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originalgravityHowDoesaHydrometerWork?HomebrewingStackExchange
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HowDoesaHydrometerWork?
Iknowhowtousemyhydrometer,andIknowwhatnumberstolookatandwritedownonmybrewingsheets,butI'mnotsurewhatthosenumbersactuallyrepresent,orevenwhatunitsIshouldusetolabelthosereadings.
What'sthesciencebehindthehydrometer?Howaretheycalibrated?WhatelseshouldIknowaboutthetheoryofusingonetomakebetterbeer?
hydrometer originalgravity science
askedNov13'10at5:43Ben594 2 7 16
3Answers
GoodAnswerbyFishtoaster.Thescienceisancient,discoveredbyArchimedes.
1:Anyobject,whollyorpartiallyimmersedinafluid,isbuoyedupbyaforceequaltotheweightofthefluiddisplacedbytheobject.
Inotherwords,ifyouputaball,withvolume1litrecompletelyunderwater,thereisanupwardsforceontheball(buoyancyorflotation)equaltotheweightof1litreofwater.(i.e.1kilogramforceor9.81Newtons)
2:(Corollary)Anyfloatingobjectdisplacesitsownweightoffluid.
Ifweplacea
objectofmass1kilo,itwilldisplaceexactly1kiloofwater,or1litreofwater.Ifthevolumeofourobjectisgreaterthan1litre(i.e.theobjectislessdensethanwater),itwillfloat.
floating
So,withaHydrometer,itisweightedsuchthattheweightofthesubmergedvolumeatthe1.000readingisexactlyequaltotheweightofthehydrometer.Ifwedissolvesolidsintothewater(sugar)thatvolumeofwaterisheavier,andlessofitneedstobedisplacedinorderforthehydrometer'sweighttobematched,andtheHydrometerfloatshigher.
editedJan21'14at13:53 answeredNov15'10at14:48ChrisCudmore601 5
9
Thegeneralprincipleisbasedondensity.Thatis,lessdensethingswillfloatontopofdenserthings.
Byanalogy,imagineahumanfloatinginapoolofwater.Nowimagineahumanfloatinginapoolofsaltwaterthehumanwillfloathigherinthesaltwater,sincethedensityofthesaltwaterishigher.
Ahydrometerworkssimilarlyit'sjustamarkedglasstubethat'sweightedsuchthatinwater,it'llsitat1.00.Ifyouputitinsomethingdenserthanwater(likewort,whichiswaterwithstuffinit),it'llsitalittlehigheroutoftheliquid,soit'satsomethinglike1.050.Thatnumberisthedensity*oftheliquid.
Onethingtonoteisthathydrometersarecalibratedtoaspecifictemperature:15C/59F.Ifyourliquidisnotthattemperature,youwanttorunyourmeasurementthroughaconversioncharttoaccountforthechangeindensityofliquidsbasedontemperature.
*Orrather,it'smeasuringthespecificgravityofthesubstance,whichisadimensionlessunitequaltotheratioofthedensityofthesampledividedbytheratioofareferenceliquid,bothatagiventemperatureandpressure.Thereferenceliquidhereispurewater,andthesampleiswaterwithstuffinit(ie,wort).
editedSep12'11at1:40drj1,589 1 8 25
answeredNov13'10at16:25Fishtoaster1,912 2 15 32
Heavyshipscanfloatbecauseoftheprinciplesofbuoyancy.Hydrometersmeasurethespecificgravityofliquids.Specificgravityisameasurementofthedensityofasubstancecomparedto
