Pressure measurement is the analysis of an applied force by a fluid (liquid or gas) on a surface.
So, the intensity of pressure which is exerted by the fluid on supporting surface on the container in which they are stored. It means pressure will act on wall also at bottom of the container.
If the fluid is flowing under pressure that is in pipes, it will be the same in all direction. We live on the earth which is surrounded by air.
This air due to its own weight exerts pressure on the surface on the earth. This pressure is also known as atmospheric pressure.
What we discuss About Pressure Measurement
- Atmospheric pressure
- Gauge pressure and vacuum pressure
- Absolute pressure
- Measurement of pressure
- Types of Manometers
- Differential manometers and Types
- Advantages and limitation of manometers
- Mechanical gauge
- Types of mechanical gauges
- Important Question Answer
This is the pressure exerted by the envelope of air surrounding the earth’s surface. The atmospheric pressure varies with the height and changes in the temperature also humidity. It is measured with the help of a barometer so also known as barometric pressure.
For example, Atmospheric pressure at sea level is 103 kN/m3. It can also be expressed as 10.3 m of water in term of equivalent water column or 760 mm of mercury in term of equivalent mercury column and also expressed as 1 bar. (105 N/m2 = 1bar)
Fluid pressures are measure either assuming atmospheric pressure at datum or absolute zero at datum. Absolute zero is also define as complete vaccume.
Gauge pressure and vacuum pressure
It is pressure measure with the help of pressure measuring instrument, in which the atmospheric pressure is taken as datum. The atmospheric pressure on the scale is marked as zero.
The pressure which is more than the atmospheric pressure is known as gauge pressure or positive pressure. If the pressure of the liquid is below the local atmospheric pressure, then the gauge is known as vacuum gauge
It is the pressure which is equally to the algebraic sum of atmospheric also gauge pressure. Absolute pressure is dependent of the changes in atmospheric pressure. A pressure of absolute zero can exist only in complete vacuum.
Any pressure measured above the absolute zero of pressure is termed as an absolute pressure.
Click Here To Read An Ideal Gas Law
- Absolute pressure = atmospheric pressure + gauge pressure
Pabs = Patm + Pgauge
- Absolute pressure = atmospheric pressure – vacuum pressure
Pabs = Patm – Pvacuum
The pressure of a liquid is measure by the following devices:-
- Principle of manometers
- Mechanical gauges principle
Principle of manometers :- By balancing the liquid column (whose pressure is to be find out) by the same or another column.
Mechanical gauges principle :- By balancing the liquid column (whose pressure is to be find out) by the spring and dead weight.
It is define as the devices used for measuring the pressure at a point in a fluid by balancing the column of fluid by the same or another column of the fluid.
They are classifies as follows :-
- Simple manometers
- Differential manometers
Simple manometers :- A simple manometers consist of a glass tube having one of its ends connect to a point where pressure is to be measure and other end remains open to atmosphere.
Types of Simple manometers are following types :-
- U- tube manometers
- Single column manometer
Piezometer :- A piezometer is the simplest form of manometer which can be use for measuring gauge pressure of moderate range.
Pressure at A = wh
W = sp. wt of liquid
H = height of liquid
U- tube manometer :- It consist of glass tube bent in u – shape , one end of which is connect to a point at which pressure is to be measure and other end remains open to the atmosphere.
Single column manometer :- It is modify form of u- tube manometer in which a reservoir , having a large cross – sectional area as compare to the area of the tube , is connect to one of the limbs of the manometer.
There are two type of single column manometer
Differential manometers are use to measure the difference of pressure between two points in the same pipe or in two different pipes.
Types of differential manometers
- Two piezometer manometer
- U- tube differential manometer
- Inverted u- tube differential manometers
- Differential micro – manometers
Two piezometer manometer :- The manometer which consist of two separate piezometer is referre as two piezometer manometers.
U – tube differential manometer :- The glass tube bent in u – shape and having both its ends connect to two gauge point between which pressure difference is to be measure and is define as u – tube differential manometer.
Invert u – tube differential manometer :– The glass tube bent in u – shape and hold invert is refer to as invert u – tube differential manometer. These are used for measuring difference of two pressure where accuracy is the major consideration. In this type of monometer , a lighter liquid is use.
Differential micro – manometer :- The micro – manometers which measure very small pressure difference or pressure differences with much accuracy is define as micro – manometer.
