Monthly Archives: May 2024

Scale Theory

Prototype of the Kilogram

By May 18, 2024 0 Comments
The International Prototype of the Kilogram is an object whose mass was used to define the kilogram from 1889, when it replaced the Kilogramme des Archives, until 2019, when it was replaced by a new definition of the kilogram based entirely on physical constants. During that time, the IPK and its duplicates were used to calibrate all other kilogram mass standards on Earth. A Kibble balance, which is used to measure weight via electric current and voltage. With this instrument, the measurement of mass is no longer dependent on a defined mass standard and is instead dependent on natural physical constants.A diagram of the second-generation tabletop Kibble balance being built at NIST. In this version, movement is restricted to two dimensions using flexures, metal strips that only bend on one axis. The mass pan holds the physical object being weighed. An electromagnetic coil sits on each side of the device. Magnets provide a magnetic field through which the electromagnetic coils move. An optical encoder, which detects light and converts it into electrical signals, measures the movement of the coil. The whole device is roughly the size of a ream of printer paper.The circuit diagram of the Kibble balance in force mode (top) and velocity mode (bottom). A digital voltmeter (DVM) is used to measure the small difference between the programmable Josephson voltage standard (PJVS) and the voltage across the resistor (top) or the induced voltage across the coil (bottom).
Scale Theory

The Kibble balance

By May 18, 2024 0 Comments
The Kibble balance, seen in last issue, is a more accurate version of the ampere balance, an early current measuring instrument in which the force between two current-carrying coils of wire is measured and then used to calculate the magnitude of the current. The Kibble balance operates in the opposite sense; the current in the coils set very precisely by the Planck constant, and the force between the coils is used to measure the weight of a test kilogram mass. Then the mass is calculated from the weight by accurately measuring the local Earth’s gravity (the net acceleration combining gravitational and centrifugal effects) with a gravimeter. Thus the mass of the object is defined in terms of a current and a voltage— allowing the device to “measure mass without recourse to the IPK (International Prototype Kilogram) or any physical object”. Basic principles of the Kibble balance, a device that can be used to measure mass.The International Prototype Kilogram, stored in a vault in Paris, was replaced in 2019 by a formula that uses the Planck constant, since the IPK was losing mass.Prior to 2019, the definition of the kilogram was based on a physical object known as the International Prototype of the Kilogram (IPK),It’s the mass of a hunk of platinum-iridium alloy that’s been housed at the International Bureau of Weights and Measures in Sèvres, France since 1889. It’s called the International Prototype Kilogram (a.k.a. Big K, or Le Grand K), and it has many copies around the world . After considering alternatives, in 2013 the General Conference on Weights and Measures (CGPM) agreed on accuracy criteria for replacing this definition with one based on the use of a Kibble balance. After these criteria had been achieved, the CGPM voted unanimously on November 16, 2018 to change the definition of the kilogram and several other units, effective May 20, 2019, to coincide with World Metrology Day. One important reason for the change is that Big K is not constant. It has lost around 50 micrograms (about the mass of an eyelash) since it was created. But, frustratingly, when Big K loses mass, it’s still exactly one kilogram as per the current definition.
Scale Theory

Electromagnetic Type

By May 18, 2024 0 Comments
Digital weighing scales are devices used to measure the weight of an object. Various principles are used for measuring the weight of objects.The following briefly describes the operating principles and features of the two most popular methods, “electromagnetic type” and “loadcell type.” Electromagnetic Type This is also called the “electromagnetic balance method.” With mechanical balances, the sample is placed at one end of the beam and the weight is placed at the other end, and the value of the weight when both are perfectly balanced becomes the mass of the sample. With electromagnetic type balances, an electrical force (electromagnetic force) is applied instead of a placed weight to balance the beam. The amount of electricity required for balancing the beam changes according to the weight of the sample that is placed. The amount of current when the beam is perfectly balanced is detected, and the mass is obtained from that detected value.
Scale Theory

EVOLUTION & PRINCIPLES OF ELECTRONIC WEIGHING

By May 18, 2024 0 Comments
EVOLUTION & PRINCIPLES OF ELECTRONIC WEIGHING
The principle in Mechanical scales is by balancing of Unknown weight byKnown counter weight. The fundamental use of weighing machine is to findthe weight of the Object. However, presentation of weight data to the customer is easily readable onelectronic display compared to mechanical scale.The weight informationalone is seldom of interest until it has been used, to print or to correlate withother data which is possible in Electronic Scales only. ART OF ELECTRONIC WEIGHING: With help of micro processors, stability and accuracy are increased. Still somestringent requirements like Long term stability and Accuracy have to beconsidered. So greatest care has to be taken in the choice of loadcells andfixing of loadcell to the scale structure requires good care, as even the bestloadcell when incorrectly fixed, will produce a wrong result. Technology Update Before microprocessor became common in weighing electronics, scales werecontrolled with hardware and hardwiring. Achieving good Accuracy was aproblem. The Electronic scale uses the loadcell transducer which converts mechanicalforce into electrical signal.
Scale Theory

