precautions while using laboratory thermometers

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Precautions while using Laboratory Thermometers

The precautions to be observed while reading a laboratory thermometer.
31. Thermometerconsistsofaliquid
Thisthermometerconsistsofaliquidfilledglassbulbandaconnectingpartiallyfilledcapillarytube.Whenthetemperatureofthethermometerincreases,thedifferentialexpansionbetweentheglassandtheliquidcausestheliquidtoriseinthecapillary.Avarietyofliquids,suchasmercury,alcohol,toluene,andpentane,andanumberofdifferentglassesareusedinthermometerconstruction,sothatvariousdesignscoverdiverserangesbetweenabout300
32. Bimetallic thermometer
Inthisthermometerthedifferentialexpansionofthindissimilarmetals,bondedtogetherintoanarrowstripandcoiledintotheshapeofahelixorspiral,isusedtoactuateapointer.Insomedesignsthepointerisreplacedwithlowvoltagecontactstocontrol,throughrelays,operationswhichdependupontemperature,suchasfurnacecontrols.
33. Filledsystemthermometer
Thistypeofthermometerhasabourdontubeconnectedbyacapillarytubetoahollowbulb.Whenthesystemisdesignedforandfilledwithagas(usuallynitrogenorhelium)thepressureinthesystemsubstantiallyfollowsthegaslaw,andatemperatureindicationisobtainedfromthebourdontube.Thetemperaturepressuremotionrelationshipisnearlylinear.Atmosphericpressureeffectsareminimizedbyfillingthesystemtoahighpressure.Whenthesystemisdesignedforandfilledwithaliquid,thevolumechangeoftheliquidactuatesthebourdontube.
34. Vaporpressurethermalsystem
This filledsystem thermometer utilizes the vapor pressure of certain stable liquids to measure temperature. The useful portion of any liquidvapor pressure curve is between approximately 15 psia (100 kilopascals absolute) and the critical pressure, that is, the vapor pressure at the critical temperature, which is the highest temperature for a particular liquidvapor system. A nonlinear relationship exists between the temperature and the vapor pressure, so the motion of the bourdon tube is greater at the upper end of the vaporpressure curve. Therefore, these thermal systems are normally used near the upper end of their range, and an accuracy of 1% or better can be expected.
35. Resistance thermometer
Inthistypeofthermometerthechangeinresistanceofconductorsorsemiconductorswithtemperaturechangeisusedtomeasuretemperature.Usually,thetemperaturesensitiveresistanceelementisincorporatedinabridgenetworkwhichhasareasonablyconstantpowersupply.Althoughadeflectioncircuitisoccasionallyused,almostallinstrumentsofthisclassuseanullbalancesystem,inwhichtheresistancechangeisbalancedandmeasuredbyadjustingatleastoneotherresistanceinthebridge.Metalscommonlyusedasthesensitiveelementinresistancethermometersareplatinum,nickel,andcopper.
36. Thermistor
This device is made of a solid semiconductor with a high temperature coefficient of resistance. The thermistor has a high resistance, in comparison with metallic resistors, and is used as one element in a resistance bridge. Since thermistors are more sensitive to temperature changes than metallic resistors, accurate readings of small changes are possible.


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