THERMODYNAMICS: Pressure Measuring Devices

7/17/2019 THERMODYNAMICS: Pressure Measuring Devices

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THER 003

PressureMeasurement Devices

Engr. Lina D. dela Cruz

Chemical Engineering Department

Technological Institute of the

Philippines



PRESSURE

MEASUREMENTLIQUID MANOMETERS

Technically a manometer is any device usedto measure pressure.

However the word manometer is commonlyused to mean a pressure sensor whichdetects pressure change !y means of

li"uid in a tu!e.



#anometers are differential pressuresensors.

$ differential pressure sensor measures the

difference !etween a pressure !eing appliedto it and a reference pressure %oftenatmospheric pressure&.



Differential pressure is a comparison of onepressure to another.

There are many forms of flow meters. 'or

flow in closed channels %such as pipes&the most commonly used and the deviceswe shall consider here are(



U-tube manometer

The )*tu!e manometer is somewhat self*descriptive. In its !asic form it consists ofa clear glass or plastic tu!e shaped into

the form of a +)+. The tu!e is partially filled with a li"uidsuch as water alcohol or mercury%although for safety reasons mercury is no

longer commonly used&. The lower the density of the li"uid the

higher the sensitivity of the manometer.





The diagram opposite shows a !asic )*tu!e manometer. ,oth ends of the tu!eare open and atmospheric pressure acts

e"ually on the li"uid through each end.



In a )*tu!e manometer the difference !etween theun-nown pressure and atmospheric pressure isthe gauge pressure.

In a )*tu!e manometer

Pgauge Pun-nown * Patm /gh

where / is the density of the li"uid in the tu!e in-g m0

g is the acceleration due to gravity that is 1.23m.s4

h is difference !etween the heights of the li"uid ineach lim! in m



If we -now atmospheric pressure or canaccept the errors which occur using thestandard value then we can find

P un-nown !y rearranging the e"uation

P un-nown Patm 5 /gh



The )*tu!e manometer is not in wide use inindustry although it is sometimes used tocali!rate other instruments.

It is mainly used in la!oratories fore6perimental wor- and demonstrationpurposes.



It can !e used to measure the pressure offlowing li"uids as well as gases !ut cannot!e used remotely.

If pressures fluctuate rapidly its responsemay !e poor and reading difficult



BAROMETERS

$ !arometer is a pressure sensor specificallyused to measure atmospheric pressure.

,ecause of this they have to !e sensitive

and measure a!solute pressure.They are mainly used for meteorological

purposes.

High atmospheric pressure is usuallyassociated with fine weather while lowpressure usually predicts poor weather



,arometers have e6istedfor many years andthere are two !asictypes.

3. Li"uid in glass!arometer

4. $neroid !arometer



Liquid in glass

barometer This !arometer is a type of li"uidmanometer.

It consists of a thin glass tu!e containing a

li"uid. The li"uid is usually mercury to -eep the

length of the !arometer as small aspossi!le



This diagram shows the !asic design of a li"uid inglass !arometer.

It consists of a )*shaped fi6ed to a !ac-ing plate.

7ne lim! forms a reservoir and is open toatmospheric pressure via a small hole

The other lim! is a thin tu!e containing a volumeof li"uid small in comparison to that contained inthe reservoir.

Li-e the inclined tu!e manometer this allows thepressure to !e read against a scale on one lim!.



It is the vacuum which allows the !arometerto measure a!solute pressure.

It is still measuring the difference in

pressure !etween the two lim!s.



However the pressure produced !y thevacuum is lower than the local airpressure and constant.

Therefore the pressure acting on the lim!containing the vacuum can !e considerednegligi!le.



The only significant pressure acting on the

li"uid in the !arometer is the atmosphericpressure.

Hence the height of the li"uid in the tu!e isproportional to atmospheric pressure



Aneroid barometer

The word aneroid simply means somethingwhich does not contain li"uid and sodistinguishes this type of !arometer from

the li"uid in glass type.The aneroid !arometer shown here senses

atmospheric pressure changes !y meansof a sealed metal capsule.



7ne or two faces of the capsule arediaphragms.

The capsule contains a partial vacuum.

,ecause the pressure inside the capsule islower than atmospheric pressure andconstant it is sensitive to changes inatmospheric pressure.

If atmospheric pressure increases thediaphragms move and the capsuleflattens.



If atmospheric pressure rises the capsulee6pands. This displacement is detected !ya !ar pressed on to the capsule !y a

strong spring. This is mechanicallyamplified !y gears and levers andtransmitted to a pointer on a cali!ratedscale.



The aneroid !arometer is simpler to readand ta-es up less space than the li"uid inglass !arometer.

It is not affected !y temperature changes assignificantly !ut is less accurate mainly!ecause of its mechanical parts.

In fact aneroid !arometers are usuallycali!rated against li"uid in glass!arometers.



$neroid !arometers are commonly used fordomestic purposes.

They are li-ely to !e cali!rated in non*8I

units such as millimetres of mercury or!ar

8ome versions are cali!rated with respectto li-ely weather conditions9



If electronic measurement is re"uired themovement of the diaphragm can !edetected in terms of capacitance as

discussed earlier in this section.This increases accuracy and sensitivity.

,ecause atmospheric pressure changeswith altitude capacitive aneroid

!arometers sometimes form the !asis ofaltimeters in aircraft.

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THERMODYNAMICS: Pressure Measuring Devices