Fluid Mechanics 3(2+1)

LESSON 5. PRESSURE MEASUREMENT

• Atmospheric pressure is usually measured by a mercury barometer. 

• A simple barometer consists of a tube more than 760 mm (30 inch) long inserted in an open container of mercury with a closed and evacuated end at the top and open end at the bottom with mercury extending from the container up into the tube. 

• A void is produced at the top of the tube which is very nearly a perfect vacuum.  Figure 2.10 below shows an example of a barometer.

• Mercury rises in the tube to a height of approximately 760 mm (30 in.) at sea level. 

• The level of mercury will rise and fall as atmospheric pressure changes; direct reading of the mercury level gives prevailing atmospheric pressure as a pressure head (of mercury), which can be converted to pressure using the relation: 

                                    P_atm =  ρgh.

5.2 Piezometer tube

Ρ=γh

• A simple vertical tube open at the top, which is attached to the system containing the liquid where the pressure (higher than atmospheric pressure) to be measured.

• As the tube is open to the atmosphere, the pressure measured is the gauge pressure.

• When Piezometric is used to measure the pressure it is called as monometers.

• Monometers are classified as:

• Simple

• Differential

• Micro monometers

5.3 U-tube manometer

• One end of the U-tube is connected to the pressure that is to be measured, while the other end is left open to atmosphere. 

• The tube contains a liquid, which is called the manometric fluid, which does not mix with the fluid whose pressure is to be measured.

• The fluid whose pressure is being measured should have a lesser density than the manometric fluid. (ρ < ρ_man )

• Better for higher pressures.

• Possible to measure pressure in gases.

• Possible to measure pressure in gases.

• Pressure change from 1  to 2 is γ_mΔh

• Pressure change from 3  to 4 is γl

• Pressure in pipe is P_p

0  +   γ_{m Δh  -  γl  =  Pp}

5.4 Differential Manometer

• In some cases, the different between the pressures at two different points is desired rather than the actual value  of the pressure at each point.

• A manometer to determine this pressure difference is called the differential manometer (see figure below).

• The liquids in manometer will rise or fall as the pressure at either end (or both ends) of the tube changes.

 

 

5.5 Pressure Gauges

• The pressure to be measured is applied to a curved tube, oval in cross section.

• Pressure applied to the tube tends to cause the tube to straighten out, and the deflection of the end of the tube is communicated through a system of levers to a recording needle.

• This gauge is widely used for steam and compressed gases.

• The pressur eindicated is the difference between that communicated by the system to the external (ambient) pressure, and is usually referred to as the gauge pressure.