PRESSURE MEASURING DEVICES

Presented by-

Biswajit Baruah

And

Bhaskar Chetia

What is Fluid Pressure? Fluid contained in a vessel exerts force on all

its sides. The force per unit area is called Fluid Pressure.

Mathematically-

p= P/A where P= the force and

A= area on which the force acts

p= intensity of pressure

S.I. unit is N/m2 or Pascal

Pressure Head- Represents the internal energy of a fluid due to

the pressure exerted on its container. It may also be called static pressure head or simply static head.

Mathematically,

h = p / γ

where h=pressure head

p=intensity of pressure

γ=specific wt. of the liquid

Hence, pressure intensity may be expressed in two ways-

1. As force per unit area &

2. As an equivalent static head.

Pascal’s Principle: Any change in the pressure of a fluid is transmitted

uniformly in all directions throughout the fluid.

A small force F1 applied to a piston with a small area produces a much larger force F2 on the larger piston. This allows a hydraulic jack to lift heavy objects.

Atmospheric,absolute and gauge pressure

Devices used for measuring pressure-

Fluid pressure may be measured by following devices-

1. Manometers-

(a) Simple manometers- PIEZOMETER, U-TUBE MANOMETER & SINGLE-

COLUMN MANOMETER.

(b) Differential manometers-

U-TUBE DIFFERENTIAL MANOMETER & INVERTED U-TUBE DIFFERENTIAL MANOMETER.

2. Mechanical gauges- BOURDON TUBE PRESSURE GAUGE, DIAPHRAGM

PRESSURE GAUGE & VACUUM GAUGE.

MANOMETER- measures pressure at a point in a fluid by balancing the column of fluid by the same or another column of fluid.

U-TUBE MANOMETER

PIEZOMETER

SINGLE-COLUMN MANOMETERS

INCLINED

VERTICAL

Differential manometers-

U-TUBE DIFFERENTIAL MANOMETER

INVERTED U-TUBE

DIFFERENTIAL MANOMETER

Simple manometers-Consists of a glass-tube whose one end is connected to a point where pressure is to be measured and the other end is exposed to the atmosphere

Includes-

Piezometers

U-tube manometers &

Single-column manometers

PIEZOMETER--It is the simplest form of Manometer,used for measuring moderate pressure.It consist of a Tube,open at one end to the Atmosphere,in which,Liquid can rise freely without overflow,the Height,to which the Liquid rises up in the Tube,gives the Pressure Head directly.

p= γ h-

-not suitable for measuring Negative Pressure because Air will enter in the pipe through the Tube

Measures gauge pressure only.

DisadvantagesCannot be used for

measuring large pressures in case of lighter liquids.

&Cannot be used for

measuring gas pressures

U-tube manometer- "U"-Tube manometer

enables the pressure of both liquids and gases to be measured with the same instrument.

The fluid whose pressure is being measured should have a mass density less than that of the manometric fluid and the two fluids must be immiscible.

The pressures in the right limb and the left limb above the datum line B-C are equalPressure head above B-C in the right limb= h2S2

Pressure head above B-C in the left limb = h+h1S1

Equating, h+h1S1= h2S2 OR, h= h2S2

_ h1S1

S2

S1

FOR POSITIVE GAUGE

PRESSURE

.

S1

S2

FOR NEGATIVE

GAUGE PRESSURE

The pressures in the right limb and the left limb above the datum line X-X are equal Pressure head above X-X in the right limb= 0Pressure head above X-X in the left limb = h+h1S1+h2S2

Equating,

h+h1S1+h2S2 = 0 OR ,h = _( h1S1+h2S2)

U-TUBE MANOMETER

Light liquid

Heavy liquid

S1

S2

A= area of crossection

Datum line

a= area of crossection

A>>a

A x Z1 = a x h2

Or, Z1= (a x h2)/A ............(1)Equating the pressure heads above the datum line Z-Z,

h +(h1+ Z1)S1 = (h2+ Z1)S2

or, h = (h2+ Z1)S2 _ (h1+ Z1)S1 or, h = Z1 (S2 _ S1) + h2S2 _ h1S1

= { (a x h2)/A } (S2 _ S1) + h2S2 _ h1S1

As A>>a, so ratio A/a is very very small and can be neglected and above equation becomes,

h = h2S2 _ h1S1…………………………………………………… (2)

SINGLE-COLUMN

MANOMETER(VERTICAL)

#useful for the measurement of small pressures and is more sensitive than the vertical tube.-# Due to inclination the distance moved by the heavy liquid in the right limb is more.

