Measurement of Level, Lecture notes of Electronic Measurement and Instrumentation

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Measurement of level

Methods:

Direct measurement of level

Indirect, measurement. (Pressure to level)

h = head or level

p = pressure

g = density of liquid

Level measurement

Measurement of head(h) is advisable

if level measurement is to determine

the volume of liquid in a vessel

…. Where A is the area

If pressure measurement must be used –

g is density of liquid, g

w

density of water

and G, specific gravity of the liquid. The

measurement depends upon Sp. Gravity.

Direct measurement of level

• Sight glass
• Float and tape
• Float and shaft
• Bubbler system
• Air trap
• Diaphragm box
• Interfacial level
• Level in pressurized tanks
• Capacitive
• Ultrasonic
• Nucleonic (gamma radiation)
• Guided wave radar
• Switches (limit/vibrating fork/electronic)

Sight glass

Scale

Open or vented vessel Pressurized vessel

Diaphragm box/Air trap

Diaphragm

box

gauge gauge Air box

(trap)

Closed

valve

Capacitive

C= KA/d

K = dielectric constant of

material between plates

Guided wave radar

Signal out

Transmitter

Transmitted pulse

Reflected pulse

Wave guide

Process fluid

Wave reflects due

to difference in

dielectric constant

Nucleonic level gauge

Liquid

Radiation

receiver

I = I

0

e

• mrx

I

0

=intensity without absorbing

material

x=thickness of material

m=ns is the absorption

coefficient in cm

n the number atoms per cm

3

of

the material

(atomic density)

s is the absorption cross

section in cm

2

x

Source

Multilevel switches

Level of solids

Pressure switch

Ultrasonic level sensor

Guided Wave Radar

Measurement of weight

Level

Material

Open

Closed

Relation between head and displacement, d is

Where

g

m

is density of manometer fluid

A

1

is area of range tube

A

2

is area of float chamber

d is the float displacement

g

l

is liquid weight density

Level measurement in pressure vessels

Measurement of interface level

g

m

= weight density of manometer

liquid

g

l

= weight density of lighter fluid

g

2

= weight density of heavier fluid

A

2

=Area of float chamber

A

1

= Area of range tube

d = float displacement

g 1 g 2 A 2

A

1

d

h

h

0

h

1

Interface level

Measurement of density

Flow regulators

Gas

h

1

h

0

Gas

Measuring tank

Reference

chamber