The Liquid Levels can be measure by different methods but following two are more important from the subject point of view. 01)Electrical Methods:-
The Direct Conversion to Liquid Level position to Electrical Signal is used in many instances.The measurements is generally done by two conversions,so
that the Liquid Level is determined indirectly.
The first conversion usually is Liquid Level to a Displacement through a Float in a Liquid or a Spring Loaded Plate in contact with the surface in the case
of Granular Solids.
This Displacement is then converted into an Electrical Signals by a Secondary Transducer connected to Float or Plate.These are however,in many applications where this is not possible and hence other methods like Optical or Acoustic means or Gamma rays are used.
The Electric Transducers used for Level Measurements are
01)Resistive
02)Inductive
03)Capacitive
Now we will see about Resistive Method first.
01)Resistive Method:-
This method uses Mercury as a Conductor.A number of Contact Rods are placed at various Liquid Levels.
As Head "h" increases, the rising Level of Mercury above the Datum,shorts successive Resistors "R" and increases the value of "h" directly.
Advantages of Resistive Method:-
01)The System uses Low Voltage to eliminate danger to the operators and to prevent Arcing at the contact points.
02)The Signal can be transmitted to any desired point.
03)The Transducers can be used in Pressured Containers without Packing Glands or Shafts.
04)The unit is simple to calibrate since the distance between the levels of Contact Rods can be accurately measured and the indicated value may be checked for each measured value.
Disadvantages of Resistive Method:-
01)Due to Arcing at the contact points,it is not safe to use this Transducer in Explosive Atmosphere.
02)In order to have a step less indication of the Liquid Level,an extremely large number of Contact Rods are needed.
03)The Contact Rods are corroded by Corrosive Liquids.In addition,the Electric Charges promote Corrosion.
04)These Systems also present difficulties when there is Saturated Vapour above the Liquid Phase.
05)Any changes in the Conductivity of the Liquid causes serious errors.
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