Weighing Scale: A device used to determine weight. Weighing scales are divided into two main categories: Spring or Load-Cell Scales and Balances. Spring, and load cell scales measure the amount of deflection on the spring or load cell caused by the applied load. A balance measures weight using a fulcrum and a balance beam with the unknown mass placed on the load-accepting element at one end, and a known, adjustable weight or electro-magnetic force applied to the other end. When equilibrium is met and the beam is in the balance position, the weights of the known mass or force and the object on the load-accepting element are equal. Spring scales and strain gauge scales are the most common, but have two sources of error that balances do not. One source of error comes from temperature which can change the elasticity of a scales spring or load cell and therefore affect the rate of deflection. Temperature compensated springs or load cells can help correct this problem to a certain degree. The other source of error comes from the difference of local gravitational forces around the world which can vary as much as 0.5% at differing geographical locations. Because balances do not measure deflection and because the forces acting on either side of the balance beam are equal regardless of local gravity, balances do not have these sources of error and are best suited for precision weighing applications. See Types of Weighing Scales.
Accuracy :The degree to which measurements of a quantity relate to its actual (true) value. In weighing scales, accuracy is a product of instrument errors such as readability, repeatability (see Precision), linearity, and sensitivity drift.
Precision: The degree to which repeated measurements under unchanged conditions show the same results. Precision is also called reproducibility or repeatability.
Adjustment :The set of operations carried out on a measuring system so that it provides prescribed indications corresponding to given values of a quantity to be measured. On a weighing scale, types of adjustment include zero adjustment and span calibration adjustment. Calibration adjustment is often mistakenly called calibration.
Calibration: The set of operations that establish, under specified conditions, the relationship between the values of quantities indicated by a measuring instrument and the corresponding values realized by reference standards. Calibration should not be confused with adjustment of a measuring system, often mistakenly called “self-calibration” or “auto-calibration” (see Adjustment).
Maximum Capacity, “Max” : The largest load that may be accurately weighed. Think of the scale like a 12in. ruler. The ruler can only measure items less than 12in. long. Similarly, a scale with a 5,000lb. capacity can only weigh items less than 5,000lb. in weight. When purchasing a scale, you should choose a model that has a maximum capacity which exceeds the weight of the heaviest item you need to weigh at one time, while also considering the scales’ division value (see Scale Division, value of, “d”). Unlike a ruler, if you weigh an item that exceeds the capacity of a scale, you may overload it and cause permanent damage (see Overload Capacity).
Scale Division, value of, “d” – The value of the scale division or interval, expressed in units of mass, is the smallest subdivision of the scale for analog indication or the difference between two consecutively indicated or printed values for digital indication or printing. Using the ruler analogy again, the scale division value would be like the distance between marks along a ruler. A ruler which has 16 marks or “divisions” per inch would have a hypothetical scale division value of 1/16 th in. Manufacturers may also refer to the scale division value as its Display Resolution, Graduation, or Readability.
Scale Division, number of, “n” – Quotient of the maximum capacity divided by the value of the scale division.
Where = Number of Scale Divisions
= Maximum Capacity
= Verification Scale Interval (in many cases e = d )
i.e. If a scale has a 500lb maximum capacity (Max) and a scale division of 0.05lb (d), it would have 10,000 scale divisions ( n=10,000d). In the ruler analogy, the number of spans between marks along the ruler’s edge would be like the number of scale divisions on a weighing scale.
Verification Scale Division, value of, “e” : A value, expressed in units of weight (mass) and specified by the manufacturer of a device, by which the tolerance values and the accuracy class applicable to the device are determined. In most cases, the verification scale division (e) is the same value as the displayed scale division (d). However, in some cases the verification scale division may be different than the displayed scale division such as in devices used for weight classifying or weighing in pre-determined amounts, and certain other Class I and II scales.
Multi-Interval Scale – A scale having one weighing range which is divided into partial weighing ranges (segments), each with different scale intervals, with each partial weighing range (segment) determined automatically according to the load applied, both on increasing and decreasing loads.
Accuracy Class: Weighing Devices are divided into accuracy classes according to the number of scale divisions (n) and the value of the scale division (d).
NTEP Certificate of Conformance, “C.C.” - The National Conference on Weights and Measures issues an NTEP Certificate of Conformance following successful completion of an evaluation of a device. It indicates that the device(s) described in the Certificate is/are capable of meeting applicable requirements of the NIST Handbook 44. This means that the device is “legal for trade” and has been approved to charge or pay based on weight.
