Weighing Scale

A device used to determine weight. Weighing scales can be divided into two primary types: spring scales and balances. Spring scales measure weight using the principal of the spring (Hooke's Law) which deforms in proportion to the weight placed on the load receiving end. Most digital scales use a special type of spring called a strain gauge load cell which measures the deformation or stress exerted using electrical current. Balances are the oldest type of weighing device and measure weight using the principal of the lever. An unknown weight is placed on one end of the lever and balanced against a known weight on the other end. Modern electronic balances use an electromagnet to balance the beam and determine the mass. This is called electromagnetic force restoration/compensation.


Accuracy

The degree to which a measurement relates to its actual (true) value. The accuracy of a weighing device is dependent on several factors including the readability, calibration, and surrounding environment. In general, when properly calibrated, most scales should be accurate to within ±2 divisions (or digits). Accuracy of a measuring device is not the same as it's precision which is also called repeatability.


Repeatability

This is the scale's ability to show consistent results under the same conditions (same device, same operator, same environment). To determine a scale's repeatability, a test weight is placed on the scale then removed several times while recording each weight result. The repeatability measures how spread out the values are around the mean or average value.


Reproducibility

This is the scale's ability to show consistent results under different operating conditions (different users, different labs)


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. Scales are subject to constant wear and tear which over time can degrade accuracy. Adjustment corrects a scales accuracy so that it is within the tolerance applied to the device.


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. Basically, calibration is the process of weighing a known weight on a scale and noting the discrepency on the display. After performing calibration, an adjustment is sometimes performed to correct the scales sensitivity.


Maximum Capacity

This is the maximum weight that can be measured using a particular scale. When selecting a scale, the heaviest item you plan to weigh should be within the scale's maximum capacity. It is a good idea to select a scale with slightly more capacity than you will need to avoid overloading. However, the higher the capacity is on a scale, the lower the readability typically will be. Therefore, you should avoid selecting a scale with a capacity much higher than the heaviest item you intend to weigh.


Readability

On electronic and digital scales, this is the smallest change in mass that corresponds to a change in displayed value. In other words, this is the smallest step that the scale will increment by as weight is added or removed. On analog (mechanical) scales, this is the smallest subdivision of the scale dial or beam for analog indication.


Verification Scale Interval

This is the smallest scale interval or step that can be used to determine price based on weight in commercial transactions for a particular scale. The value of the verification scale interval (e) is determined by the scale manufacturer when submitting a device for type approval through a program such as NTEP (or CE for EU countries).


Uncertainty of Measurement

This is a parameter that is used to state the quality of a measurement. Because no measuring instrument is 100% accurate, when recording measured data the measurement uncertainty is used to give the range over which the true value is likely to lie. It is calculated by taking into consideration all the possible errors (variations) that arise from the measurement such as repeatability, linearity, etc.


Accuracy Class

Scales that are intended to be used in commerce are grouped into accuracy classes according to the number of scale divisions (n) and the value of the scale division (d or e). These accuracy classes are meant to determine the intended area of use for a particular scale and also dictate the tolerances applied to the device during testing.

Class
Value of the Verification Scale Interval (e) in SI Units
Number of scale divisions (n)
 
Intended Application
Minimum
Maximum
I
≥1 mg
50,000
 
Precision Laboratory Weighing
II
 
1 to 50 mg, inclusive
100
100,000
Laboratory weighing, precious metals and gem weighing, grain test scales, medical cannabis
 
≥100 mg
5,000
100,000
III
 
0.1 to 2 g, inclusive
100
10,000
All commercial weighing not otherwise specified, grain test scales, retail precious metals and semi-precious gem weighing, animal scales, postal scales, vehicle on-board weighing systems with a capacity less than or equal to 30,000 lb, and scales used to determine laundry charges
 
≥5 g
500
10,000
IIIL
≥2 kg
2,000
10,000
Vehicle scales, vehicle on-board weighing systems with a capacity greater than 30,000 lb, axle-load scales
IIII
≥5 g
100
1,200
Wheel-load weighers and portable axle-load weighers used for highway weight enforcement

NTEP Approval

NTEP is a program administered by NCWM for evaluating weighing devices for their conformity to NIST Handbook 44. Scales that pass NTEP certification are deemed “legal for trade” and can be used in commercial transactions based on weight. When a device is submitted to NTEP, extensive testing is performed to insure it passes accuracy tests and meets the specifications listed by the manufacturer. A Certificate of Conformance is issued to a scale manufacturer upon successful completion of testing. You can search the complete database of issued Certificates of Conformance by following this link: http://www.ncwm.net/certificates


Approval Seal

A label, tag, stamped or etched impression, or the like, indicating official approval of a device. This is placed on legal for trade scales out in the field after they have been inspected and shown to perform within the acceptable tolerances for their accuracy class. A local inspector from the Department of Weights and Measures will periodically conduct inspections of scales used in commercial transactions similar to how they inspect and seal gas pumps being used in commercial transactions at your local gas station. This is why it is important for businesses that use scales in commercial transactions to purchase one that is NTEP approved and have it professionally calibrated periodically. If a local sealer believes that your business may be using a scale to provide goods or services, they may conduct a random inspection. If a non-NTEP approved scale is being used, they may impose heavy fines and require that the owner purchase an NTEP approved scale before they can conduct business. If an NTEP approved scale is found to be out of calibration, the device may be labeled "out of service" by the sealer until it has its calibration properly adjusted.


