FLUID MECHANICS

Manometers

31st August, 2010.

A study of the different types of manometers and the principleS on which

they are based

MANOMETRY

It is A standard technique for measuring pressure that involves the use of liquid columns in vertical or inclined tubes.

Pressure measuring devices based on this technique are called manometers.

Definition of Pressure

Pressure is defined as the amount of force exerted on a unit area of a substance.

Pam

N

area

forceP

2

Types of pressure:

Absolute Pressure:Measured relative to a perfect vacuum

Gage Pressure:Measured relative to the local atmospheric pressure

Relationship between Absolute pressure and gage pressure:

Absolute pressure = Atmospheric pressure + gage

pressure

Note:

Absolute pressures are always positive.

Gage pressure can be positive or negative, depending on whether the pressure is above atmospheric pressure (positive value) or below atmospheric pressure (negative value).

A negative gage pressure is also referred to as a suction or vacuum pressure.

Types of Manometers:

1. Piezometer tube.

2. U-Tube manometer.

3. Inclined-Tube manometer.

Piezometer Tube:

The simplest manometer is a tube, open at the top, which is attached to the top of a vessel containing liquid at a pressure (higher than atmospheric) to be measured. An example can be seen in the figure . This simple device is known as a Piezometer tube. As the tube is open to the atmosphere the pressure measured is relative to atmospheric so is gauge pressure.

Working:

ra-r

atm=gh

Or for the gage pressure at A

ra=gh

Where g= specific gravity of material

Although the piezometer tube is a very simple and accurate pressure measuring device, it has several disvantages.

It is only suitable when the pressure in the container is greater than atmosphic pressure (otherwise air would be sucked into the system).

U-Tube Manometer:

Using a "U"-Tube enables the pressure of both liquids and gases to be measured with the same instrument. The "U" is connected as in the figure below and filled with a fluid called the manometric fluid. The fluid whose pressure is being measured should have a mass density less than that of the manometric fluid and the two fluids should not be able to mix readily - that is, they must be immiscible.

Working:Pressure in a continuous static fluid is the same at any horizontal level so,Pressure at B= Pressure at C

Pb=P

c

From left hand arm:

Pb=P

a+ρgh

1

From right arm:

Pc=P

atmospheric+ρ

mangh

2

Since we're measuring gage pressure, we can substract Patmospheric

, givingPb=Pc

Pa=ρ

mangh

2-ρgh

1

If the fluid being measured is a gas, the density will probably be very low in comparison to the density of the manometric fluid i.e. r

man >> r. In this case the term

ρgh1 can be neglected, and the gauge pressure give by:

Pa=ρ

mangh

2

Inclined Tube Manometers:

To measure small pressure changes, this type of manometers are used.One leg of the manometer is inclined at an angle, and the differential reading is measured along the inclined tube.

Manometer used in steam engines

A manometer with digital interface

End of slideshow

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