Monday, 25 December 2017

SIMPLE MANOMETER: PIEZOMETER, ADVANTAGES AND DISADVANTAGES OF PIEZOMETER

SIMPLE MANOMETER: PIEZOMETER, ADVANTAGES AND DISADVANTAGES OF PIEZOMETER

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.

Do you have any suggestions? Please write in comment box.

Reference:

Strength of material, By R. K. Bansal
Image Courtesy: Google

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