Digital kitchen scales.
A 'weighing scale' (usually just "scale" in common usage) is a device for measuring the
weight of an object. These scales are often used to
measure the weight of a
person, and are also used in
science to obtain the
mass of an object, and in many industrial and commercial applications to determine the weight of things ranging from feathers to loaded tractor-trailers.
Balance
A 'balance' (also 'balance scale', 'beam balance' or 'laboratory balance') is used to measure 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 measuring pan, standard
weights are added to the scale pan until the beam is in
equilibrium as closely as possible. Then a slider weight usually present is moved along a scale on or parallel to the beam (and attached to it) until fine balance is achieved. The slider position gives a fine correction to the mass value.
Table-top mass balance
Very
precise measurements are achieved by ensuring that the
fulcrum of the beam is
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 weights to be applied by
movement of a small weight along the measuring arm of the beam, as described above. For greatest 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 measures ''mass'', which is independent of the
force of
gravity. The
moments of force on either side balance, and the acceleration of gravity on each side cancels out, so a change in the strength of the local gravitational field will not change the measured weight. Mass is properly
measured in
grams,
kilograms,
pounds,
ounces, or
slugs.
Public mass balance for weighing people
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 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). 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 weights, an off-center beam can be used. A scale 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 weights 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 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 beam.
Digital bathroom scales
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 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 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 scales 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.
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. Rack and pinion mechanisms are often used to convert the linear spring motion to a dial reading.
Spring scales typically measure force, which can be measured 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 measurement of mass (the quantity measured for weight in commerce) in the location in which they are used. They can give an accurate measurement in kilograms or pounds for this purpose.
Strain gauge scale
The deflection of a load-supporting beam can be measured using
strain gauge, which is a length-sensitive electrical resistance. The capacity of such devices is determined by the resistance of the beam to deflection and the results from several supporting locations may be added electronically and so this type of measurement is especially suitable for determining the weight of very heavy objects, such as trucks and railcars, as is done in a modern
weigh bridge.
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.
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 .5% over the surface of the earth, the
distinction between weight and mass is relevant for accurate calibration of scales for commercial purposes. Usually the goal is to measure the mass of the sample rather than its weight at that particular location.
Traditional mechanical balance-beam scales intrinsically measured
mass. But ordinary electronic scales intrinsically measure the
gravitational force between the sample and the earth, 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.
Analytical balance
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. 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 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.
Supermarket / Retail scale
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 modern 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 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.
Sources of error
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 cheat in measurement)
★ Air gusts, even small ones, may push the scale up or down.
★
Friction in the moving components may prevent the scale from reaching 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 instead of free movement circle/point (used to cheat in measurement)
★
★ Shortening the arm by putting chain for the pan over the beam (used to cheat in measurement).
★ Mechanical misalignment due to
thermal expansion/contraction of components of the balance.
★ Earth's
magnetic field may act on iron components in the balance.
★ Magnetic fields from nearby electrical wiring may act on iron components.
★
★ Placing a weak magnet under the object to be measured (used to cheat in measurement)
★ 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 reactivity 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 Earth's rotation.
★ Gravitational anomalies (''i.e.'' using the balance near a mountain; failing to level and recalibrate the balance after moving it from one geographical 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)
Symbolism
The weighing scales (specifically, a beam 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".
See also
★
Apparent weight
★
Roberval Balance
★
Steelyard balance
★
Weigh lock - for weighing canal barges
★
Weighbridge - for weighing vehicles (such as trucks) and railcars
External links
★ National Conference on Weights and Measures, NIST Handbook 44, ''
Specifications, Tolerances, And Other Technical Requirements for Weighing and Measuring Devices'', 2003
★
Analytical Balance article at ChemLab
★
Laser beams pluck nano-strings
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