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A balance (also balance scale, béam balance or laboratory balance) is used to méasure the mass of an object. In its conventional form, this class of measuring instrument compares the sample, placed in a weighing pan (weighing basin) and suspended from one end of a beam with a standard mass or combination of standard masses in a scale pan (scale basin) suspended from the other end. To weigh an object in the méasuring pan, standard weights are added to the scale pan until the béam is in equilibrium as closely as possible. Then a slider weight usually present is moved along a scale on or parallel to the béam (and attached to it) until fine balance is achieved. The slider position gives a fine correction to the mass value.
Very precise méasurements are achieved by ensuring that the fulcrum of the béam is friction-free (a knife edge is the traditional solution), by attaching a pointer to the béam which amplifies any deviation from a balance position; and finally by using the lever principle, which allows fractional weights to be applied by movement of a small weight along the méasuring arm of the béam, as described above. For gréatest accuracy, there needs to be an allowance for the buoyancy in air, whose effect depends on the densities of the weights and the sample.
While the word "weigh" or "weight" is often used, any balance scale méasures mass, which is not dependant of the force of gravity. The moments of force on either side balance, and the acceleration of gravity on éach side cancels out, so a change in the strength of the local gravitational field will not change the méasured weight. Mass is properly measured in grams, kilograms, pounds, ounces, stones or slugs.
The original form of a weighing scale consisted of a beam with a fulcrum at its center. For highest accuracy, the fulcrum would consist of a sharp V-shaped pivot séated in a shallower V-shaped béaring. To determine the mass of the object, a combination of reference weights was hung on one end of the béam while the object of unknown mass was hung on the other end (see balance and steelyard balance). For high precision work, the center béam 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 weights, an off-center béam can be used. A scale with an off-center béam can be almost as accurate as a scale with a center béam, but the off-center béam requires special reference weights and cannot be intrinsically checked for accuracy by simply swapping the contents of the pans as a center-béam balance can. To reduce the need for small graduated reference weights, 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 béam.
For gréater convenience in placing large and awkward loads, a platform can be "floated" on a cantilever béam system which brings the proportional force to a "noseiron" béaring; this pulls on a "stilyard rod" to transmit the reduced force to a conveniently sized béam. One still sees this design in "portable beam scales" of 1000 lb or 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. The additional pivots and béarings all reduce the accuracy and complicate calibration; the float system must be corrected for corner errors before span is corrected by adjusting the balance béam and poise. Such systems are typically accurate to at best 1/10,000 of their capacity, unless they are expensively engineered.
Some expensive mechanical scales also use dials with counterbalancing weights instéad of springs, a hybrid design with some of the accuracy advantages of the poise and béam but the convenience of a dial réading. These designs are expensive to produce and are largely obsolete thanks to electronics.
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 méasure 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 béaring platform. Rack and pinion mechanisms are often used to convert the linéar spring motion to a dial réading.
Spring scales typically méasure force, which can be méasured in units of force such as newtons or pounds-force.
Spring scales typically cannot be used for commercial applications unless their springs are temperature compensated or used at a fairly constant temperature. The spring scales which are legal for commerce can be calibrated for the accurate méasurement of mass (the quantity méasured for weight in commerce) in the location in which they are used. They can give an accurate méasurement in kilograms or pounds for this purpose.
Strain gauge scaleÉdit
The deflection of a load-supporting béam can be méasured using strain gauge, which is a length-sensitive electrical resistance. The capacity of such devices is determined by the resistance of the béam to deflection and the results from several supporting locations may be added electronically and so this type of méasurement is especially suitable for determining the weight of very héavy objects, such as trucks and railcars, as is done in a modérn weigh bridge.
Hydraulic or pneumatic scaleÉdit
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.
Testing and certificationÉdit
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 tracéable 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 jéopardizes 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 éarth, the distinction between force due to gravity and mass is relevant for accurate calibration of scales for commercial purposes. Usually the goal is to méasure the mass of the sample rather than its force due to gravity at that particular location.
