Scales and Balances Information
Scales and balances are instruments which reflect an object's mass — it's matter independent of gravity, often represented by grams, kilograms, or pounds (mass); or its weight — the object's mass multiplied by gravitational acceleration (W=mg), which is best represented by ounces, pounds (force), tons, or Newtons. Scales are used to quantify the weight of items, be it for commercial, scientific, or investigative purposes. As such, scales are relied upon in nearly every industry to provide measurements ranging from micrograms to megagrams, or ounces to tons.
Scale vs. Balance
Balances were common in prehistory to determine relative weights rather than absolute weight. The most common type of balance relies upon a horizontal lever of equal arm lengths which pivot around a minimal-friction fulcrum. Balances with off-center fulcrums are available, but require adjustable reference masses and are more difficult to check for accuracy. Balances remain an effective means of precision weighing as they are not subject to variables which may cause a misreading on a scale (temperature, local gravity, etc.). In fact, center beam balances are still frequently used to calibrate the reference masses used to adjust scales.
Scales, devices which provide a weight measurement without contrasting it to another object, were uncommon until the mid-1800s when postal workers utilized spring scales to speed package handling. Until the technological advances of World War II scales often suffered from inaccuracy, and not until decades later had scales developed to the point of widespread use.
Mechanical Scales and Balances
The most common type of mechanism utilized by mechanical scales and balances is the spring. The item to be weighed is either suspended from an extension spring, or placed on top of a compression spring. Spring scales require a spring with a known-constant based upon Hooke's law. The degree of deflection is indicative of the weight of the item, and some scales use a rack and pinion mechanism to translate this deflection into a dial reading.
Other mechanical scales rely upon a pendulum and fluid mechanics to determine weight. Pendulum balances supply a platform or hook attached to a fulcrum. Opposite the platform is a weighted, pointed indicator. When an object is weighed the indicator rises until the load is balanced. Hydraulic/pneumatic scales depend upon the predictive characteristics of compressed fluids. A load depresses a piston and the compressed fluid is fed into a Bourdon tube analog gauge or is registered by an electronic sensor. Fluid-based scales are common on cranes and other heavy lifting equipment.
Electronic Scales and Balances
The most common type of electronic scale is the digital scale. Digital scales are supplied with a load cell; this device supplies the measurement, and turns the mechanical energy into an electrical effect. As the load cell compresses under burden, so does the attached strain gauge, which is an insulated, flexible metal foil pattern (commonly made of constantan alloy). As the strain gauge's geometry changes, so does its electrical resistance. This change in resistance is measured by a Wheatstone bridge, and the scale provides a proportional readout based upon the change in resistance. Frequently, light loads require amplification for accuracy. Occasionally, spring scales utilize a strain gauge as well, as depicted in the image below.
Scale and Balance Accuracy
Scales express accuracy as a percentage of the overall scale capacity. This percentage provides a tolerance of what can be expected from a calibrated scale. For industrial and scientific applications, the precision of a scale should be considered as well. Many scales allocate measurements by separating a burden, regardless of weight, into divisions of 100—100,000. For example, a 20,000 kg scale may be manufactured to calculate weight in divisions of 5,000, and therefore it would display a weight in intervals of 4 kg; this is also known as resolution. Using these considerations, scales are separated into scale classes. The precision of a scale's resolution often correlates with the cost of the scale.
Table credit: AW Scales
For this reason, considering a scale capacity that is just above the heaviest expected burden assures operators of the most accurate readings. Scales are also judged on their accuracy with two principles of operation. Repeatability is the scale's potential to show consistent results under the same conditions. Re-producibility is a scale's ability to show consistent measurements under changed conditions.
Other mechanical and environment variables may cause misreadings. Though they may be inconsequential for heavy loads, scientific scales can have severely incorrect readings as a result of any of the following.
- Worn components -- Defective or old equipment is unlikely to provide correct measurements
- Temperature: An increase or decrease in temperature affects the ability of springs and strain gauges to deflect. Items that are warmer than the scale will be slightly heavier-than-displayed due to convection currents, but if the exact temperature of the item is known than a misreading can be mathematically adjusted.
- Air pressure/altitude -- For every meter in altitude the item is raised, the change in air pressure will result in approximately -.065 mg decrease in weight.
- Local gravity -- Readouts can change ±.05% depending upon the global location in which the object is measured.
- Air currents -- Drafts and wind can affect the readouts of very sensitive scales, and in laboratories a draft shield is recommended.
