gas thermometers

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    A mercury-in-glass thermometer, also known as a mercury thermometer, was invented by German

    physicist Daniel Gabriel Fahrenheitin 172 and is a thermometerconsistin! o" mercuryin a !lasstube.

    #alibrated marks on the tube allow the temperatureto be read by the len!th o" the mercury within the

    tube, which varies accordin! to the heat !iven to it. $o increase the sensitivity, there is usually a bulb o"

    mercury at the end o" the thermometer which contains most o" the mercury% e&pansion and contraction

    o" this volume o" mercury is then ampli"ied in the much narrower bore o" the tube. $he space above the

    mercury may be "illed with nitro!en or it may be less than atmospheric pressure, which is normally

    known as a vacuum.

    'ercury will solidi"y ("ree)e* at +-.- #(+7.-/ F* and so may only be used at hi!her temperatures.

    'ercury, unlike water, does not e&pand upon solidi"ication and will not break the !lass tube (unlike

    water, when it is "ro)en then 0uickly heated, will break, makin! it di""icult to notice when "ro)en*. " the

    thermometer contains nitro!en, the !as may "low down into the column and be trapped there when thetemperature rises. " this happens, the thermometer will be unusable until returned to the "actory "or

    reconditionin!. $o avoid this, some weather services re0uire that all mercury+in+!lass thermometers be

    brou!ht indoors when the temperature "alls to +7 # (+. F*. n areas where the ma&imum

    temperature is not e&pected to rise above +-.- # (+7.-/ F*, a thermometer containin! a mercury+

    thalliumalloymay be used. $his has a solidi"ication ("ree)in!* point o" +1.1 # (+7- F*.

    Immersion Styles

    A total immersion thermometer is desi!ned to indicate temperatures correctly when the bulb and the

    entire li0uid column are immersed in the substance bein! measured. A minimal len!th o" thethermometer must be visible.

    A partial immersion thermometer has a line around it indicatin! ma&imum immersion depth.

    For ma&imum accuracy, select the proper thermometer. A total immersion style o""ers the best accuracy

    when used properly. A partial immersion style is appropriate "or use when immersion depth is limited.

    3hen used correctly, either type will yield reliable results.

    Parts of a Glass Thermometer

    4ulb 5 reservoir "or the thermometric li0uid

    6tem 5 !lass capillary tube throu!h which the mercury or or!anic li0uid "luctuates with chan!es intemperature

    6cale 5 the scale is !raduated in de!rees, "ractions or multiples o" de!rees

    #ontraction #hamber5 an enlar!ed capillary bore which serves to reduce a lon! len!th o" capillary, or

    prevent contraction o" the entire li0uid column into the bulb

    &pansion #hamber5 an enlar!ement o" the capillary bore at the top o" the thermometer to prevent

    build up o" e&cessive pressures in !as+"illed thermometers

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    INDUSTRIAL GLASS THER!ETERS

    $he 3eiss 78 and /8 9ari+an!le: is molded o" G 9alo&: polyester, ;< !lass=mineral rein"orced. $his

    particular material has e&cellent stren!th, impact and sti""ness properties which makes it per"ect "or

    the tou!hest applications. 6ome o" the uses are process pipin!, tanks, pipelines in buildin! construction,

    boilers and air ducts. $he 9ari+an!le is a versatile thermometer that can be ad>usted easily "or optimumreadability. Furnished in ran!es up to and includin! ;;F.

    : 9ari+an!le is a re!istered trademark o" 3eiss nstruments, nc.

    Gas thermometer

    A gas thermometer measures temperature by the variation in volume or pressure o" a !as.?1 ne

    common apparatus is a constant volume thermometer. t consists o" a bulb connected by a capillary

    tubeto a manometer. $he bulb is "illed with a !as such that the volume o" the !as in the bulb remains

    constant. $he volume is related to temperature by k, known as #harlesBs Caw. $he pressure o" the !as inthe bulb can be obtained by measurin! the level di""erence in the two arms o" the manometer.?2Gas

    thermometers are o"ten used to calibrate other thermometers

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    Filled+6ystem $hermometers

    Many physical properties change with temperature, such as the volume of a liquid, the length of a metal

    rod, the electrical resistance of a wire, the pressure of a gas kept at constant volume, and the volume of a

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    gas kept at constant pressure. Filled-system thermometers use the phenomenon of thermal expansion of

    matter to measure temperature change.

    The filled thermal device consists of a primary element that takes the form of a reservoir or bulb, aflexible capillary tube, and a hollow Bourdon tube that actuates a signal-transmitting device and/or a local

    indicating temperature dial. A typical filled-system thermometer is shown in Figure. In this system, the

    filling fluid, either liquid or gas, expands as temperature increases. This causes the Bourdon tube to uncoil

    and indicate the temperature on a calibrated dial.

    The filling or transmitting medium is a vapor, a gas, mercury, or another liquid. The liquid-filledsystem is the most common because it requires a bulb with the smallest volume or permits a

    smaller instrument to be used.

    The gas-filled system uses the perfect gas law, which states the following for an ideal gas:

    T = kPV (7-5)

    where:

    T = temperature

    k = constant

    P = pressure

    V = volume

    If the volume of gas in the measuring instrument is kept constant, then the ratio of the gas

    pressure and temperature is constant, so that

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    (7-6)

    The only restrictions on Equation 7-6 are that the temperature must be expressed in degreesKelvin and the pressure must be in absolute units.

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