Weighing scale

This Mettler digital analytical balance gives the mass of an object at a precision of 0.1 milligram.

A weighing scale ("scale" in common usage) is a device for measuring weight, often of a person. Balances measure the mass of an object and are used in science to obtain the mass of an object. In many industrial and commercial applications, scales and balances to determine the weight and/or mass of things ranging from feathers to loaded tractor-trailers.

Contents

Occasionally, an appropriate weighing scale may be used to measure force rather than mass.

Balances

This electronic balance is used to measure mass 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 (known 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. 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.

When the weights on the plates of this balance are equal, the needle mid-rod points straight up.

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, which 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. 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. This design can still be seen 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 scales

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 (1809-1884), a professor at the University of Munich who invented the balance about 1874), use a spring with a known spring constant, 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. They also generally 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 scales

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 rail cars, as is done in a modern weigh bridge.

Hydraulic or pneumatic scales

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 regulations generally require periodic inspections by licensed technicians using weights which have a calibration that 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 labeled "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 percent 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. So, such a scale has to be re-calibrated after installation, for that specific location, in order to obtain an accurate weight measurement.

Laboratory balances

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.

Supermarket scale

A supermarket scale is used in bakery, deli, seafood, meat, produce and other perishable departments. Supermarket scales 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 Hobart Corporation, Bizerba, DIGI/Teraoka, Mettler Toledo, Cas, Berkel and Ishida.

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.
  • 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 mis-calibrated.
  • Mechanical components may be mis-aligned.
  • 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.
  • 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 (for example, 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.

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."

References

External links

All links retrieved August 9, 2013.

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