Roberval Balance

By May 18, 2024 0 Comments
Roberval Balance
The Roberval Balance is used in Top-Loading Electronic Balances.The Roberval Balance is a mechanism invented by the 17th century French mathematician Gilles Personne de Roberval. The sample position on the pan does not affect the weight measurement. The following describes the mechanism of this Roberval Balance, which should be called the basic principle of top-loading balances. What Would Happen If the Roberval Balance Did Not Exist? “With the balance in, pans are fixed in place on the beam”. By the principle of leverage, balances of this structure require the length from the fulcrum to the weight and the length up to the sample to be the same. If the position where the sample is placed on a pan is changed, as shown by the dotted line in the figure A, this balance becomes out-of-balance even if the placed sample is the same. This means that this mechanism cannot be used as a balance since the balance point shifts depending on where the sample is placed. For this reason, the Roberval Balance has a parallelogram frame structure with freely moving sides, as shown in figure B. If pans are placed on its vertical beams and the sample and weight are placed on top of these pans, they will be balanced regardless of where the sample and weight are placed if their masses are the same. The sample may be placed anywhere on the pan. This is the basic structure of the Roberval Balance. In top-loading electronic balances, further improvements have been made to this basic structure to achieve superior accuracy (i.e. little eccentric error).
Scale Theory

large scale

By May 18, 2024 0 Comments
Large-scale and complex weighing systems often require multiple loadcells to record accurate measurements. These multiple loadcell devices are arranged symmetrically around the corners of the measurement platform.This distributes the weight over a wider physical area. In this case, a junction box or summing box combines the measurement signals from these distributed loadcells. The result is a useful, combined or “summed” digital output.These junction boxes are built to withstand tough environments and are easily installed with most measurement applications.
Scale Theory

Loadcells

By May 18, 2024 0 Comments
Loadcells (Elliott) Elliott Bros. manufactured loadcells of various kinds and also incorporated them in their own crane-weighers, belt-weighers and weighbridges, accurate to within Weights & Measures Regulations’ tolerances. The more usual cell was of the compression type with the billet in the shape of a ring dynamometer for the lower ranges and like a pillar for higher loads, as shown in Fig. which illustrates one of each variety upon which have been mounted the four wire-wound strain-gauges,two active and two passive-connected in a bridge network and furnishedwith output leads. Resistance strain gauge loadcells Elliott Type D is a cutaway illustration of thebillet and casing of their D-type cell which is designed for medium accuracywork.
Scale Theory

filling machine

By May 18, 2024 0 Comments
Last week continuation : Here is a list of the main automatic weighing instruments that must meet the OIML
(International Organization of Legal Metrology) requirements and recommendations: Continuous totalizer – OIML R 50 An automatic instrument that determines the mass of a bulk product placed on a conveyor belt, without the need for a systematic subdivision of the product and without interrupting the movement of the belt. Railway vehicle weighbridge – OIML R 106 An automatic instrument equipped with a load receiver including tracks, on which the railway vehicles are being weighed. Technology Update Weight/price labeler – OIML R 51 An automatic instrument that labels individual items with their weight and price information. Gravimetric filling machine – OIML R 61 An automatic instrument that fills containers with a predetermined and virtually constant mass of bulk production. Weighing in motion is also a common application in rail transport. Trains are weighed, either on the main line or at yards.Weighing in Motion systems installed on the main lines measure the complete weight (distribution) of the trains as they pass by at the designated line speed. Weighing in motion on the mainline is therefore also referred to as “coupled-in-motion weighing”: all of the railcars are coupled. Weighing in motion at yards often measure individual wagons. Itrequires that the railcars are uncoupled on both ends in order to weigh. Weighing in motion at yards is therefore also referred to as “uncoupled-in-motion weighing”. Systems installed at yards usually works at lower speeds and are capable of higher accuracies.
Scale Theory

loadcell

By May 18, 2024 0 Comments
The types of strain gauge loadcells commonly used in platform scales includesingle point, planar beam, bending beam, shear beam, dual shear beam, and Stype loadcells.

Shear Beam LoadCell
A shear beam loadcell has a piece of metal that elastically deforms under the force of a load. The strain is picked up by a strain gauge in the metal and changed to an electrical signal.. Shear beam loadcells are used for medium loads of less than five tons.
Single Point LoadCell
Single point loadcells have an aperture that is the cut out of the body of the loadcell. The aperture controls the thickness of the metal across the loadcell body. When a load is applied, the shape of the loadcell deforms, which is detected by strain gauges as the body of the loadcell is distorted. The deformation creates a voltage signal that is proportional to the force of the weight.
Scale Theory

Resistance Strain Guage Loadcells

By May 18, 2024 0 Comments
Figure shows the inside of a column-type loadcell. The positions of the foil type active and passive strain gauges and their connections can clearly be discerned. These gauges together with their trimming and temperature compensating resistors are wired into a Wheatstone bridge network. The column and base are machined from a one piece forging of special heat-treated high tensile steel. Since that time, however, strain gauges have proliferated both as mechanical scale components and in stand-alone load cells. Today, except for specific laboratories where precision mechanical balances are still used, strain gauge load cells dominate the weighing industry. Resistance Strain Guage Loadcells The loadcell is completed by a cast iron case which encloses the gauged portion, being sealed by O-rings at the top using the machined grooves. The case also carries the output cable socket. The typical cell having a nominal capacity of 20 tons is about 3.5 inches high including the cap and suffers a compression of about 0.006 inch at full load to give an output of some 18mV with a standard input of 10V. An 800 tons cell would be about 17.5 inches high and would compress about 0.025 inch for a similar output. Combined errors are claimed not to exceed + or- 0.1%. The problems of non-axial loading almost disappear if the loadcell can be used in tension instead of compression. In many cases this is not convenient, but when it is, loadcells of the kind illustrated in fig. are available for the purpose.

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