Approval Seal : A label, tag, stamped or etched impression, or the like, indicating official approval of a device.
Calibration Certificate: A Calibration Certificate is a document provided and signed by a calibration technician that documents the completion of a successful calibration. The certificate will typically list the standard that was used to calibrate the device and provides traceability to the internationally defined standard. For scales and balances, calibration certificates are only valid in the state in which the device
was calibrated. This is due to the change of local gravitational acceleration which can vary as much as 0.5% at various locations around the world.
Load-Receiving Element :That element of a scale that is designed to receive the load to be weighed; for example, platform, deck, rail, hopper, platter, plate, scoop. The dimensions of the load-receiving element or platform should be considered when selecting a scale. You can often use a scale with a platform slightly smaller than the object(s) being weighed as long as the load is stable and does not lean against anything except the load-receiving element, and is under the scales max capacity. You can also use an expansion tray or container to effectively increase the size of the weighing platform or load- receiving element on smaller, compact scales. Strain Gauge Load Cell – An electronic device (transducer) that is used to convert a force into an electrical signal using a strain gauge. Strain gauges are small measuring devices which contain a metallic foil pattern on a flexible backing. As the load cell deforms from the weight placed on it, the strain gauge’s foil is deformed, causing a change in its electrical resistance. This change is measured by a Wheatstone bridge configuration.
Weight – The force that results from the action of gravity on matter.
Mass – The measure of the amount of matter in a body.
Mass vs. Weight – In everyday situations, mass and weight mean the same thing. In scientific situations, weight and mass represent different quantities. The weight of an object is the magnitude, W, of the force of gravity on that object and is proportional to its mass.
Where = weight
= mass
= local gravitational acceleration ( 9.8 m/s 2 avg. for Earth)
Kilogram, “kg” – the base unit of mass in the International System of Units (SI Units). It is equal to the
mass of the International Prototype Kilogram (IPK).
International Prototype Kilogram, “IPK” - . The kilogram was originally the mass of a cubic decimeter of water. In 1889, the 1st CGPM sanctioned the international prototype of the kilogram, made of platinum-iridium, and declared: This prototype shall henceforth be considered to be the unit of mass .The International Prototype Kilogram is stored and maintained at the International Bureau of Weights and Measures (French Abbreviation: BIPM) along with its six official copies. The kilogram is the only SI unitstill defined by a physical artifact. Efforts are being made though to produce a future, more stable kilogram standard that can be reproduced in a laboratory using written specifications. One such project uses a sphere of a specific number of silicon atoms to define the kilogram. Experiments from this project have produced some of the most near-
perfect man-made spheres to date. Other projects use an electronic approach, such as the NIST’s watt balance which measures the electric power necessary to oppose the weight of a kilogram test weight under earth’s gravity.
Gross Weight – The total weight of the object being weighed including its vehicle, packaging, or container. Gross weight is typically required for calculating the shipping or transportation charge.
Net Weight – The weight of an object being weighed, discounting the weight of its vehicle, packaging, or container. Net weight is useful for calculating the charge, tax, or payment required for items.
Tare Weight – The weight of an empty vehicle, package, or container. Tare weight is sometimes written on the outside of railcars or shipping and packing containers for quick determination of the net weight during weighing operations.
Tare Mechanism – A mechanism (including a tare bar) designed for determining or balancing out the weight of packing material, containers, vehicles, or other materials that are not intended to be included in net weight determinations. Most electronic scales have a tare button or feature which is used to reset the display value to zero when a container is placed on its load-receiving element. If the vehicle or
container is then filled, the displayed weight will be that of the contents alone (see Net Weight). If the vehicle or container and its contents are removed from the scale, a negative weight value is typically displayed equal in absolute value to the Tare Weight.
Zero-Setting Mechanism – Means provided to attain a zero balance indication with no load on the load- receiving element. Three types used in scales are:
Automatic Zero-Tracking Mechanism – Automatic means provided to maintain the zero balance indication within certain limits, without intervention of an operator.
Manual Zero-Setting Mechanism – Nonautomatic means provided to attain a zero balance indication by the direct operation of a control. indication requiring a single initiation by an operator. Span – The distance between adjoining sections of a scale.
Some Various Types of Weighing Scales
Analytical Balance – One which measures mass to a very high degree of precision and accuracy. Most analytical balances have a scale division of 0.1mg or better.
Animal Scale – A scale designed for weighing single heads of livestock.