- Moisture -- Items to be weighed are best kept at ambient temperature and humidity levels to reduce the chance of condensation/evaporation affecting readouts.
- Electrostatic -- Static electricity may be generated by the handling and processing of items. Keeping environmental humidity levels at 60% will help minimize static buildup.
- Magnetic forces -- Measurements should be conducted away from magnetic fields whenever possible. If unavoidable for the application, demagnetizing equipment or magnetic shielding should be considered.
- Air buoyancy -- As a displaced fluid, air will supply some buoyancy to the item being weighed. This is true more so with items of larger volumes, and the effect of displaced air is frequently overlooked.
- Dust -- Dust and other negligible impurities collected on the weighing platform will skew the measurement.
- Vibratory disturbances -- Any physical environmental alteration may disrupt a scale's measurement, however temporary.
- Operator interaction -- Operators transfer dust, electrostatic particles, and moisture to items when handling them, and they may affect air currents, ambient temperatures, and magnetic forces as well. Operators may also simply mismeasure the item.
Calibration is the sequence of operations to determine the discrepancy between a known reference and the amount by which the scale misrepresents that weight. One of the most common methods of determining miscalibration is with a linearity test; several reference masses weighed together should weigh the same as when those weights are measured individually and summed.
The process of reconfiguring the scale to be within tolerable accuracy is known as adjustment. If the adjustment is completed by a calibration technician, a calibration certificate may be authorized to assure users of the scale's accuracy. Such certificate can only be obtained when the scale has been adjusted at its installation site. Such certificate also has a traceable and definable record, and may be required for some industries. For commercial and scientific functions, the adjustment of a scale is best handled by Calibration and Repair Services, and scale calibration is recommended at least once a year.
Scales are frequently adjusted by the use of levers, screws, or an operator interface. For non-commercial and non-scientific uses, infrequent testing of a scale using informal known references (dumbbells, household items, etc.) will typically suffice in calibrating a scale. When calibrating and adjusting tractor trailer scales, a test truck with a weight cart is utilized as depicted in the video below.
Video credit: madmaxdaddy via YouTube
The overall footprint of the scale is often considered to assure that it is well-accommodated by its workspace. A minimal clearance zone should be kept around the scale to ease loading and unloading. Scales come in varying magnitudes, and size is often relative to scale capacity.
Benchtop/countertop scales are designed to be positioned on elevated, flat surfaces. They typically weigh objects which can be maneuvered by the operators hand.
Suspended scales feature a weighing platform hung from the calculating device. These scales can be used for many objects and a range of burdens, but carry some peril if the device is overloaded or malfunctions, or if the operator is negligent. However, suspended scales can save valuable floor/bench space.
Integrated scales have a specific form factor for assimilating the scale into other devices, such as conveyor systems, vehicles suspensions, or material handling vehicles.
Floor scales are meant to be installed on the flat, finished floors of a factory, workshop, or retail business. Floor scales are often large devices capable of accurately calculating thousands of pounds.
Vehicle scales are commonplace on roadsides and in railways. These scales determine the cargo weight of trailers and boxcars to ensure legislative compliance and mechanical efficiency. Their load capacity is often measured in tens of tons, and can have a weighing area of hundreds of square feet.
The load bearing platform of the scale must effectively support the dimensions of the object to be weighed. An object on the scale does not necessarily have to be at rest, but its entire weight should be concentrated on the platform. Using a platform with a smaller surface area than the length of the object is often acceptable provided the load is balanced and stable. Expansion trays and tare containers can be useful for measuring large or bulk items. Roadway vehicle scales typically require the vehicle to be positioned on skids or a weighbridge, and railway cars are loaded onto rails integrated to the track gauge. Vehicle scales are frequently capable of in-motion weighing.
The maximum load weight a scale can withstand before plastic deformation occurs.
A scale will provide either an analog or digital representation of measured weight. It may be necessary to provide a numeric output away from the vicinity of the scale itself, so models with a remote display or remote add-on are available.
- Hostile environment -- The scale can be used in a variety of adverse conditions that should not affect its performance. Extreme temperatures, excessive moisture, and exposure to chemicals, corrosives, and vibration may require a more robust scale.
- Draft shield -- Sensitive scales may require a polycarbonate cover to prevent air currents or dust from disrupting a measurement.
- Counting -- The scale is capable of recalling a reference weight in order to estimate a bulk count of similarly-weighted items.
- Tracking -- The scale self generates a weight report and assigns it an item of freight. The scale may do this through an integrated RFID reader or an automated ticket composer.
