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Digital kitchen scale, which works by the spring scale principle.
Digital kitchen scale, which works by the spring scale principle.

A weighing scale (usually just "scale" in common usage; except in Australian English where "scales" is more common) is a measuring instrument for measuring the weight of an object (the gravitational attraction between the object and the Earth). Australian English ( AuE, AusE, en-AU) is the form of the English language used in Australia. In the Physical sciences Quality assurance, and Engineering, Measurement is the activity of obtaining and comparing physical quantities In the Physical sciences weight is a Measurement of the gravitational Force acting on an object They use one of two techniques. A spring scale measures weight by the distance a spring deflects under its load. SPRING directs here for other uses of the word see Spring SPRING is GIS and remote sensing image processing system with an object-oriented A balance, described below, compares the unknown weight to a standard weight using a horizontal lever. Weighing scales are used in many industrial and commercial applications, and products from feathers to loaded tractor-trailers are sold by weight. Specialized medical scales and bathroom scales are used to measure the body weight of human beings. A weighing scale (usually just "scale" in common usage except in Australian English where "scales" is more common is a Measuring instrument for

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Balance

A balance (also balance scale, beam balance or laboratory balance) is used to measure the weight of an object. In the Physical sciences weight is a Measurement of the gravitational Force acting on an object It was the first weight measuring instrument invented. In its traditional form, it consists of a pivoted horizontal lever of equal length arms, called the beam, with a weighing pan, also called scale (hence the term "scales") scalepan, or bason (obsolete [1]) suspended from each arm. Beam may refer to Beam (structure, a construction element Beam (nautical, the most extreme width (or breadth of a nautical vessel or a The unknown weight is placed in one pan, and standard weights are added to the other pan until the beam is in equilibrium as closely as possible. In precision balances, a slider weight is moved along a graduated scale on the beam until fine balance is achieved. The slider position gives a fine correction to the weight value. Since the mass of an object is proportional to its weight, a scale can be labeled to display the object's mass. Mass is a fundamental concept in Physics, roughly corresponding to the Intuitive idea of how much Matter there is in an object

Traditional balance
Traditional balance

Balances are used for precision weight measurement, because unlike spring scales their accuracy is not affected by differences in the local gravity, which can vary by almost 0. 5%[2] at different locations on Earth. A change in the strength of the gravitational field caused by moving the balance will not change the measured weight, because the moments of force on either side of the balance beam are affected equally. In Physics, the moment of force (often just moment, though there are other quantities of that name such as Moment of inertia) is a Pseudovector

Very precise measurements are achieved by ensuring that the fulcrum of the beam is essentially friction-free (a knife edge is the traditional solution), by attaching a pointer to the beam which amplifies any deviation from a balance position; and finally by using the lever principle, which allows fractional masses to be applied by movement of a small weight along the measuring arm of the beam, as described above. Friction is the Force resisting the relative motion of two Surfaces in contact or a surface in contact with a fluid (e Generally an amplifier or simply amp, is any device that changes usually increases the amplitude of a signal. In Mathematics, a fraction (from the Latin fractus, broken is a concept of a proportional relation between an object part and the object In Physics, motion means a constant change in the location of a body For greatest accuracy, there needs to be an allowance for the buoyancy in air, whose effect depends on the densities of the masses and the sample. In Physics, buoyancy ( BrE IPA: /ˈbɔɪənsi/ is the upward Force on an object produced by the surrounding liquid or gas in which it is

The original form of a balance consisted of a beam with a fulcrum at its center. Beam may refer to Beam (structure, a construction element Beam (nautical, the most extreme width (or breadth of a nautical vessel or a For highest accuracy, the fulcrum would consist of a sharp V-shaped pivot seated in a shallower V-shaped bearing. To determine the mass of the object, a combination of reference weights was hung on one end of the beam while the object of unknown mass was hung on the other end (see balance and steelyard balance). A steelyard balance or steelyard is a straight-beam balance with arms of unequal length For high precision work, the center beam balance is still one of the most accurate technologies available, and is commonly used for calibrating test weights.

To reduce the need for large reference masses, an off-center beam can be used. A balance with an off-center beam can be almost as accurate as a scale with a center beam, but the off-center beam requires special reference masses and cannot be intrinsically checked for accuracy by simply swapping the contents of the pans as a center-beam balance can. To reduce the need for small graduated reference masses, a sliding weight called a poise can be installed so that it can be positioned along a calibrated scale. A poise adds further intricacies to the calibration procedure, since the exact mass of the poise must be adjusted to the exact lever ratio of the beam.

