Force-measuring (weight) scales Weighing scale

1 force-measuring (weight) scales

1.1 history
1.2 mechanical scales

1.2.1 spring scales
1.2.2 hydraulic or pneumatic scale


1.3 digital scales

1.3.1 digital bathroom scale
1.3.2 strain gauge scale
1.3.3 supermarket , other retail scale


1.4 testing , certification

1.4.1 sources of error

force-measuring (weight) scales
history

a simple balance 19th century

although records dating 1700s refer spring scales measuring mass, earliest design such device dates 1770 , credits richard salter, scale-maker. spring scales came wide usage in united kingdom after 1840 when r. w. winfield developed candlestick scale weighing letters , packages, required after introduction of uniform penny post. postal workers work more spring scales balance scales because read instantaneously , did not have balanced each measurement.

by 1940s various electronic devices being attached these designs make readings more accurate. load cells, small nodes convert pressure (or force) digital signal, have beginnings late nineteenth century, not until late twentieth century became accurate enough widespread usage.

mechanical scales
spring scales

a spring scale measures mass reporting distance spring deflects under load. contrasts balance, compares torque on arm due sample weight torque on arm due standard reference mass using horizontal lever. spring scales measure force, tension force of constraint acting on object, opposing local force of gravity. calibrated measured force translates mass @ earth s gravity. object weighed can hung spring or set on pivot , bearing platform.

in spring scale, spring either stretches (as in hanging scale in produce department of grocery store) or compresses (as in simple bathroom scale). hooke s law, every spring has proportionality constant relates how hard pulled how far stretches. weighing scales use spring known spring constant (see hooke s law) , measure displacement of spring variety of mechanisms produce estimate of gravitational force applied object. rack , pinion mechanisms used convert linear spring motion dial reading.

spring scales have 2 sources of error balances not: measured mass varies strength of local gravitational force (by as 0.5% @ different locations on earth), , elasticity of measurement spring can vary temperature. proper manufacturing , setup, however, spring scales can rated legal commerce. remove temperature error, commerce-legal spring scale must either have temperature-compensated springs or used @ constant temperature. eliminate effect of gravity variations, commerce-legal spring scale must calibrated used.

hydraulic or pneumatic scale

it common in high-capacity applications such crane scales use hydraulic force sense mass. test force applied piston or diaphragm , transmitted through hydraulic lines dial indicator based on bourdon tube or electronic sensor.

digital scales
digital bathroom scale

a digital bathroom scale type of electronic weighing machine, used measure many readings including body fat, bmi, lean mass, muscle mass, water ratio along body mass. digital bathroom scale smart scale has many functions smartphone integration, cloud storage, fitness tracking, etc.

strain gauge scale

in electronic versions of spring scales, deflection of beam supporting unknown mass measured using strain gauge, length-sensitive electrical resistance. capacity of such devices limited resistance of beam deflection. results several supporting locations may added electronically, technique suitable determining mass of heavy objects, such trucks , rail cars, , used in modern weighbridge.

supermarket , other retail scale

these scales used in modern bakery, grocery, delicatessen, seafood, meat, produce , other perishable goods departments. supermarket scales can print labels , receipts, mark mass , count, unit price, total price , in cases tare. modern supermarket scales print rfid tag can used track item tampering or returns. in cases, these types of scales have sealed calibration reading on display correct , cannot tampered with. in usa, scales certified national type evaluation program (ntep), in south africa south african bureau of standards , in uk international organization of legal metrolog.

testing , certification


scales used trade purposes in state of florida, scale @ checkout in cafeteria, inspected accuracy fdacs s bureau of weights , measures.

most countries regulate design , servicing of scales used commerce. has tended cause scale technology lag behind other technologies because expensive regulatory hurdles involved in introducing new designs. nevertheless, there has been trend digital load cells strain-gauge cells dedicated analog converters , networking built cell itself. such designs have reduced service problems inherent combining , transmitting number of 20 millivolt signals in hostile environments.

government regulation requires periodic inspections licensed technicians, using masses calibration traceable approved laboratory. scales intended non-trade use, such used in bathrooms, doctor s offices, kitchens (portion control), , price estimation (but not official price determination) may produced, must law labelled not legal trade ensure not re-purposed in way jeopardizes commercial interest. in united states, document describing how scales must designed, installed, , used commercial purposes nist handbook 44. legal trade (lft) certification approve readability repeatability/10 ensure maximum margin of error of 10%.

because gravity varies on 0.5% on surface of earth, distinction between force due gravity , mass relevant accurate calibration of scales commercial purposes. goal measure mass of sample rather force due gravity @ particular location.

traditional mechanical balance-beam scales intrinsically measured mass. ordinary electronic scales intrinsically measure gravitational force between sample , earth, i.e. weight of sample, varies location. such scale has re-calibrated after installation, specific location, in order obtain accurate indication of mass.

sources of error

some of sources of error in weighing are:

buoyancy – objects in air develop buoyancy force directly proportional volume of air displaced. difference in density of air due barometric pressure , temperature creates errors.
error in mass of reference weight
air gusts, small ones, push scale or down
friction in moving components causes scale reach equilibrium @ different configuration frictionless equilibrium should occur.
settling airborne dust contributing weight
mis-calibration on time, due drift in circuit s accuracy, or temperature change
mis-aligned mechanical components due thermal expansion or contraction of components
magnetic fields acting on ferrous components
forces electrostatic fields, example, feet shuffled on carpets on dry day
chemical reactivity between air , substance being weighed (or balance itself, in form of corrosion)
condensation of atmospheric water on cold items
evaporation of water wet items
convection of air hot or cold items
gravitational differences scale measures force, not balance.
vibration , seismic disturbances