- Intrinsically safe -- The use of the scale does not pose an explosion or fire risk because it has low power consumption, uses inert materials, and can interface with other hazardous process control equipment.
- Tare ability -- The scale has the capability to subtract/divide predetermined amounts from the load placed on the scale. This may be done to subtract a container's weight from the net load of freight.
- Battery powered -- The scale retains full operation when relying on batteries for power.
- Printer/scanner -- A printer is attached to the scale to provide a hardcopy record of the measurement. This is common in many applications, ranging from supermarket scales and postage meters to vehicle measurements. A scanner can read reports to monitor cargo.
- Multihead weighing -- A scale is able to sum components to meet a target weight. This is most common in production lines.
- Remote control -- The scale has an associated RF/IR controller for adjusting and reading the scale away from the primary interface.
- In-motion weighing -- To be weighed, an item does not need to be stationary. Instead, the scale compensates for the movement of the items. This is frequently used in belt conveyor systems and roadside trailer weighstations.
- Software -- A scale comes with associated computer software to help users interface with the scale.
- Ramps: The load platform is easy to load/unload by the use of inclines.
- Checkweighing -- In quality testing instances, the scale is integrated into a system so products that are over/underweight are removed from manufacturing.
- Unit selection -- A scale has the capability to calculate weights in two or more different units.
- Washable -- Scales can be exposed to a variety of contaminants, and a scale which can be cleaned is beneficial to many industries.
Types of Scales and Balances
|An abridged list of common scales.|
Axle scales weigh individual vehicle axles.
Commonly found in airline/train terminals.
Measures and deposits a material into a product packaging, in this instance a bag.
Used as part of a manufacturing process to ensure products are within a specified weight.
Measures in-motion components on a conveyor line.
This device is placed upon the hoist of a crane and measures the degree of stress.
Drum scales are a type of floor scale designed to efficiently weigh drums and their contents.
Fertilizer scales are used to ensure fertilizer mixtures are optimized for the agricultural industry.
Forklift scales measure the load burden of these material handling vehicles.
Hopper scales monitor the load of a storage basin. They are also applied to bins and containers.
Livestock scales are used to measure the weight of large animals. They typically feature a pen and slip-resistant platform.
Medical scales weight people with the assistance of medical professionals.
Mobile scales can be moved from site to site for weighing operations. They may be wheeled or disassembled, and are common for warehouses, freight handling, and truck stops.
This scale determines the amount of moisture in an item. It calculates the difference between an untouched and a dried sample.
On-board scales weigh the load of vehicles via an integrated load cell in the suspension.
Pallet truck scale
A scale integrated into a pallet jack provides a weight of the cargo lifted by the pallet forks.
Personal scales are meant for self-weighing and other household weighing operations.
Pocket scales have a small footprint that allows user to easily transport the scale.
A postal scale is used to weigh mail and parcels, and assign a postage value to the item based upon weight.
Railroad scales weigh the loads of train cars.
Retail scales are commonplace in instances where item cost is determined by weight, such as in a grocery.
These scales require a load to be hung from its bottom hook, where the tension results in deflection translated into a numeric value.
Truck scales are used to regulate the loads of heavy freight vehicles such as tractor trailers, box trucks, and dump trucks.
These scales weigh roadway vehicles one wheel at a time.
Accreditation and Standards
One of the most important certifications is by NTEP, which is a program run by the National Conference on Weights and Measures to certify that all scales utilized in commerce meet acceptable tolerances. A compliance certificate is bestowed upon any device which meets NTEP approval, and the conference keeps an updated database of conforming devices.
Many scientific and commercial scales are IP Code-rated as well. The Ingress Protection Code provides certification that an electronic device is invulnerable to certain degrees of dust, water, and mechanical intrusion. The code is represented by a two-digit numeral; the former digit indicates the degree of dust resistance, while the latter digit represents its endurance against liquids. An alphabetic character may follow the numeral for various amendments.
Table credit: Electrical-Installation.org
Similarly, scales that rely on an enclosure for protection -- in non-hazardous outdoor and indoor installations -- are frequently subject to NEMA standard 205.
A variety of industrial standards also regard the installation and application of scales and balances.
A-A-50059 -- Government acquisition standards for person-weighing scales
A-A-50062 -- Suspended spring scales
A-A-50093 -- Food packaging scales
A-A-50503 -- Balance beam scales (capacity >2610g)
A-A-58088 -- U.S. Air Force standards for aircraft scales
NIST HDBK 44 -- Technical requirements of weighing devices
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