. The Engineering Support experts at Minebea Intec have many years of experience with such extreme requirements.

With its high heat resistance, the PR 6212 load cell, for example, ensures reliable processes in sectors such as the chemical and steel industries. The compact product is available in an optional high-temperature version for up to 180°C and absorbs temperature fluctuations through an integrated lifting protection with extended horizontal play. In addition, a high level of vibration resistance ensures precise measurement results even when oscillations occur.

Temperature fluctuations and deviations from the adjusted temperature application range should also be avoided to achieve correct weighing results. If, for example, an industrial scale has an established sensitivity temperature effect of ≤ 2 × 10^-6/K and has been adjusted for use at 18°C, its sensitivity deviation could be up to 0.6 mg/100 g if the temperature of the usage location is 21°C.

Another effect dependent on temperature and the moisture content of the air occurs when humid air flows over cold surfaces, causing moisture to settle on the surfaces in the form of condensate. Incorrect measurement results are particularly likely to occur in this way when small objects are weighed on high-resolution scales.

How undesirable air changes lead to incorrect results

The possible influence of air currents, air buoyancy and wind forces on weighing results is often underestimated. These disruptive factors occur primarily in outdoor areas, where moving air, for example in tall silos, creates dynamic pressure that makes repeatable weighing results significantly more difficult. Strong winds can also lead to lifting forces which make precise weighing problematic as well, particularly in the case of horizontal tanks. The extent to which air currents distort weighing results also depends on factors such as the shape and size of a vessel, the smoothness of its surface and the distance from other tall buildings that can bring about wind shadows.

In addition to considering possible negative influences on the weighing results, it is extremely important when developing outdoor weighing solutions to design all system elements to be resilient and stable enough for the wind forces that occur to ensure reliable operation with correct measurement results. This includes, among other things, appropriate dimensioning of the capacity of the selected load cells, the mechanical constrainers and the lifting protection elements.

The Inteco®/PR 6203 load cell – the logical evolution of the renowned PR 6201 from Minebea Intec – is suitable for use in many outdoor applications. The company's FlexLock mounting kits minimise the influence of movements on the weighing result, thereby particularly reducing the negative influences of wind forces on weighing processes in outdoor areas. This flexible, innovative weighing solution for silo and vessel weighing also features a high level of vibration resistance.

Electrostatics and magnetism – and which measures ensure precise results

Electrostatic charges exert forces on the material to be weighed, the weighing load plate or the load cell, leading to a significant distortion of the measurement results and poor repeatability of the results with repeated measurements. This also applies to magnetic forces that arise when materials with different magnetic properties meet.

For many materials, static electricity can be eliminated using an anti-static spray or an ionising fan. Magnetic forces should be counteracted by keeping the measurement object away from all magnetic or magnetised parts or by demagnetising or shielding the object. If this is not possible, an underfloor scale could be a good workaround.

The weighing processes in plastics production in particular often present a challenge with regard to electrostatic charge, which can distort the weighing result. The electrostatic charge can be neutralised by using an ioniser in the dynamic checkweighers, such as in the case of the WK+ checkweigher: The checkweigher combines practical features, such as a shielding cap to protect against draughts and an ioniser for minimising charge, with high-precision and reliable weighing. With such tailored solutions, Minebea Intec has been able to neutralise static in plastic parts and thus achieve precise weighing results in numerous applications.

Vibrations, friction, overload and seismic forces – weighing solutions for extreme conditions

Vibrations, friction, overload and impact loads are factors that can hugely influence a measurement result in weighing processes and should therefore be prevented or at least limited where possible. As an example, friction occurs when connecting pieces in scales harden and become immobile after long periods of operation due to product deposits, or when hoses and cable ducts lose their flexibility over time. Regular cleaning and maintenance aid in largely avoiding the measurement errors caused in this manner.

Earthquakes are among the most destructive forces on earth. Even small seismic loads can distort weighing processes, and severe earthquakes pose the risk of entire structures being damaged or destroyed. When measuring technology is used in earthquake-prone areas, special mounting kits for load cells offer a certain degree of security with regard to the thermal or mechanical expansion or contraction of vessels to avoid the risk of collapsing constructions or toppling silos and thus offer suitable protection.

The PR 6241 series load cells are specially designed for weighing small and medium-sized process vessels and for high-precision dosing. Their unique design principle – the combination of the "real" shearing force principle and the pendulum support principle – in conjunction with the FlexLock mounting kits enables the compensation of movements arising from mechanical or thermal contraction or expansion of the vessel or the support structure. Novego, the innovative weighing module, offers another solution. Uniquely in the market, Novego guarantees measuring accuracy even when there are lateral forces of up to 20 percent of the load. The remarkable resistance to transverse forces eliminates inaccuracies caused by vibrations, making it ideal for weighing vessels with mixers, for example. In addition, the integrated overload protection of many Minebea Intec systems ensures robust solutions that enable reliable operation even under extreme mechanical loads.

Load cells: important criteria for optimal precision

The accuracy of measurement results can be affected by more than just the general or environmental factors described above. The quality of the measurement is sometimes already negatively influenced by selecting an incorrect component, installation errors or adjustment and calibration errors.

One powerful solution, for example, is the Flexus checkweigher from Minebea Intec. It is available with the high-precision load cell WZED on request, which is protected from external influences under the weighing belt. In addition, a tailored infeed and outfeed belt construction ensures that weighing is virtually free of vibrations. In addition to higher speed and precision when weighing, this solution also offers electro-magnetic force compensation and lower susceptibility to interference from factors such as temperature, humidity or wind. Products with an low minimum weight of five grams or more can therefore be verifiably weighed in accordance with all standards. WZED has received MID approval for applications where verification is required and with belt speed ranges of up to 3 m per second, and can therefore also be used in high-speed applications with up to 600 objects per minute. This load cell is thus particularly suitable for dynamic weighing in the confectionery industry or for canned products, as well as for use in industries such as pharmaceuticals, chemicals or cosmetics production.

With the industrial scales of the Combics® series, Minebea Intec also guarantees reliable measuring results in the long term through the use of proven, high-resolution strain-gauge technology. Combics® scales feature resolutions of up to 60,000 d, depending on the model. Combics® industrial scales cover a very wide variety of weighing tasks in numerous different industries and are certified according to GMP, ISO, ATEX, OIML and FDA.

Resolutions of up to 620,000 d are offered by the Signum® industrial scales and digital IS platforms from Minebea Intec in the Supreme version, which achieve outstanding mass determination precision on the basis of their electro-magnetic force compensation. The Signum® is extremely sturdy and robust and can be easily and perfectly adapted to all ambient conditions in industrial environments owing to its highly user-friendly interface.

Advice from specialists

A wide range of internal and external influences can lead to incorrect results when determining mass. Minebea Intec has been tackling this problem for decades and, owing to this experience, can provide the ideal support to users when choosing the right weighing technology, as well as for the correct installation, adjustment and regular calibration of this technology. The company's comprehensive range of products and services for automatic and manual weighing and inspection processes offers tailored solutions for a wide variety of applications and industries, which enable precise weighing results even in the face of disruptive influences.

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