Let, l = length of the heavy liquid moved in the right limb, θ = inclination of right limb horizontal and Z2 = vertical rise of liquid in the right limb from X-X= l sin θ Putting the value of Z2 in equation (2) we get,

h = l sinθ x S2 _ h1S1

SINGLE-COLUMN

MANOMETER(INCLINED)

Differential U-tube manometer-

a differential manometer is used to measure the difference in pressures between two points in a pipe, or in two different pipes.

-most commonly used types-1.U-tube differential manometer.2.Inverted U-tube differential manometer

Case 1.Figure (a) below shows a differential manometer whose two ends are connected with two different points A and B at the same level and containing same liquid.

We know that the pressures in the left limb and the right limb , above the datum line X-X, are equalPressure head in the left limb = hA + (h +h1)S1

Pressure head in the right limb = hB + h1S1 + hSEquating,

hA + (h +h1)S1 = hB + h1S1 + hS or, hA - hB = h ( S – S1)

i.e. difference of pressure head, hA - hB = h ( S – S1)

case 2- Figure (b) below shows a differential manometer whose two ends are connected with two different points A and B at different levels and containing different liquids. 

We know that the pressures in the left limb and the right limb , above the datum line X-X, are equalPressure head in the left limb = hA + (h +h1)S1

Pressure head in the right limb = hB + h2S2 + hSEquating,

hA + (h +h1)S1= hB + h2S2 + hSor, hA - hB = h ( S – S1) + h2S2 - h1S1

Thus, difference of pressure at A and B, hA - hB = h ( S – S1) + h2S2 - h1S1

Mechanical gauges- Used for measuring medium and high

pressures. Employs elastic systems such as tubes,

diaphragms, bellows,etc. Types-

Bourdon tube pressure gauge.

Diaphragm gauge &

Vacuum gauge.

Bourdon tube pressure gauge

The bourdon tube pressure instrument is one of the oldest pressure sensing instruments in use today. The bourdon tube consists of a thin-walled tube that is flattened diametrically on opposite sides to produce a cross-sectional area elliptical in shape, having two long flat sides and two short round sides. The tube is bent lengthwise into an arc of a circle of 270 to 300 degrees. Pressure applied to the inside of the tube causes distension of the flat sections and tends to restore its original round cross-section. This change in cross-section causes the tube to straighten slightly.Since the tube is permanently fastened at one end, the tip of the tube traces a curvethat is the result of the change in angular position with respect to the centre. Within limits, the movement of the tip of the tube can then be used to position a pointer to indicate the applied internal pressure.

BOURDON TUBE PRESSURE GAUGE

Diaphragm gauge- This type of gauge employs a

metallic disc or diaphragm instead of a bent tube. This tube or diaphragm is used for actuating the indicating device.

Referring to the figure below, when pressure is applied on the lower side of the diaphragm it is deflected upward. This movement of the diaphragm is transmitted to a rack and pinion. The latter is attached to the spindle of needle moving on a graduated dial. The dial can again be graduated in a suitable scale.

Vacuum Gauge- Bourdon gauges discussed earlier can

be used to measure vacuum instead of

pressure. Slight changes in the design

are required for this purpose. Thus, in

this case, the tub is bent inwards instead

of outwards as in pressure gauges.

Vacuum gauges are graduated in

millimetres of mercury blow the

atmospheric pressure. In such cases,

therefore, absolute pressure in

millimetres of mercury is the difference

between barometer reading and vacuum

gauge reading.

Vacuum gauges are used to measure

the vacuum in the condensers, etc. if

there is leakage the vacuum will drop.