For greater convenience in placing large and awkward loads, a platform can be "floated" on a cantilever beam system which brings the proportional force to a "noseiron" bearing; this pulls on a "stilyard rod" to transmit the reduced force to a conveniently sized beam. One still sees this design in "portable beam balances" of 500 kg capacity which are commonly used in harsh environments where electricity is not available, as well as in the lighter duty mechanical bathroom "scale" (misnamed, since it is actually a balance). The additional pivots and bearings all reduce the accuracy and complicate calibration; the float system must be corrected for corner errors before span is corrected by adjusting the balance beam and poise. Such systems are typically accurate to at best 1/10,000 of their capacity, unless they are expensively engineered.

Some expensive mechanical balances also use dials with counterbalancing weights instead of springs, a hybrid design with some of the accuracy advantages of the poise and beam but the convenience of a dial reading. These designs are expensive to produce and are largely obsolete thanks to electronics.

Spring scale

A spring weighing scale can measure forces transmitted through the scale in any direction.
A spring weighing scale can measure forces transmitted through the scale in any direction.
Main article: Spring scale

In a typical Spring Scale, the spring stretches (as in a hanging scale in the produce department of a grocery store) or compresses (as in a simple bathroom scale) in proportion to how hard the Earth pulls down on the object. A spring scale is a Weighing scale used to measure Force, such as the force of Gravity, exerted on a Mass or the force of a person's grip Every spring has a proportionality constant that relates how hard you pull it to how far it stretches. Some weighing scales such as a Jolly balance (named after Philipp von Jolly who invented the balance about 1874) use a spring with a known spring constant (see Hooke's law) and measure the displacement of the spring by any variety of mechanisms to produce an estimate of the gravitational force applied by the object, which can be simply hung from the spring or set on a pivot and bearing platform. Johann Philipp Gustav von Jolly ( September 26 1809 – December 24 1884 in Munich) was a German Physicist and Mathematician Year 1874 ( MDCCCLXXIV) was a Common year starting on Thursday (link will display the full calendar of the Gregorian calendar (or a Common A spring is a flexible elastic object used to store mechanical Energy. In Mechanics, and Physics, Hooke's law of elasticity is an approximation that states that the amount by which a material body is deformed (the Gravitation is a natural Phenomenon by which objects with Mass attract one another Rack and pinion mechanisms are often used to convert the linear spring motion to a dial reading.

Spring scales measure force, which is measured in units of force such as Newtons or pounds-force. The newton (symbol N) is the SI derived unit of Force, named after Isaac Newton in recognition of his work on Classical This article deals with the unit of force For the unit of mass see Pound (mass.

Spring scales typically cannot be used for commercial applications unless their springs are temperature compensated or used at a fairly constant temperature. Spring scales which are legal for commerce can be calibrated for the accurate measurement of mass in the location in which they are used. They can give an accurate measurement in kilograms or pounds for this purpose.

Strain gauge scale

In electronic versions of spring scales, the deflection of a beam supporting the unknown weight is measured using a strain gauge, which is a length-sensitive electrical resistance. A strain gauge (alternatively strain gage) is a device used to measure the strain of an object Electrical resistance is a ratio of the degree to which an object opposes an Electric current through it measured in Ohms Its reciprocal quantity is The capacity of such devices is only limited by the resistance of the beam to deflection. The results from several supporting locations may be added electronically, so this technique is especially suitable for determining the weight of very heavy objects, such as trucks and railcars, and is used in a modern weigh bridge. Truck scales are large floor mounted weighing systems that can weigh entire vehicles and their contents

Hydraulic or pneumatic scale

It is also common in high-capacity applications such as crane scales to use hydraulic force to sense weight. The test force is applied to a piston or diaphragm and transmitted through hydraulic lines to a dial indicator based on a Bourdon tube or electronic sensor. Many techniques have been developed for the measurement of Pressure and Vacuum.

Testing and certification

Most countries regulate the design and servicing of scales used for commerce. This has tended to cause scale technology to lag behind other technologies because expensive regulatory hurdles are involved in introducing new designs. Nevertheless, there has been a recent trend to "digital load cells" which are actually strain-gage cells with dedicated analog converters and networking built into the cell itself. Such designs have reduced the service problems inherent with combining and transmitting a number of 20 millivolt signals in hostile environments.

Government regulation generally requires periodic inspections by licensed technicians using weights whose calibration is traceable to an approved laboratory. Scales intended for casual use such as bathroom or diet scales may be produced, but must by law be labelled "Not Legal for Trade" to ensure that they are not repurposed in a way that jeopardizes commercial interest. In the United States, the document describing how scales must be designed, installed, and used for commercial purposes is NIST Handbook 44.

Because gravity varies by over 0. 5% over the surface of the earth, the distinction between force due to gravity and mass is relevant for accurate calibration of scales for commercial purposes. In the Physical sciences weight is a Measurement of the gravitational Force acting on an object Usually the goal is to measure the mass of the sample rather than its force due to gravity at that particular location.