Checkweighing Scale – One used to verify predetermined weight within prescribed limits. These scalesare typically used in weighing operations where the operator must fill and weigh a product to ensure uniform weight. Some checkweighers will activate remote switches when the target weight has been met.
Counting Scale – One used to weigh multiple objects of uniform weight and display a total piece count.
Computing Scale – One that indicates the money values of amounts of commodity weighed, at predetermined unit prices, throughout all or part of the weighing range of the scale.
Crane Scale – One with a nominal capacity of 5000 pounds or more designed to weigh loads while they are suspended freely from an overhead, track-mounted crane.
Jewelers’ Scale – One adapted to weighing gems and precious metals.
Postal Scale – A scale (usually a computing scale) designed for use to determine shipping weight or delivery charges for letters or parcels delivered by the U.S. Postal Service or private shipping companies.A weight classifier may be used as a postal scale.
Point-of-Sale Scale – A scale used to complete a direct sales transaction.
Prescription Scale – A scale or balance adapted to weighing the ingredients of medicinal and other formulas prescribed by physicians and others used or intended to be used in the ordinary trade of pharmacists.
Vehicle Scale – A scale adapted to weighing highway, farm, or other large industrial vehicles (except railroad freight cars), loaded or unloaded.
Weight Classifier – A digital scale that rounds weight values up to the next scale division. These scales usually have a verification scale division (e) that is smaller than the displayed scale division (d). Example: A postal weight classifier that has a verification scale division value (e) of 1 g which. For the purpose of postal rate calculation, the scale can be set to display weight with a scale interval (d) of 0.05oz (Inch-
Pound Units).
Wheel-Load Weigher – Compact, self-contained, portable weighing elements specially adapted to determining the wheel loads or axle loads of vehicles on highways for the enforcement of highway weight laws only.
Sources of Error in Weighing Instruments
Environmental Factors – A scale’s accuracy and precision are highly dependent on the environment in which it is installed. Several environmental factors can affect the scales measurement including: Air Currents / drafts – These account for most large random errors. Be sure to use your weighing device in an area free of any drafts or air currents that may affect the weight readout.
On high precision analytical balances (0.1mg or better), glass draft shields are required. Care should also be taken when weighing objects that are hot or cold inside a draft chamber. The effect of convection currents can make cold objects appear heavier, and hot objects appear
lighter.
Air Buoyancy – The upward force, caused by atmospheric pressure. The net upward buoyancy force is equal to the magnitude of the weight of air displaced by an object. Air buoyancy is mostly a concern when weighing objects of relatively low density.
Temperature – Spring scales and load cell scales deflect at a lower rate and consequentlyperform poorly under cold conditions. Most springs and load cells are temperature compensated to counteract this source of error to a degree. The scale should always be used within the manufacturer’s recommended operating temperature. For most scales this is between 32°F and 104°F. When moving a scale from one climate to another, you should allow the internal components to acclimate their new environment before performing calibration.
Zero Error – Occurs when the weighing curve shifts by a constant amount. You can eliminate this error by using the zero-setting mechanism before a weighment.
Sensitivity Error – Quotient of the change in an indication of a measuring system and the corresponding change in a value of a quantity being measured. Sensitivity of a measuring system can depend on the value of a quantity being measured. That is, the error increases with heavier loads. Sensitivity errors can occur from temperature drift, aging, adjusting with an incorrect calibration weight, or incorrect compensation of an off-center load error.
Linearity Error – Defines how well the device's actual performance across a specified operating range approximates a straight line.
Random Error – The sample standard deviation of the error (indicated values) for a number of
consecutive automatic weighings of a load, or loads, passed over the load receptor, shall be expressed
mathematically as:
or Where: = error of a load indication
= the number of loads
=
Scale Characteristic Curve
Works Cited
Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices, NIST
Handbook 44.; National Institute of Standards and Technology: Gaithersburg, MD., 2010
“Base unit definitions: Kilogram.” International System of Units from NIST. Oct. 2000. Web. 8 Jan. 2010.
<http://physics.nist.gov/cuu/Units/kilogram.html> .
“Terms and Definitions.” Weighing Systems. 2005. Web. 8 Jan. 2010. <http://www.weighing-
systems.com/TechnologyCentre/terms.html>.
“Accuracy.” Wikipedia, The Free Encyclopedia. Wikimedia Foundation, Inc. 3 Jan. 2010. Web. 8 Jan.
2010. < http://en.wikipedia.org/wiki/Accuracy> .