The purpose of the instructions is to define general procedure to be applied during supervision of weighing of grain in bulk.
This procedure describes the following types of scales:
- Weigh hoppers and automatic spilling systems
- Motor truck and railroad track scales
Though it is clear that weighing should conduct to a determination of the weight of goods, it is important to take into account for what purpose the result is needed. The use to be made of the weights specifies either the acceptability of a certain type of scale or the accuracy to be required.
• Process and inventory controls
Only ask for a rough and rapid determination of the quantity produced or stored. A process control may also be needed to regularizes the flow of materials. Such installations are normally incorporated in the bulk handling conveyor system. Accuracy tolerances up to +/- several percents are acceptable.
• Determination of weights for calculation of freight costs
Railway companies of certain countries determine the weight of wagons and/or complete trains by in-motion weighing. Most weighbridges demand uncoupling of railway wagons for individual weighing.
Shipping companies base freight costs very often on weights determined roughly by a draft survey.
• Determination of commercial weights
Is the most frequent weighing supervised by Inspection company. Since payment of material costs depends directly on the result obtained, such weighing must meet the legal requirements for accuracy. In addition to legal prescriptions the acceptable tolerances must be either in connection with the value of the goods or in accordance with contractual specification. As a general rule only scales recognized by government agencies, Weights and Measures Authorities, etc. can be used. Owners of such scales are obliged to ask for periodical checking by this authority and must be in possession of the stamped and signed test sheets.
This study is limited to the types of scales named hereafter and to which detailed recommendations are given in the following paragraphs:
- Weigh hoppers and automatic spilling systems
- Motor truck and railroad track scales
Items 1 and 2 permit only discontinuous weighing.
Weigh hoppers and automatic spilling systems
Many types and models with various designs are found in the market. All of them are provided for weighing bulk material in dusty or granulated form. Design must be adapted to the granulation, moisture content, etc. of the products to be weighed.
Standard sizes in hopper scales range from 1 to 50 tons; in tank scales from 1 to 150 tons.
The feeding hopper must be of correct design permitting free flow out of material without tendency to obstruct and/or to adherence of goods on the walls or in the corners.
The feeding hopper must have a capacity of 1.5 to 2 times that of the weigh hopper to assure feeding of the scale out of the feeding hopper and not by the conveyor system, i.e. to assure a regular flow into hopper.
Mechanical feeders have also to be of adequate design.
Weighing hoppers can have two main principles:
The tipping bucket, which dumps out as soon as the accurate quantity of goods has poured in. The advantage of this system is its working without any outlet gate, hence no spilling of material during the filling period of the hopper is possible.
Such tipping buckets are applicable for relatively small capacities only (up to about 1 ton), because above certain limits the dynamic forces would become invincibly high.
By weighing hoppers with bottom outlet an outlet, gate with manual, electrical, pneumatic etc. drive controls the evacuation of the material. In that case the inspector has to check the connect operation of the outlet gate and its tightness.
Attention has to be paid to the change of the tare by deposition of dust on the hopper and the lever system.
For both systems the correct design permitting free flow and avoiding obstruction and adherence of goods is most important. If there is the tendency that material remains in the weighing hopper, the scale has to be declared unfit for weighing the goods. Fitting an adequate electric knocker on the weighing hopper may in some cases improve the situation. For adhesive goods a simple system weighing the gross weight only must be refused. In such cases only a double system, weighing the gross weight and the tare is acceptable.
Evacuation of goods weighed in a collecting hopper with an adequate capacity is necessary when the weigh hopper discharges the material all at once as the case with the tipping buckets or with quick opening outlet gates. If the collecting hopper is too small there is the risk that not all material is evacuated out of the weighing hopper and will be weighed twice.
Direct feeding from the weigh hopper on to a conveyor is only applied when the output capacity is small or when two or more parallel weigh hoppers are installed, as for instance in automatic bulk weighing systems. The inspector has also in the case of direct feeding to check that the weighing hopper are empty before a new filling cycle starts.
Dust removal by suction is a normal attachment to these types of scales since filling of the hoppers as well as evacuation of the material is impossible without an intense development of dust. Inspectors have to check the adjustment of these installations to avoid:
- removal of material with the dust as a result of too strong exhaust or suction mouth too near to the drop of material.
- any force on the weighing hopper produced by the suction.
The dust collected in the area of feeding of the weighing hopper has not been weighed and must therefore be conducted back in a conveyor or tube preceding the scale.
The disposition and layout of conveyance of the goods preceding and following the scale has to be checked for location of by-passes, deviation gates and leaking points. Inspectors should check that no part of material of the total flow goes round the scale or is branched off from the flow to a different final destination after weighing.
In case of weighing hoppers discharging the material all at once the drop can create an important suction effect in the evacuation tube. Adequate air inlets must be provided to avoid exertion of a force on the weighing hopper.
Inconsistent material flow will result in inconsistent weight values. Direct feeding of the weigh hopper by a conveyor must be avoided, but a properly designed supply bin with a constant minimum head of material and, if necessary, a device such as an agitation in the scale inlet chute provided.
Vibrations influence the accuracy of weighing and can produce premature deterioration of essential parts of the scale, as for instance the articulations, edges and seats. Scales must have a flexible connection to the top hopper and be supported directly by main girders of the building.
Manual control is no more used on modem hopper scales. Some very old plants may still be equipped by such weighers. Operational cost of such scales is very high.
Inspectors having been called to supervise manually controlled hopper scales, integrated in a conveyor system, know the possibilities to influence as desired the weight of each filling of the hopper by adequate manipulation of the feeding and/or the outlet gates.