Traditional mechanical balance-beam scales intrinsically measured mass. Mass is a fundamental concept in Physics, roughly corresponding to the Intuitive idea of how much Matter there is in an object But ordinary electronic scales intrinsically measure the gravitational force between the sample and the earth, i. Newton 's law of universal Gravitation is a physical law describing the gravitational attraction between bodies with mass e. the weight of the sample, which varies with location. In the Physical sciences weight is a Measurement of the gravitational Force acting on an object So such a scale has to be re-calibrated after installation, for that specific location, in order to obtain an accurate indication of mass.

Analytical balance

Mettler digital analytical balance with 0.1 mg precision.
Mettler digital analytical balance with 0. 1 mg precision.

An analytical balance is an instrument used to measure mass to a very high degree of precision. Mass is a fundamental concept in Physics, roughly corresponding to the Intuitive idea of how much Matter there is in an object The weighing pan(s) of a high precision (. 1,. 01, mg or better) analytical balance are inside a transparent enclosure with doors so dust does not collect and so any air currents in the room do not affect the delicate balance. For other uses of the words gram or gramme see Gram (disambiguation. Dust is a general name for minute Solid particles with Diameters less than 500 micrometers. The use of a Vented Balance Safety Enclosure that has uniquely designed acrylic airfoils allows a smooth turbulence-free airflow that prevents balance fluctuation and the weighing of mass down to 1μg without fluctuations or loss of product. Also, the sample must be at room temperature to prevent natural convection from forming air currents inside the enclosure, affecting the weighing. Room temperature (also referred to as ambient temperature) is a common term to denote a certain Temperature within enclosed space at which humans are accustomed Convection in the most general terms refers to the movement of molecules within Fluids (i

Analytical precision is achieved by maintaining a constant load on the balance beam, by subtracting mass on the same side of the beam that the sample is added. The final balance is achieved by using a small spring force rather than subtracting fixed weights.

Vented balance safety enclosure

Supermarket / Retail scale

A supermarket / retail scale is used in bakery, deli, seafood, meat, produce and other perishable departaments. Vented balance safety enclosures are used in pharmaceutical chemical biological and toxicological laboratories to provide maximum containment for weighing operations in Weighing scales A bakery (also called baker's or bakehouse) is an establishment which produces or/and sells Bread, pies pastries Cakes Biscuits Seafood is any Sea Animal or Seaweed that is served as Food, or is suitable for eating particularly saltwater animals such In modern English usage meat most often refers to Animal tissue used as food mostly Skeletal muscle and associated Fat, but it may also refer Produce is a generalized term for a group of Farm -produced goods generally limited to Fruits and Vegetables More specifically the term "produce" Supermarket scales can print labels and receipts (in bakery specially), marks Weight/Count, Unit Price, Total Price and in some cases Tare, a supermarket label prints weight/count, unit price and total price. A bakery (also called baker's or bakehouse) is an establishment which produces or/and sells Bread, pies pastries Cakes Biscuits Some manufacturers are Adam Equipment, AEW Delford, Hobart Corporation, Bizerba, DIGI/Teraoka, Avery India, Mettler Toledo, CAS, Avery Berkel, Ishida and ATP-Instrumentation. Hobart is a manufacturer of equipment used in Food service operations Bizerba GmbH & Co KG is a German food processing equipment enterprise Mettler-Toledo ( is a manufacturer of scales and analytical instruments Avery Berkel, a major manufacturer of weighing systems and Food processing equipment was formed in 1993 when the British conglomerate GEC Some of the more modern supermarket scales will print an RFD tag which can be used to track the item for tampering or returns. In most cases these type of scales have a sealed calibration so that the reading on the display is correct and cannot be tampered with - in the USA the approval is NTEP, for South Africa it is SABS, the UK it is OIML. The International Organization of Legal Metrology or Organisation Internationale de Métrologie Légale (OIML is an international treaty organization

Sources of error

An old two pan balance.
An old two pan balance.

Some of the sources of potential error in a high-precision balance include the following:

Symbolism

The scales (specifically, a two pan, beam balance) are one of the traditional symbols of justice, as wielded by statues of Lady Justice. JUSTICE is a Human rights and law reform organisation based in the United Kingdom. "Lady Justice" or "Lord Justice" is also the title of judges on the Court of Appeal of England and Wales. This corresponds to the use in metaphor of matters being "held in the balance".

See also

External links

Footnotes

  1. ^ "A Practical Dictionary of the English and German Languages" (1869) p.1069
  2. ^ Hodgeman, Charles, Ed. (1961). Handbook of Chemistry and Physics, 44th Ed. . Cleveland, USA: Chemical Rubber Publishing Co. .   p. 3480-3485
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