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Home > what is weighing?

 

History of Weighing

 To define the weight of a mass, namely weighing whose history stands on thousands of years is one of the main areas of interest for metrology.  Weighing is done by equilibrating the mass whose weight is unknown and another mass whose weight is known within equal distances in proportion to a brace. We call this mechanism as equal arm scale.  Such a mechanism with leather scales, which has been made of 80mm quartz in Old Egypt, is exhibited in London Science Museum. 

         Actually, the need for weighing something was arisen from a commercial concern as in today?s world.  It is believed that the above mentioned mechanism has been used to scale the golden that came out of the sands of the Nile. The golden should have been scaled in order to enter trading life and be exchanged and this was realized by equating its weight with the known weights. In Old Egypt, barleycorns have been used as known weight- because, may be it was abundant. The weight of 200 barley seeds was a standard unit and primitive equivalent to standard unit of today.

         In Middle Age, world richness and even Money was scaled for measurement. At that time, barleycorns that have also been the thousands of years before are used as standard unit. Those seeds are used to measure the small lengths as well.

         Wool, spice and silk can be counted among precious things like gemstones and golden for that time. Relatively heavy standard units were required to scale them. In Pharmacy, smaller standard units were required. In time, new standard units were created and used because of these requirements, but all of them were based on and equivalent to the oldest standard unit, barleycorns. 

Birth of Metric System and First Kilogram

  After French Revolution, metric system was born. In 1790, 16.Louis has established a committee that consists of leading scientists and asked them to generate a new measurement-weighing system. 

         The committee has finalized its work on May 19, 1791. Recommended system was based on unit length.  Unit length was described as one in twelve million of the distance between the pole and equator and designated in meters. This weight is defined as the weight of a unit of water volume, which is calculated with this unit of length at freezing point.  Louis enacted the work of committee on March 26, 1791.

         The committee came together for the second time in 1799. The mass standard was agreed to be defined as the weight of one cubic decimeter of water at 4 °C. This was called as kilogram. Therefore, one gram has the weight of one cubic centimeter of water.  Brass weights that equal to new standard mass were manufactured.  Lately, platinum mass was manufactured and calibrated so as to be equivalent to this weight.  This weight was begun to be known as kilogram des archives and become the standard weight of many European countries.

         In 1870 and 1872, French government called to discuss metric standard and the third meeting was held.  In this meeting in which eighteen countries has attended, the protection and the verification authority for metric standard was given to Comite International des Poids et Mesures (International Committee for Weights and Measures) and Bureau International des Poids et Mesures (International Bureau for Weights and Measures). As a result of this meeting, it was decided to manufacture the new kilogram from 90% platinum and 10% iridium. 

         George Matthey of Johnson (Matthey & Co) has manufactured three cylindrical parts at above proportions and submitted to metallurgist Claire Deville. These three parts alloy were calibrated and compared with kilogram des archives. Then, one of these parts was accepted as the international prototype for kilogram and designated as K.  In 1882, 40 pieces of a kilogram was ordered from Matthey & Co. These parts were manufactured in 1884 and distributed to 20 countries after calibration. 

         This is the brief history of kilogram.  The laboratories in International Institute of Meteorology (UME), which have been accredited with institute laboratories in different countries, operate in our country. 

 Weighing Techniques and Load Cells

 Since 5000 BC, equal arm scale has been used for weighing.  The other weighing method is spring scales. In spring scale, there is a pointer attached to spring and this pointer is used to display weight. The advantage of such scales is that they do not require any standard mass after calibration. In terms of repeatability and reliability, selecting the spring material appropriately, setting the spring sufficiently are important for the indicator to answer linearly. 

         Load Cell, which is subject to our work, is another method for weighing. The best transducer to display the weight on the screen is load cell.

         Load Cell based weighing indicators or shortly electronic scales as spring scales technically steel and aluminum is used as spring material, however the material is formed in different ways, which are convenient to measure.  (Even if the spring component of a load cell is not similar with a spring, it is same in principle.)

         Load cells are strain gage based transducers. Strain gages are electronic structures in various sizes and forms, which are used to analyze static and/or dynamic resistance of a system or detached structure.  In commercial load cells, strain gage in 4 or 5 different sizes and forms is used (Special strain gages that are produced for analysis of various composites, marble, ceramic, glass and concrete etc. are off topic)

         Load cell is a detached structure, whereas weighing is a static measurement in general. It works on just a single axis, since it is applied in weighing.  (Weight is formed as a result of a mass gravitated by the world with acceleration and this gravity changes depending on where you are.) Another name for it is transducer. Force is gotten by multiplying mass with acceleration. Weight is equal to gravity force of a mass in ground-vertical axis.