• As a general guide it shall be advised that the scale-beam must come to a standstill in zero-position before the outlet is opened and that it is closed only after evacuation of all material out of the hopper.
Fully automatic control is today generally applied. It can be performed by a completely mechanical lever system, partly mechanical combined with electrical or electronical equipment or by completely electronic weight sensing arrangements.
Testing and calibration of weigh hopper and automatic spilling systems have to be performed carefully and periodically before commencement of weighing and repeatedly during supervision of a consignment and in every case when quality or kind of material changes.
The Scale Code of the National Bureau of Standards, must be met by all scales used in commerce. These prescriptions demand that the indicated weight of a scale match the actual weight of a calibrated test weight placed upon the scale within specific tolerance limits. This tolerance should not exceed 0.1% of the test load.
The above applies for static weighing only which apparently is the case in weigh hoppers and automatic spilling systems. But since these scales have to perform automatic and periodic operations, many variables are introduced into its weighing cycle resulting in the conclusion that a static testing with calibrated test weights is insufficient.
Only periodical weighing of the hopper content performed during the automatic functioning of the scale permit checking of the real accuracy and to take into account all variable factors. Therefore, all scale installations of this type must have a simple device permitting deviation of the out-flowing material for one hopper filling on a test scale.
The results of these test weighing will never be identical. A comparable accuracy can be determined by a statistical method. For a rapid determination the average value of a sufficient number of tests will by exact enough.
On basis of the final result the necessary adjustments have to be made by a skilled man.
Motor truck and railroad track scales
Motor truck and railroad track scales are used to weigh materials in receiving and shipping for customer billing and for tariff determinations. All kinds of goods can be weighed.
Correct utilisation of the weighing capacity is very important since the best accuracy is ascertainable between about 20% and 80% of the full or nominal load of a scale. Every overload must strictly be avoided.
Cleanliness of the scale has to be observed carefully. An inspector will often encounter a scale pit having never been cleaned since the installation of the scale. Water evacuation from the scale pit must be well dimensioned and in good condition.
The platform and surrounding area must be clean and free from dust or material deposits.
Installation, construction and foundation have to be accepted or refused by the inspector according to his findings since he has no possibility to change anything when called for supervision of weighing. The following main considerations have to be observed:
• Foundation should be adapted to the actual soil conditions. Inspector has to check if sets and/or cracking can be found.
• Entrance to the scale pit should be protected against unauthorised personnel.
• For electronic load cell scales: all cables should be shielded and all shields interconnected and carried to a common and separate ground. Cables of such circuits should be grounded at one end only and insulated with non-hygroscopic materials, junction boxes, etc. protected against moisture.
• Power sources for electronic instruments should be on separate circuit back to the distribution transformer.
• Inspection report concerning commissioning tests made by Weights and Measures Authorities should be available.
Scale maintenance, inspection and service should take place regularly by trained personnel automatically according an agreed schedule. Inspector has to check if schedule and performance reports are available.
Approaches to scales should be straight and level paved, at each end at least 10 ft (3m) in length for static weighing and at least 75 ft (25m) for in motion weighing. Particular requirements of local Weights and Measures Authorities should be consulted.
Inspector has to ascertain that load and scale are completely at state of rest when the weight is read or registered (truck motor stopped, no oscillations of suspension springs, etc.).
Weighing the axles separately, double weighing and additioning the axle loads can only be applied when many precautions are observed.
• Double weighing should be refused for liquid material in tanks because the slightest difference between the level of the approach and that of the platform, which is already produced by the movement of the platform, in the order of less than one millimetre, displaces the load.
• For weighing solid materials important errors arise when the truck is placed on the scale with its brake applied or if the trailer remains coupled to the truck with the coupling strained.
In-motion weighing is generally only applied for railroad tracks, on special dedicated scale.
Mechanical control is a service-proved and still often encountered system. Weighing on these scales can more or less easily be supervised by an inspector. All types provide some form of weight readout either as a visual or printed total of weight.
Full-load cell scales replace more and more the mechanical controlled ones. The load cell provides a weighing method that overcomes the limitation and inconveniences of mechanical levers. Readout must no more be located near the weighing, connection of several scales to one readout device or vice versa is possible.
A correctly installed electronic system procures accurate and constant readings if all the considerations under are carefully observed.
Testing and calibration to be performed by skilled people.
• Zero-balance has to be adjusted since a correct starting point or zero-load condition is essential for accurate weighing. A scale that will not retain its zero-load balance within reasonable limits is not reliable or proper instrument. The same factors and causes that effect the ability of a scale to maintain its zero-load balance are apt to affect its ability to repeat accurately its indications from repeated applications of the same load. The causes for shifts of zero-load has to be made by a scale expert. Only after demonstration that the mechanism is in good condition without faults in assembly or installation, then, and only then, should attention be directed to the adjustable features of the scale.
The scale should be tested at 1/2, 1/2 and 3/4 of the total calibrated capacity and the test weighing placed to each end and to the centre of the platform. Additionally to tests attended by the inspector, he should check the last reports established by Weights Measures Authority.
Modern scales are no more mechanical installations, more or less understandable in their operation. They are equipped in a large extent with electronic appliance and therefore quite sophisticated objects.
Most of weighers available on the market have only limited applicability. This requires that inspector is first of all able to judge if the results produced by a certain scale are acceptable or if the weight has to be determined by another procedure.
The accuracy of a scale, applicable for the weighing concerned, depends from many external factors not related to the scale itself. The inspector should have the necessary back-ground to discover these factors, to ask for necessary modification or to refuse acceptance of weights determined on inadequate installations.