Weighing scale
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.
Weighing scales are also sometimes used to measure
force rather than mass.
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Electronic mass balance, used in a school laboratory. |
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.
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.
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. 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.
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.
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A spring weighing scale can measure forces transmitted through the scale in any direction. |
Some weighing scales such as a
Jolly balance (named after
Phillipp Gustav 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.
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.
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.
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 issue of "weight" vs. "mass" becomes relevant for accurate calibration of scales for commercial purposes. The goal is to measure the weight (actually, the mass) in relation to the legal reference standards (not the true scientific local weight at that particular location).
Traditional mechanical balance-beam scales intrinsically measured weight compared to standards. But modern ordinary electronic scales intrinsically measure downward force, the "local weight" at that location -- not a good thing! So such a scale has to be re-calibrated after installation, for that specific location, in order to obtain an accurate weight measurement.
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When the weights on the plates of this balance are equal, the needle mid-rod points straight up. |
A
balance (also
beam balance or
laboratory balance) is used to accurately measure the
mass of an object. This class of
measuring instrument uses a comparison technique in its conventional form of a
beam from which a weighing pan and scale pan are suspended. To weigh an object, it is placed on the measuring pan, and standard
weights are added to the scale pan until the beam is in
equilibrium.
While the word "weigh" or "weight" is often used, any balance scale actually measures
mass, which is not dependent upon the
force of
gravity, as opposed to a scale with a spring, which measures weight. Mass is properly
measured in
grams,
kilograms,
pounds,
ounces, or
slugs; while weight is in
newtons or
pounds force.
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.
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.
*
Roberval Balance*
Apparent weight*
Weigh lock - for weighing canal barges
*
Weigh bridge - for weighing trucks and railcars
*
Digital Scales Magazine (review source) * National Conference on Weights and Measures, NIST Handbook 44,
Specifications, Tolerances, And Other Technical Requirements for Weighing and Measuring Devices, 2000
*
Analytical Balance article at ChemLab*
Laser beams pluck nano-strings*
Mettler Toledo - Manufacturer of scales*
My Weigh - Manufacturer of scales*
Jennings - Manufacturer of scales*
Tanita - Manufacturer of scales*
Scalesmart - Manufacturer & Retailer of scales*
MWS Ltd - Manufacturer of scales