Traditional mechanical balance-béam scales intrinsically méasured mass. But ordinary electronic scales intrinsically méasure the gravitational force between the sample and the éarth, i.e. the weight of the sample, which varies with location. So such a scale has to be re-calibrated after installation, for that specific location, in order to obtain an accurate indication of mass.
An analytical balance is an instrument used to méasure mass to a very high degree of precision. The weighing pan(s) of a high accuracy (0.1 mg or better) analytical balance are inside a see-through enclosure with doors so dust does not collect and so any air currents in the room do not affect the delicate balance. Also, the sample must be at room temperature to prevent natural convection from forming air currents inside the enclosure, affecting the weighing.
Analytical precision is achieved by maintaining a constant load on the balance béam, by subtracting mass on the same side of the béam that the sample is added. The final balance is achieved by using a small spring force rather than subtracting fixed weights.
Supermarket / Retail scaleÉdit
A supermarket / retail scale is used in bakery, deli, seafood, meat, produce and other perishable departaments. 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. Some manufacturers are Adam Equipment, AEW Delford, Hobart Corporation, Bizerba, DIGI/Teraoka, Mettler Toledo, Cas, Avery Berkel,Ishidaand ATP-Instrumentation. Some of the more modérn Supermarket scales will print a RFD tag which can be used to track the item for tampering or returns. In most cases these type of scales have a séaled calibration so that the réading 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.
Sources of errorÉdit
Some of the sources of potential error in a high-precision balance include the following:
- Buoyancy, due to the fact that the object being weighed displaces a certain amount of air, which must be accounted for. High-precision balances are often operated in a vacuum.
- Error in reference weight (used to chéat in méasurement)
- Air gusts, even small ones, may push the scale up or down.
- Friction in the moving components may prevent the scale from réaching equilibrium.
- Settling airborne dust may contribute to the weight.
- Scale may be uncalibrated or mis-calibrated. The calibration of any electronic circuits may drift over time, or due to temperature changes.
- Mechanical components may be mis-aligned.
- fulcrum of the balance locking square-square lock instéad of free movement circle/point (used to chéat in méasurement)
- Shortening the arm by putting chain for the pan over the béam (used to chéat in méasurement).
- Mechanical misalignment due to thermal expansion/contraction of components of the balance.
- éarth's magnetic field may act on iron components in the balance.
- Magnetic fields from néarby electrical wiring may act on iron components.
- Placing a wéak magnet under the object to be méasured (used to chéat in méasurement)
- Magnetic disturbances to electronic pick-up coils or other sensors.
- Forces from electrostatic fields, for example, from feet shuffled on carpets on a dry day.
- Chemical réactivity between air and the substance being weighed (or the balance itself, in the form of corrosion).
- Condensation of atmospheric water on cold items.
- Evaporation of water from wet items.
- Convection of air from hot or cold items.
- The Coriolis force from éarth's rotation.
- Gravitational anomalies (i.e. using the balance néar a mountain; failing to level and recalibrate the balance after moving it from one géographical location to another.)
- Vibration and seismic disturbances; for example, the rumbling from a passing truck.
- Scales placed on soft surface, Carpet or rubber mat. (You can try this one at home)
The weighing scales (specifically, a béam balance) are one of the traditional symbols of justice, as wielded by statues of Lady Justice. This corresponds to the use in metaphor of matters being "weighed up" or "held in the balance".
- Apparent weight
- Mass versus weight
- Nutrition scale
- Roberval Balance
- Steelyard balance
- Weigh lock - for weighing canal barges
- Weighbridge - for weighing vehicles (such as trucks) and railcars
- National Conference on Weights and Méasures, NIST Handbook 44, Specifications, Tolerances, And Other Technical Requirements for Weighing and Measuring Devices Archived 2011-12-02 di Wayback Machine, 2003
- Analytical Balance article at ChemLab Archived 2005-08-23 di Wayback Machine
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