We were discussing the basic definition and
significance of Pascal’s Law along with its derivation , Vapour
pressure and cavitation, Absolute
pressure, Gauge pressure, Atmospheric pressure and Vacuum pressure and
also the basic concept of pressure measurement in our previous posts.
Today we will understand here the basic concept of
simple manometer to measure the pressure at a point in fluid in the subject of
fluid mechanics with the help of this post.
We will discuss here the pressure measuring
instrument i.e. Piezometer in detail with the help of this post. We will also see
here the advantages and drawbacks of pressure measuring instrument Piezometer
tube.
Further, we will discuss another simple manometer
i.e. U-Tube manometer in our next post. So let us brief here simple manometer
and Piezometer.
Simple Manometer
Simple manometer is basically, a pressure measuring
device, used to measure the pressure at a point in fluid by balancing the
column of fluid by the same or another column of fluid.
Simple manometer will have one glass tube. One end
of glass tube will be connected with a point where we need to measure the
pressure and another end will be open to atmosphere.
Types of simple manometer
There are following three types of simple manometers
as mentioned here
Piezometer
U-Tube manometer
Single column manometer
Piezometer
Piezometer is the simplest form of simple manometer
which is used to measure the gauge pressure of a fluid in a container by using
the concept of hydrostatic pressure.
Piezometer is only suitable to measure the pressure
of fluid in a container, if pressure in the container is more than the
atmospheric pressure i.e. Gauge pressure.
We must have to remind it that fluid in the
container, in which pressure need to be measured, should be liquid rather than
gas. In order to reduce the error, glass tube diameter should be kept at least 12
mm.
Let us consider we have one container filled with a
liquid and we need to measure the pressure of liquid at point A in the
container. Let us consider that we are using the pressure measuring device “Piezometer”
here to measure the pressure of liquid at point A as displayed here in
following figure.
Rise of liquid in the glass tube of Piezometer will
provide us the pressure head at point A and could be written as mentioned here.
Rise of liquid in the glass tube of Piezomter will also be termed as
Piezomteric head.
Gauge pressure at point A = ρ x g x h
Where,
ρ = Density of liquid
g = Acceleration due to gravity
h = Rise of liquid in Piezometer glass tube
Absolute pressure at Point A = Pa + ρ x g x h
Where, Pa is the atmospheric pressure
Advantages of Piezometer tubes
There are following advantages of using Piezometer
tube for measuring the pressure at a point of fluid in a container and these
are as mentioned here.
Technique of using Piezometer is very simple
Pressure measurement with Piezometer tube is quite
economical
Piezometer tube will provide the accurate pressure
reading
Disadvantages of Piezometer tubes
There are following disadvantages of using
Piezometer tube for measuring the pressure at a point of fluid in a container
and these are as mentioned here.
Piezometer tube will only be used for measuring the
pressure at a point of liquid. It could not be used to measure the pressure at
a point of gas. In simple, Piezometer tube will not be used to measure the
pressure of gas.
Piezometer tube will not be used to measure the
negative pressure. Therefore pressure at point A, where we are measuring the
pressure, must be greater than the atmospheric pressure.
Liquid will be exposed to atmosphere in Piezometer
tube and therefore liquid must be clean and non-toxic.
Insensitive when piezomteric head will be small as
it will be very difficult to measure the pressure if piezomteric head will be
small.
We will discuss another type of simple manometer
i.e. U-Tube manometer in detail in our next post.
We will see there the advantages and disadvantages of U-tube manometer too.
We will see there the advantages and disadvantages of U-tube manometer too.
Do you have any suggestions? Please write in comment
box.
Reference:
Fluid mechanics, By R. K. Bansal
Image Courtesy: Google
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