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While looking for a suitable model of platform scales, one may see how many different scales that can be used for weighing are available. With this purchase advice, we would like to give you an overview of different designs and functions to make the process of selecting the right platform scales easier.
First and foremost, the question of the platform size is crucial. Platform scales can have different platform sizes. The platform sizes are designed according to the sensors installed there and, if possible, should not be smaller than the goods to be weighed.
- What is the measuring task like? (Type of scales)
- Is the weighing operation legal-for-trade/subject to verification? (Legal requirement)
- What are the ambient conditions?
- What is the heaviest item to be weighed? (Maximum weight)
- How light is the lightest item to be weighed? (Minimum weight)
- What is the desired accuracy? (Resolution / repeatability / accuracy)
- What is the aim of the weighing? (Functions)
- Should the measurement data be further processed after weighing? (Interfaces)
Due to the different applications, platform scales have a very wide range of use. Due to different shapes, functions and accessories, standard platform scales can easily be adapted to the needs of the respective place of use.
First of all, the user should define the application of the platform scales, which means to primarily determine the main task field. Enquiries about the platforms scale can be used to cover all the task at the enterprise are not quite correct.
With this knowledge, the user can make a preselection of the platform scales.
First and foremost, PCE platform scales are robust and durable.
The platform scales consist of an evaluation unit (display) and an external platform (weighing plate). The platform is mainly mounted on or in the work table. The platform size plays a particularly important role in the platform scales. There are two types of the platform scales, which should be used according to the specific application. The less expensive platform scales have only one weighing cell (single point) in the middle of the platform. However, this construction is significantly higher compared to the platform with 4 load cells, as the struts of the construction have to be more stable. The single point weighing plate is limited in size, as the leverage forces could otherwise easily damage the cell. In the case of the platform scales with 4 load cells, one is more flexible in the platform size, as the load is distributed over 4 points and thus larger platform dimensions can be made possible. When choosing the right platform scales, the price should play a subordinate role, as the fulfilment of the application requirements should be in the foreground. The platform scales are secured differently against transverse and leverage forces and should be selected in accordance with the requirements at the place of use in order to be able to guarantee a durable working application.
If the shape and design of the platform scales have been specified in more detail, further requirements must be clarified. An important piece of information that strongly influences the selection of the appropriate scales is the question of “legal-for-trade” or “mandatory for verification”. The legal-for-trade weighings must be carried out with the calibrated platform scales; otherwise there is a risk of fines in the event of non-compliance. The calibrated platform scales are mandatory where a price is determined through the determined weight.
However, if the weighing values are intended for internal purposes, verification is not mandatory and should be avoided if possible. Verification restricts some functions of the platform scales that may not be compliant with regulations, but may prove to be very helpful in the work environment.
Once the legal-for-trade requirement of the platform scales has been clarified, the ambient conditions at the place of use should be determined. Platform scales can be found in a wide variety of areas, so that different requirements are made for the material and the technology. For example, there may be a specific requirement for the alloy used, as is often the case in the chemical industry. In the food industry, for example, the industrial scales made of stainless steel with a high IP protection class are required, which are also HACCP-compliant. In foundry operations, for example, there are special requirements for heat protection and in potentially explosive areas only platform scales with an appropriate explosion protection may be used.
Once the previous general conditions have been clarified, the next step is to select the platform scales that match the goods to be weighed. In most cases, depending on the application, the aim is to achieve the largest possible weighing range (MAX) with the best possible resolution and the smallest possible measurement deviation. Taking into account the operating environment and the aspects already described, certain compromises may have to be made when selecting the platform scales.
As the platform scales are used in various fields, they have a variety of functions. Functions that are particularly on demand in industry include the totalizing function, container weighing function, batching function, TARGET and ACTUAL monitoring function, counting function. Some functions are programmed into the platform scales according to the customer specifications and thus fulfil the individual requirements of the customer.
In most cases, the platform scales are extended with the help of additional interfaces. Depending on the customer's requirements, we can equip the platform scales with a bidirectional RS232, USB, LAN, RS485 interface or with a 0-10 V, 4-20 mA analogue output as well as several switching contacts for dosing and monitoring functions. The selection of these options should be made also with consideration of the future before ordering, as retrofitting of the platform scales is associated with considerable costs.
Since every industry has its own special requirements for the platform scales and the internal process sequences can also differ greatly, PCE Instruments offers the option of manufacturing an individual model of the platform scales. This is individually tailored to the customer's requirements and includes both design and software adaptations. If no platform scales from the standard range are suitable, please contact our technicians. However, the above-mentioned selection criteria for the platform scales should already have been thought through so that the project can be implemented cost-effectively and promptly.
In all the areas where the platform scales have to be cleaned frequently or come into contact with moisture or aggressive media for other reasons, stainless steel versions are the best choice. Stainless steels are quenched and tempered steels that are specially treated and often enriched with additives in the manufacturing process to achieve certain properties. The current steel list contains 159 unalloyed and 1315 alloyed stainless steels. In a common parlance, stainless steel is often equated with rust-resistant steel. However, rust-resistant steels are only a subgroup of stainless steels. Rust-resistant steels contain at least 10.5 per cent chromium and no more than 1.2 per cent carbon. However, the grades with higher chromium contents and additional alloying components are often used. Especially if the platform scales frequently come into contact with acids or dissolved salts, the resistance of the steel grade used to the products used should be clarified before purchasing new scales.
