This ppt contains Differential Method for measuring of liquid level of storage tanks and vessels. Differential method is one of the indirect method for liquid level measurement..

1. INSTRUMENTATION &
PROCESS CONTROL
TOPIC: LIQUID LEVEL MEASUREMENT

2. LEVEL MEASURING METHODS
 Direct method:-
In this method level of liquid is directly measured on the scale. Direct
methods use varying liquid level as the means of level measurement.
 Indirect Method:-
In this type of method level is measured in terms of the variables which
changes with change in level of liquid.
Such variables are hydrostatic pressure, attenuation of radioactive
energy, electrical properties.

3. LEVEL MEASURING METHODS
A)Direct Methods:-
a) Based on direct visual observation:
1) Gauge Stick
2) Hook Gauge
3) Gauge Glass
b) Buoyancy Methods:
1) Float Gauges
2) Displacer Gauges

4. LEVEL MEASURING METHODS
B)Indirect Methods:-
a) Based on HYDROSTATIC HEAD developed by the liquid:
1) Pressure Gauge Method
2) Air Trap Method
3) Diaphragm Box Method
4) Air Purge OR Bubbler Method
5) Differential Pressure Method
b) Electrical Capacitance method:
c) Radiation method:

5. Indirect Methods
Hydrostatic Methods:-
 Principle: When the liquid is steady in the tank it exerts pressure on the
wall of the tank. Such a pressure due to height of liquid present above a certain
reference is called as HYDROSTATIC PRESSURE.
P = ρgh
 Types of Hydrostatic Method:-
1) Pressure Gauge Method
2) Air Trap Method
3) Diaphragm Box Method
4) Air Purge OR Bubbler Method
5) Differential Pressure Method

6. Hydrostatic Methods
 Differential Pressure Method:-
 All the other hydrostatic level measuring techniques can be used only when
the vessel containing liquid is exposed to atmosphere.
 But in case of liquid in a pressure vessel like Boiler Drum, the bottom
pressure corresponds to Hydrostatic Pressure + External Pressure. Hence
bottom level can’t represent liquid level unless external pressure is balanced.
 This balancing is achieved by connecting Enlarged Leg Manometer to the
pressure vessel as shown in the figure.
 The manometer reading corresponds to the hydrostatic pressure only hence
manometer itself can be calibrated in terms of liquid level.

7. Differential Pressure Method:-
Figure:-