Air Pressure - Electrical and Computer Engineering

1 Chapter5 AirPressureThereis a sumptuousvarietyabouttheNew Englandweatherthatcompelsthestranger sadmiration- alwaysdoingsomethingthere;alwaysattend-i ngstrictlytobusiness;alwaysgettingupnew designsandtryingthemonthepeopletoseehow they getsthroughmorebusinessinspringthaninany have , heatedbytheengineofthesunandcooledbyradi ationintospace, , someasuringtheairpressureis a ,a suddendropinairpressureoftensignalstheon setofstormyweather; themercurybarometer,a columnofliquidthatis supportedbyatmosphericpressure,figure?

2 ?.Aclosedtubeisfilledwithmercuryandtheni nvertedintoa ,forminga vacuumabove itstopsurface, ,butmercuryis attractive ,theobserver mustcarefullydeterminetheheightofthecolu mnabove thelevel inthereservoir, , thedownwardpressureY Bofthemercurycolumnis YV( )ThepressureofthemercurycolumnisYB DC@B B( )161162 MercuryColumnVacuum30inches(1016mB) :MercuryColumnBarometerwhereC@Bis theforceexertedbythecolumn Bis thecross-sectionalareaofthecolumnThecolu mnforceisCB B ( )where Bis themassofthemercurycolumn is thegravitationalconstant,980 B FEB B B( )whereEBis thedensityofthemercury, #in thecgssystem Bis thecross-sectionalareaofthemercurycolumn Bis theheightofthecolumnCollapsingtheseequat ions,weobtaina EB B B B( ) EB B ( )Forexample,theso-calledstandardpressure ofphysicsandchemistrycausesa anatmosphericpressureofYV i)v v xw i.

3 UnitsA varietyofunitsofpressurehave evolvedover definedas : ,sostandardairpressureis ,thestandardunitofpressureisthepascal, Airpressureis convenientlydescribedinkilopascals,orkPa . , :PressureUnitsTheAneroidBarometerWithcar efulattentiontodetail,a , forhouseholdusewhereaccuracy is lesscritical,theaneroidbarometeris morepractical. :AneroidBarometerAsshownschematicallyinf igure??, thesensingelementofthebarometeris a sealedchamberis equippedwitha , mechanicallyamplifiedandcausesa typicalaneroidbarometerscale(unrolled)is showninfigure?

4 ?. :BarometerScaleWe willusethisinsection?? is alsointerestingtoconsidertheabsoluteextr emesofmeasuredairpressure,toseeif [?], ,Mayo,YukonTerritory,1 ,Agata,Siberia, ,StAnthony,Nfld, , EyeofPacificOceantyphoonJune,19 Nov :AirPressureRecordsNoticethattherecordhi ghpressurestendtooccurinarticregions,whe retheairmassis occurredintheeyeofa ,butweshoulddesignforthemif thecostpenaltyis andAltimetryAirpressuredecreaseswithheig ht, ,a changeofaltitudebya knownamount(anelevatorride,forexample) ,weneedtoknow theairdensity, whichis givenby: G I( )where = densityofair, grams/cm#Y= airpressure,dynes/cm.

5 (orKilopascals 1000) = gasconstantforair, *)w : CI= airtemperature, @ H 5 ( )where5Y= changeinairpressure,millibars = gravitationalconstant,981 = changeinheight,cmExampleFindthechangein airpressureover a changein heightof30metresif theairtemperatureis 20 C , : q : : x *)w : 7 ) #gms/cm#Then,substitutingfor inequation??,5Y wq $ x) : # 7 x x x dynes/cm. i) result,suitableairpressuresensorshave becomeavailableat basedontheMotorolaMPX100 APsensor, a sensorforabsolutepressuresbetween0 and1000millibars1(figure?

6 ?). Thepressuresensoris essentiallya miniatureaneroidbarometer. Themembraneis a thinsilicondiaphramintowhichhasbeendiffu seda networkoffourresistorsina strainguages,changingresistanceasatmosph ericpressuredeformsthediaphram,figure??. Theresistanceofa conductoris givenby I( )where1We requirea maximumpressuremeasurementof1050millibar s,whichexceedsthemaximumratingofthesenso rbysome5%.Aswewillseeinthedesignnotes,us inga sensorratedforhigherpressurewouldhalve thesensorelectricaloutputsignalandrequir edoublethevoltagegainfromthesensoramplif ier.

7 Eventhe5%overloadis wellbelowthemaximumratingoftheMPX100AP(2 000millibars),soweareinnodangerofdamagin gthesensor. Hopefully, itsoutputremainslinearinthe5% :PressureSensor is theresistivityoftheconductor is thelengthIis thecross-sectionalareaWhena resistoris stretched,itslengthincreasesandcross-sec tionalareadecreases, ,thiseffectis , strainguagebridge,theresistorsarelocated sothatdiagonallyoppositeresistorsinthebr idgechangeresistanceinthesamedirection,e ither L 5 or L '5 .Thedifferentialoutputvoltageis thensimply.

8 Lu , and ! .If themaximumdifferentialoutput q.>u, at fullpressure,is ,determinethecorrespondingvaluesofthebri dgeresistorsat :From !.Lu ! / / // $ 5 $ '5 42 $ '5 $ 5 13 Vcc !.Lu+ :ResistorBridgeso5 !.Lu q) xv q) Then $ 5 1 $ i) x x : and $ '5 1 $ ' i) x x specificationsfortheMPX100 APpressuresensorareasfollows:Notes:Press ure RangeWe thethreevoltsupplyis obtainedbydropping2 voltsacrossresistorsinserieswiththesenso r,it turnsoutthatthetemperaturedriftofthesens oris substantiallyreduced(reference[?]

9 ]). :PressureSensorSpecificationsFullScaleSp anFromthesefigures, causedbymismatchofthebridgeresistorsanda ppearsasa fixedvoltageat .Sensitivity BThisis a somewhatredundantstatementofthetransduce rgain, numberofdesignchallengeswhichneedtobeadd ressedinthissystem:PowerSupplyTheavailab lepowersupplyis+ convertermustbeavailabletogeneratetheusu alpositive andnegative DC-DCconverterandsomemethodofensuringtha ttheoutputoftheopampdoesnotexceedthe0-5v oltrangeoftheHC11 ,we ve ,see[?].AmplifierOutputSwingTheoutputvol tageofa singlesupplybipolaropampsuchastheNationa lLM324orMotorolaMC34074is +5voltpowersupply.

10 SomeCMOS opamps,suchastheNationalLMC660CN,willpro ducea +5voltsupplyandloadgreaterthan2K , sothisis a DriftAback-of-the-envellopecalculationsh owsthatthesensoramplifierwillrequirea voltagegainintheorderof300 driftinoffsetvoltage,bias, ,thesensoritselfis sensitive barometerratherthana thermometer. References[?] and[?] mentiontheproblemoftemperaturedrift,a warningthatit temperaturedrift,weshouldchoosetheminimu mvoltagegainthatmeetstherequirements, Table??onpage??show thatthesensorgaincanvaryovera 2:1range, toprovidetwo higherthana fixedresistoranda is preferable,if at allpossible, theBarometerScaleshownin figure?