However, it is important to pay attention not only to the material of the housing, but also to how well the platform scales are protected against the ingress of water and foreign bodies. This protection can be classified according to EN 60529 (VDE 0470-1) and is known internationally as IP protection class or IP protection rating. The higher the two digits are, the better the protection of the enclosure against the ingress of foreign bodies and water is. Here, the first digit stands for the foreign bodies – IP6X means, for example, dust-proof and protected against contact. The second digit of the IP code stands for the protection against the ingress of water. The platform scales with IPX7 are protected against temporary submersion, with IPX8 against permanent submersion and with IPX9 even against cleaning with a high-pressure cleaner.
Stainless steels are easy to clean, compared to the other materials, and are therefore preferred wherever hygiene regulations have to be observed. The stainless steel platform scales can be used in the food processing and chemical industries, in laboratories, in commercial kitchens, in laundries and dry cleaners. Also, the application outdoors, for example in a field, a pasture or a company storage area is possible, without the platform scales having to be elaborately protected from moisture.
The verification of the measuring instruments and measuring tools serves to protect the consumers. It is intended to ensure that in trade and when calculating customs duties, the fees or penalties are calculated based on the correct measurement results. But also in the medical and pharmaceutical sector, verification is mandatory for the measuring instruments in hospitals, doctors' practices and laboratories where the measurement values have an impact on medical treatment. Surely everyone has checked the calibration mark on a beverage glass – consciously or unconsciously. With the help of this mark, the buyer can immediately see whether the ordered and calculated quantity of the drink – for example 0.2 or 0.3 litres – has been filled into the glass. In contrast, in the case of the goods that are weighed on the platform scales, it is not apparent whether the weight is correctly displayed. Therefore, for the scales used in legal metrology, regular inspection by the authorized for this purpose body is mandatory. In Germany, the responsibility for monitoring lies with the local calibration offices.
The place of installation influences the accuracy of the weighing result for the platform scales that determine the mass with the help of the weight. After all, the weight results from the mass multiplied by the location-dependent acceleration due to gravity. Only the scales that always compare the mass to be determined with a known mass are independent of the force of gravity. This applies, for example, to the Justitia beam balances with its symmetrically suspended weighing pans. Also, modern electronic platform scales, which have an internal reference weight and an automatic self-adjustment function, also weigh independently of the place of installation. The force of attraction of large masses is generally referred to as gravity. It acts not only on the earth, but also on other celestial bodies. In the gravity-dependent platform scales, the more divisions there are, the stronger the influence of gravity is and thus, the more accurately the scales indicate. Therefore, the geographical area in which it may be used – the zone of use – then becomes smaller. In Germany, for example, the verification can be with the gravitational value g = 9.81 m/s² completely for the Federal Republic of Germany, for the two federal states of Baden-Württemberg and Bavaria-North with g= 9.8081 m/s² or for a specific place of installation, such as Meschede, with g = 9.811523 m/s². If the scales are to be used at another location or outside the permissible zone, it must be recalibrated and then calibrated for the new regional area of validity. The calibration periods are specified for all the measuring instruments in the Measuring and Calibration Ordinance (MessEV). In general, the verification period is two years. However, unlike the main inspection and exhaust emission test for motor vehicles – i.e. the technical inspection for motor vehicles – it is always valid until the end of the year in which the prescribed period expires. This means that irrespective of the day in 2020 on which the platform scales were inspected at the Calibration / Weights and Measures Office, the verification is always valid until 31.12.2022 in the case of a 2-year period. According to Annex 7 of the Measuring Ordinance, different verification periods apply to the following non-automatic weighing instruments:
- Platform scales with a maximum load of 3000 kg or more (except building material scales) 3 years – platform scales as non-self-contained fine or precision scales 4 years – commercial scales as platform scales with less than 350 kg maximum load 4 years – platform scales for weighing people in hospitals (except bed scales) 4 years – Personal scales that are not installed in hospitals; verification is unlimited – baby scales 4 years – container trolleys for liquefied gases with shock-free application and removal 4 years – platform scales as cattle scales in farms 4 years
Not each model of the platform scales is legal for trade. For this purpose, certain requirements must be met in the design. Above all, it must be ensured that the scales cannot be manipulated after calibration without damaging a calibration seal. The users who have to use the verified platform scales for their weighing operations must make sure when selecting new scales that it is already verified or conformity-assessed with a verification mark. Whether and when the scales have been verified can be seen on the type plate. Each time the scales are verified, a sticker indicating the year and the name of the verifying authority is affixed there. The following information must be permanently and tamper-proof displayed on the type plate or on the display or housing of the verified or conformity-assessed platform scales:
• Manufacturer's brand or name
• Model designation for the scales
• Number of the EC pattern approval
• The serial number of the scales
• Accuracy class It is indicated in Roman numerals from I to IV and is enclosed in an oval frame.
• Maximum load “Max” The weighing results above the maximum load may not be used, usually an overload symbol is displayed instead.
• Minimum load “Min” The weighing results below the minimum load, even if displayed, must not be used for legal-for-trade measurements.
• Division value / graduation value “d” The division value corresponds to the possible display steps and thus to the reading accuracy.
• Verification value “e” The verification value for the platform scales of accuracy class III corresponds to the division value "d". In accuracy class II, the verification value often corresponds to 10 times the graduation value and in accuracy class I, the calibration value can correspond to 100 times the graduation value. The additional decimal places are marked as additional digits in the display – similar to the last digit in the electricity meters.
• The temperature limits within which the scales maintain the specified accuracy.
The platform scales that have been placed on the market with conformity assessment according to Directive 2014/31/EU is considered to be verified. It bears the CE mark and, directly behind it, the metrology mark M with the last two digits of the year of marking. The M and the year are framed by a rectangle. The EU declaration of conformity is mandatory for all conformity-assessed platform scales. If the declaration of conformity has been issued for a series of scales, it can also be provided as part of the operating instructions.