Advantages and limitation of manometers
- Easy to fabricate and relatively inexpensive.
- Good accuracy.
- Require little maintenance.
- High sensitivity.
- Not affected by vibration.
- Specially suitable for low pressure and low differential pressure.
- It is easy to change the sensitivity by affecting a change in the quantity of manometric liquid in the manometer.
- Usually bulky and large in size.
- Being fragile , gets broken easily.
- Readings of the manometers are affect by the change in temperature , altitude and gravity.
- A capillary effect is create due to surface tension of manometric fluid.
- For better accuracy meniscus has to be measure by accurate means.
The pressure measuring device which embody an element which deflects under the action of the apply pressure are referred as mechanical gauge.
These are use for measuring high pressure for example in stream boilers , water mains and where high accuracy is not require.
Types of mechanical gauges
- Bourdon’s tube pressure gauge.
- Diaphragm pressure gauge
- Dead weight pressure gauge
Bourdon’s tube pressure gauge :- Bourdon’s tube pressure gauge can measure gauge pressure as well as vacuum pressure.
Its consist of an elliptical tube XYZ , bent such as an arc of a circle. It is define as bourdon’s tube.
When one end of the tube is attach to the fluid , the internal stress causes the tube to expand , thus the tube tries to straighten itself. These are use for measuring high as well as low pressure.
If bourdon’s tube is made of bronze , then it is use for measuring low pressure and if the bourdon’s tube is made of nickel steel , then it is use for measuring high pressure.
Diaphragm pressure gauge :- These are also used for measuring gauge pressure as well as vacuum pressure. It consist of metallic disc or corrugated diaphragm instead of bent tube.
When is gauge is connect to the fluid pipe at A, it undergoes elastic deformation.
It is communicated to a pointer with the help of a pinion arrangement. The pointer in turn moves on a graduated scale which indicates the pressure intensities.
Barometer reading = 760 mm of mercury
Specific gravity of mercury = 13.6
Intensity of pressure = 40 kpa
Express the intensity of pressure in SI unit both gauge and absolute.
Ans. Given intensity of pressure = 40 kPa
= 40 kN/m2
Sp. Gr. Of mercury = 13.6
Sp. Wt. of mercury = 13.6 * 9.81 = 133.416
1)Gauge pressure :-
In terms of pressure intensity
= 40 kN/m2
= 0.4 * 105 N/m2
= 0.4 bar [ 105 N/m2 = 1 bar]
In terms of water head
H = p/w = 0.4 * 105/9810 = 4.077 m of water
In term of mercury head
H = p/wm = 0.4 * 105/13.6 * 9810 = 0.299 m of mercury
2)Absolute pressure :-
Given barometer reading (atmospheric perssure)
= 760 mm of mercury
= 0.760 m of mercury
= 0.760 * 13.6 * = 10.336 m of water
Absolute pressure = Patm + Pgauge
In term of pressure intensity
Abs. pressure = Patm + P gauge
=0.76 * 9.81 * 13.6 + 40
= 141.396 kN/m2
In term of water head
Abs. pressure = 10.336 + 4.077
= 14.413 m of water
In term of mercury head
Abs. pressure = 0.760 + 0.299
= 1.059 m of mercury
Ans. Given h1 = 25 – 15 = 10 cm = 0.10 m
H2 = 25 cm = 0.25 m
S1 = 0.85
S2 = 13.6
Let Pa be pressure intensity in the pipe and let it be expressed in term of water column.
pressure at B = pressure at C
Pa/w + s1h1 = s2h2
Pa/w = s2h2 – s1h1
= 13.6 * 0.25 – 0.85 * 0.10
= 3.315 m of equivalent water column
Pa/w = w * 3.315
= 9.81 * 3.315
= 32.52 kN/m2
= 32.52 Kpa (kilo pascal)
1)Pressure in the pipe in N/m2
2)Pressure head in meter of water
Sp. Gr. Of oil, S = 0.90
H = 0.60 m
Let Pa be the pressure intensity in pipe.
Hence starting from the right end.
Patm + sh = Pa/w
Patm = 0
Pa = SH
= 0.90 * 0.60
= 0.54 m of equivalent water column head.
And Pa = 0.54 * w
= 0.54 * 9810
= 5297.4 N/m2
Pa = 5297.4 Pa