PRESSURE MEASUREMENT
INTRODUCTION
In
our day to day life, we are meeting various applications of pressure. Have you
ever noticed the flow of wind, the force of water coming out of the hose? All
these are the different examples for atmosphere earth’s
surface these
Nearly all industrial processes use
liquids and gases. Controlling those processes requires the measurement and
control of liquid and gas pressure. Thus pressure measurement is one of the
most important processes of measurements . If you are pealing an apple then the
pressure is the key variable: if the knife is sharp, then the area of contact
is small and you can peel with less force exerted on the blade . if an object
weigh 100 pounds and rests on 1 sq inch of earth’s surface , It is exerting 100
pounds/sq inch . All objects on earth have weight and therefore exert pressure
on earth that can be expressed in unit of weight in unit area .
Pressure can be defined as the amount
of force applied or distributed over the surface and it can be measured as
force per unit area
P=F/A
CLASSIFICATION
OF PRESSURE SCALES
Atmospheric
pressure
The
atmosphere may be supposed to be divided into a number of horizontal layers
parallel to the surface of the earth. Each layer bears the weight of all other
layers above it and thus subjected a pressure due to their weight. This
pressure is called as the atmospheric pressure. Evidently this pressure is greatest
at the surface of the earth and decreased as move higher.
The normal atmospheric pressure at the
sea level is: 1.03kg/cm2 or
Barometer
The
Hg in the tube settles down, leaving a vacuum above it. The height of the
mercury column in the tube above the level of the mercury in the pan indicates
the atmospheric pressure in inches of Hg.
Gauge
pressure
It
is the pressure, measured with the help of a pressure measuring instruments, in
which the atmospheric pressure is taken as the datum or in other words, the
atmospheric pressure on the gauge scale is marked as zero. Most liquid pressure
gauges use atmospheric pressure (14.7 PSI) as the datum point. They indicate a
pressure of 0 PSI at the surface of a liquid even though the pressure is
actually 14.7 PSI or 1.03 Kg/cm2. Generally, the gauge pressure is
above the atmospheric pressure.
Manometer
Monometer
is the simplest measuring device used for measuring gauge pressures at very low
range by balancing the pressure against the weight of the column of the liquid.
It is generally used within a range of 2Kg/cm2. The action of all manometers
depends on the effect of pressure exerted by the fluid at a depth.
U-tube
manometer
It
consists of a transparent glass tube constructed in the form of an elongated U
and partially filled with a liquid, most commonly water or Hg.
Absolute
pressure
It
is the pressure equal to the algebraic sum of atmospheric pressure and gauge
pressures.
Vacuum
pressure
Gauges
that indicate pressure below zero is called vacuum pressure. Absolute pressure
gauges cannot indicate pressure below zero because this type of gauges, zero is
perfect vacuum.
TYPES
OF PRESSURE
Static
pressure:
When
a fluid is in equilibrium the pressure at a particular point is identical in
all directions and independent of orientation. The air compresses to its
minimum volume and exerts a force in all direction within the cylinder. This is
the static pressure excreted by gas. As the weight increases the pressure
within the cylinder increases.
Dynamic
pressure:
It
is the pressure above static pressure caused by the movement of fluids. The
dynamic pressure can be produced by gravity or mechanically by the pump.
Differential
Pressure:
It
is the pressure difference between two related pressures. Measuring two related
pressure and calculating the difference between the two measurements can determine
the differential pressure. Differential pressure is frequently used to
determine fluid flow rate.
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