Weighing scale
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Doctors_scale.jpg
A weighing scale (usually just "scale" in common usage) is a device using 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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Balances
A balance (also balance scale, 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 (weighing bason) and scale pan (scale bason) 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.
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. See analytical balance for the type used in high precision measurements.
While the word "weigh" or "weight" is often used, any balance scale actually measures mass, which is not dependent upon 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 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. 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.
In order to reduce the need for large reference weights an off-center beam can be used. This design can be almost as accurate as the center beam, but 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. This 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 / 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 very 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
Some weighing scales 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 accurage 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.
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.
Electronic scales
There are two technologies commonly in use to measure weight electronically. The most obvious is force restoration, in which an electromagnet is used to pull against the weight (usually after it has been transmitted to some form of stilyard by an appropriate lever or flexure system). The current in the electromagnet is adjusted by a feedback mechanism until a "null sensor" is centered; at this point the current being fed to the electromagnet is directly proportional to the weight. Force restoration is used in the most accurate and expensive electronic balances, offering resolutions to 1 part in 1,000,000 and beyond. It is rarely used in less expensive systems.
The more common electronic force sensor is the strain gage load cell. In a load cell, flexures or bearings are positioned to transmit the weight force to a beam in such a manner as to eliminate the effects of side loading, torque, and so on. The beam bends like a very stiff spring when weight is applied. Strain gages are very thin flexible resistive elements which are bonded to this sense beam, generally two so that they will be stretched and two so that they will be contracted as weight is applied. The four gages are then connected in a Wheatstone Bridge. The analog signal generated by such a cell is typically around 20 millivolts at full capacity. The full usable capacity of such a cell will depend on its mechanical construction, and the usable resolution is usually about 1/10,000 of that capacity.
Currently, the industry standard "electronic scale indicator" is designed to read the signal from such a Wheatstone-bridge cell, and bridges or platforms need not be matched to indicators made by the same manufacturer for compatibility.
A large scale such as a truck scale or large platform can be placed directly on top of appropriately designed load cells, or a smaller load cell can be placed inline with the stilyard rod of a mechanical system. Because they are more expensive to manufacture and harder to service large mechanical lever systems are rarely produced today, though they still have application in corrosive and explosive environments.
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.
See also
External links
- National Conference on Weights and Measures, NIST Handbook 44, Specifications, Tolerances, And Other Technical Requirements for Weighing and Measuring Devices (http://ts.nist.gov/ts/htdocs/230/235/h44.htm), 2000
- body fat scale,kitchen scale (http://www.scalemanufacturer.com)
