32901016 manometer power point

8/12/2019 32901016 Manometer Power Point

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MANOMETER

Bulacan Agricultural State College

(BASC)

Preferred By: Erwin Blas

BSIT-2B


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Manometer

A manometeris an device employed tomeasure pressure.


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Variety of manometer designs

simple, common design is to seal a lengthof glass tubing and bend the glass tube

into a U-shape. The glass tube is thenfilled with a liquid, typically mercury, sothat all trapped air is removed from the

sealed end of the tube. The glass tube isthen positioned with the curved region atthe bottom.


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After the mercury settles to the bottom ofthe manometer, a vacuum is produced inthe sealed tube (the left tube in thepicture). The open tube is connected tothe system whose pressure is beingmeasured. In the sealed tube, there is nogas to exert a force on the mercury

(except for some mercury vapor).


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In the tube connected to the system, thegas in the system exerts a force on themercury. The net result is that the columnof mercury in the left (sealed) tube is

higher than that in the right (unsealed)tube. The difference in the heights of thecolumns of mercury is a measure of the

pressure of gas in the system.


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In the example at the left, the top of theleft column of mercury corresponds to 875mm on the scale. The top of the right

column of mercury corresponds to 115mm. The difference in heights is 875 mm -115 mm = 760. mm, which indicates thatthe pressure is 760. mm Hg or 760. torr.


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This method for measuring pressure led tothe use of millimeters of mercury (mm Hg)as a unit of pressure. Today 1 mm Hg is

called 1 torr. A pressure of 1 torr or 1 mmHg is literally the pressure that produces a1 mm difference in the heights of the two

columns of mercury in a manometer.


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F = m g

In this equation, m is the mass of mercury

in the column and g = 9.80665 m/sec2 isthe gravitational acceleration. This force isdistributed over the cross-sectional area ofthe column ( A ). The pressure resulting

from the column of mercury is thusP= mg/ A


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The mass of mercury is given by theproduct of the density of mercury ( dHg)and the volume of mercury ( V). For acylindrical column of mercury, the volume

of mercury is the product of the cross-sectional area and the height of thecolumn ( h). These relationships product

the following equation. P= mg/ A= dHgVg/ A=dHgAhg/A=dHghg


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Vertical U-Tube Manometer


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The pressure difference in a vertical U-Tubemanometer can be expressed as

pd = h (1)

where

pd = pressure

= specific weightof the fluid in the tube(kN/m3, lb/ft3

h = liquid height (m, ft)

The specific weight of water, which is the mostcommonly used fluid in u-tube manometers, is9.8 kN/m3or 62.4 lb/ft3.
http://www.engineeringtoolbox.com/static-pressure-head-d_610.htmlhttp://www.engineeringtoolbox.com/density-specific-weight-gravity-d_290.htmlhttp://www.engineeringtoolbox.com/density-specific-weight-gravity-d_290.htmlhttp://www.engineeringtoolbox.com/static-pressure-head-d_610.html


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Inclined U-Tube Manometer


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The pressure difference in a inclined u-tube can be expressed as

pd = h sin() (2)

where = angle of column relative thehorizontal plane

Inclining the tube manometer will increasethe accuracy of the measurement.


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Example - Differential PressureMeasurement in an Orifice

A water manometer connects the upstream anddownstream of an orifice located in an air flow. Thedifference height of the water column is 10 mm.

The pressure difference head can then be expressed as:pd = (9.8 kN/m3) (103 N/kN) (10 mm) (10-3 m/mm)

= 98 N/m2 (Pa)

where

9.8 (kN/m3) is the specific weight of water in SI-units.

l iff i l


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Example - Differential PressureMeasurement with an Inclined U-Tubemanometer

We use the same data as in the exampleabove, except that the U-Tube is inclined

to 45o.The pressure difference head can then beexpressed as:

pd = (9.8 kN/m3) (103 N/kN) (10 mm) (10-3 m/mm) sin(45)

= 69.3 N/m2 (Pa)


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32901016 manometer power point

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