Basics of Level Measurement - Working Principle and Types of Level Measurement


Level Measurement Definition

Level measurement is a single dimension from a reference point. As shown tank level is measured, either by Inage method or Outage method. Each Manufacturer has a different Instruction Set.

Level Measurement Principle

Level devices operate under three main different

  1. The position (height) of the liquid surface
  2. The pressure head
  3. The weight of the material

There are two methods used to measure the level of a liquid:

  1. Direct Methods
  2. Indirect or inferential Methods
  1. Direct Methods ( ) Visual Methods)
    The direct method measures the height above a zeropoint by any of the following methods point by any of the following methods.
    Direct methods for level measurement are mainly used where level changes are small and slow such as;
    Sump tanks and Bulk storage tanks.
    Direct methods are simple and reliable. There are four
    types of direct level measurement devices:
    1.Dip-sticks & Dip-Rods
    2.Weighted gauge tape
    3.Sight Gauges, and

Sight Gauges
There are various types of sight gauges, the two most
common types being used are:
1.Sight glass; flat tubular and reflex Sight glass; flat, tubular and reflex
2.Magnetic sight gauges.

The flat glass
The flat glass type, is used for non pressurized vessels.
It is a glass window or windows that forms part of the vessel.
A typical application is in hot oil tanks, where excessive foam
contaminated oil may be easily detected.

Tubular Sight Glasses

Light is refracted from the vapor portion of the column and is shown generally generally as white color. Light is absorbed absorbed by the liquid portion in the column and is shown generally as a dark color.
 They are used mainly for non-corrosive, non-toxic inert at moderate temperatures and pressures.

Magnetic type Sight gauges
The magnetic level gauge, consisting of a magnetic float that travels travels up and down on the inside of a long, non-magnetic magnetic (usually stainless steel) pipe. The pipe is connected to flanged nozzles nozzles on the side of the tank. The pipe column is provided provided
with a visual indicator, consisting of triangular wafer elements
 These elements flip over (from green to red, or any other color) when the magnet in the float reaches their level. Alarm switches and transmitter options are available with similar
magnetic coupling schemes.

Operational considerations for Sight Glasses
Sight glasses are usually installed with shutoff valves and a drain valve for the purpose of maintenance, repair and replacement.
o An important safety feature of these external sight glasses is the inclusion of ball check valves within the isolation valves.
The purpose of these check valves is to prevent the escape dangerous fluids if the glass breaks. Therefore it is important that the isolation valves are left fully open when the sight glass
is in use, otherwise the operation of the check valves may be inhibited.
o Sight gauges must be accessible and located within visual range.

They are not suitable for dark liquids Dirty liquids will prevent They are not suitable for dark liquids. Dirty liquids will prevent the viewing of the liquid level.


Floats give a direct readout of liquid level when they are connected to an indicating instrument through a mechanical linkage.

A simple example of this is the weighted tape tank gauge, the position of the weighted anchor against a gauge board gives an indication of the liquid level in the tank.

The scale of the gauge board is in reverse order i e the zero The scale of the gauge board is in reverse order, i.e. the zero level indication is at the top and the maximum level indication is
at the bottom of the gauge board.

Indirect (inferential) Methods

The indirect or inferential method of measurement uses the changing position of the
liquid surface to determine level with reference to a datum line.

It can be used for low & high levels where the
use of the direct method instruments is

  1. Hydro static Pressure Methods
    2.Displacement devices
    3.Capacitance Level Instrumentation
    4.Radiation-Based Level Gauges

Hydrostatic Pressure Methods
A vertical column of fluid exerts a pressure due to the column’s weight. The relationship between column height and fluid pressure at the bottom of the column is constant for any
particular fluid (density) regardless of vessel width or shape.
 This principle makes it possible to infer the height of liquid in a vessel by measuring the pressure generated at the bottom:

The level of liquid inside a tank can be determined from the pressure reading if the weight density of the liquid is constant.

Level measurement involving the principles of hydrostatics have taken numerous forms, including:

1.The diaphragm-box system
2. differential-pressure t meters
3.The air-bubble tube or purge system