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. Calibration certificates for weighing devices can only be issued by testing the device at the site in which it will be used. This is due to the change of local gravity which can vary as much as 0.5% at various locations around the world. A calibration certificate is no longer valid if the device is shipped to another location.


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 scale.


Electromagnetic Force Restoration

Traditional equal arm balances work on the principal of the fulcrum and lever. An unknown mass is placed on a pan at one end of a lever, while a set of known masses or test weights are placed on a pan at the other end to create a balance. Electromagnetic force restoration balances also use a lever system but a magnetic field is used to generate the force on the opposite end of the lever and balance out the unknown mass. The current used to drive the magnetic coil is proportional to the mass of the object placed on the platform. Most analytical and laboratory balances are of the EMFR type. EMFR balances are characterized by high accuracy, high repeatability, and high complexity compared to other weighing sensors.


Load Cell

A load cell is a type of transducer that converts force into an electrical signal. Strain gauge based load cells are the most common type. They consist of (in most cases) four strain gauges that are attached to a beam or other structure. As weight is added to the load receiving end, the beam or structure deforms. When load cells first emerged, they were mainly used for industrial applications where courser resolutions were suitable. Today though, modern advancements in weighing technology have made load cells capable of much higher resolutions. Load cells are characterized by high durability, high reliability, and low cost.


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, to make things easy, we pretend that the strength of gravity is the same everywhere on Earth and that mass and weight are interchangeable. This is a lie though. In reality, local gravity varies slightly depending on your latitude, longitude, altitude and other geological features. The same mass might have a different weight depending on where you weigh it. In other words, a 500g mass on Earth is going to weigh much more than a 500g mass on the Moon due to the much weaker gravity. Although scales measure the weight of an object, they are calibrated to display in units of mass. When a scale is calibrated at its location of use, a standard mass is placed on the scale and its weight is measured. The scale is then adjusted so that it's readings display the correct mass and any differences in gravity between its new location and the last location it was adjusted are compensated for. This is why calibration certificates for precision scales must be issued at their location of use and are not valid if the scale is shipped to another location.


Kilogram

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).


Place Values For Gram


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 unit still 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 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.


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 (0.0001g).

Animal Scale - A scale designed for weighing single heads of livestock.

Checkweighing Scale - One used to verify predetermined weight within prescribed limits. These scales are typically used in weighing operations where the operator must fill and weigh a product to ensure uniform weight. Some checkweighers will activate remote switches or sound a buzzer 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 Microbalance - A special balance which has a readability of 1 microgram (1µg) or better. A microgram is one millionth of a gram (0.000001g). These devices require special care to minimize weighing errors associated with weighing quantities.

Multi-Interval Scale (also Multi-Range, Dual Range) - 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.

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 - 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 - Digital scales have an internal value that is rounded to give the final display output. On most scales, the rounding "breakpoint" is midway between scale intervals. A weight that falls between the scale intervals may round up or down to the nearest scale interval. Since weight classifiers are meant to be used in postal and shipping applications, the breakpoint for displayed weight is at the scale interval rather than between. Any partial unit of internal resolution above a given weight is rounded up to the next scale interval for the final output. Example: Normal rounding instrument with e=d=0.1 will indicate: 1.0 if the load is 0.96 to 1.04, and 1.1 if the load is 1.06 to 1.14. Postal or shipping weight classifier instruments with e=d=0.1 will indicate: 1.0 if the load is 0.91 to 1.00, and 1.1 if the load is 1.01 to 1.10.

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 consequently perform 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. For the most part, you can avoid this error by using the re-zero function before performing a weighing.

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 increasing linearly 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 - This is the ability of a scale's characteristic curve to approximate a straight line. Linearity can be tested by weighing several test weights of increasing value up to maximum capacity and plotting them as points in a graph. The linearity would be the maximum amount that the points deviate from a straight line going from zero to max capacity.


 

References

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.

"The Fundamentals of Weighing Technology: Terms, Methods of Measurement, Errors in Weighing." Sartorius AG. 1996. Web. 8 Jan. 2010

"Accuracy." Wikipedia, The Free Encyclopedia. Wikimedia Foundation, Inc. 3 Jan. 2010. Web. 8 Jan. 2010.

"Markings and Tolerance Application for Weight Classifiers." Steven Cook, NIST, May 2004. Web. 5 May. 2011