CHAPTER 6 : MEASUREMENT OF LIQUID LEVEL

TO MEASURE THE QUANTITY OF LIQUID HELD IN A CONTAINER OR VESSEL

AFFECTS BOTH PRESSURE AND RATE FLOW

DIRECT METHOD

LIQUID LEVEL IS DIRECTLY INDICATED BY A SIMPLE MECHANICAL TYPE OF DEVICE

DIP-STICK METHOD

DIPPING A GRADUATED ROD IN A LIQUID

APPLICATION : LEVEL OF OIL IN CAR ENGINE / PETROL STATION

ADVANTAGE : ECONOMIC ll DRAWBACKS : NOT ACCURATE

oil-on-dipstick

SIGHT GLASS METHOD ( PIEZO-METER )

GRADUATED GLASS TUBE MOUNTED ON THE SIDE OF THE LIQUID CONTAINIG VESSEL FOR PROVIDING A VISUAL INDICATION OF THE LIQUID

ADVANTAGES : SIMPLE & DIRECT ll DRAWBACKS : NOT SUITABLE FOR DIRTY LIQUID, MANUALLY RECORDING NEEDED

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HOOK GAUGE METHOD

IN A LARGE TANK FOR ACCURATE MEASURE OF A VERY SMALL CHANGES

ADVATANGES : ACCURACY UP TO + - 0.1MM

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IN A LARGE TANK/RESERVOIR, A SMALL CHANGE IN LEVEL WOULD MEAN LARGE METRIC CHANGES

IN THIS DEVICE, A VERTICAL TUBULAR ROD IS PROVIDED A VERNIER SCALE TO BE CLAMPED AT A SUITABLE HEIGHT AT THE UPPER END AND A U-SHAPED HOOK AT THE LOWER END

THIS ROD MOVES IN A GUIDE BRACKET FIXED TO A RIGID BODY AT THE DATUM OR REFERENCE LEVEL HAS A MAIN GRADUATED SCALE IN IT

IT IS THEN GRADUALLY RAISED UNTIL THE TOP OF THE HOOK BREAKS THROUGH THE SURFACE OF THE LIQUID

THE MOVABLE ROD IS THEN CLAMPED AND THE LEVEL IS READ OFF SCALE

FLOAT GAUGE METHOD

DETERMINING THE LIQUID LEVEL THAT ALWAYS FOLLOW THE VARYING LIQUID LEVEL. THE FLOAT USUALLY USED ARE HOLLOW METAL SPHERES, CYLINDRICAL CERAMIC FLOAT OR DISC SHAPED FLOATS OF SYNTHETIC MATERIAL 1

THE INSTRUMENT CONSISTS OF A FLOAT, COUNTER WEIGHT AND A FLEXIBLE CONNECTION THAT MAY BE A CHAIN OR A THIN METALLIC PERFORATED TAPE

THE COUNTER WEIGHT KEEPS THE CHAIN/TAPE TAUT AS THE LIQUID RISES OR FALLS WITH ANY CHANGE IN THE LIQUID LEVEL

THE CHAIN ATTACHED ON A GEAR OR SPROCKET WHEEL TO WHICH THE POINTER IS ATTACHED

ANY MOVEMENT OF THIS WHEEL WOULD INDICATE ON A SUITABLY CALIBRATED SCALE THE LEVEL OF THE LIQUID IN THE TANK

ADVANTAGES OF FLOAT GAUGES ARE SUFFICIENTLY ACCURATE WHEN PROPERLY CALIBRATED AFTER INSTALLATION

DISADVANTAGES OF FLOAT GAUGES IS A PROPER CORRECTION REQUIRED IF THERE IS THE CHANGE IN THEN LIQUID DENSITY DUE TO A CHANGE IN TEMPERATURE

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ULTRASONIC LEVEL SENSORS 3

USED FOR NON-CONTACT LEVEL SENSING OF HIGHLY VISCOUS LIQUIDS AS WELL AS BULK SOLIDS

THEY ARE ALSO WIDELY USED IN WATER TREATMENT APPLICATION FOR PUMP CONTROL AND OPEN CHANNEL FLOW MEASUREMENT

SOUND WAVES ARE DIRECTED TOWARDS THE FREE SURFACE OF THE LIQUID FROM AN ULTRASOUND TRANSMITTER

THESE WAVES GET REFLECTED FROM THE SURFACE OF THE LIQUID AND ARE RECEIVED BY THE RECEIVER

HYDROSTATIC PRESSURE LEVEL MEASUREMENT DEVICE 4

THE HYDROSTATIC PRESSURE CREATED BY A LIQUID IS DIRECTLY RELATED TO THE HEIGHT OF THE LIQUID COLUMN (P=PGH)

THEREFORE, A PRESSURE GAUGE IS INSTALLED AT THE BOTTOM OR ON THE SIDE OF THE TANK CONTAINING THE LIQUID

THE RISE AND FALL OF THE LIQUID LEVEL CAUSES A CORRESPONDING INCREASE OR DECREASE IN THE PRESSURE P WHICH IS DIRECTLY PROPORTIONAL TO THE LIQUID LEVEL H

THE DIAL OR SCALE OF THE PRESSURE GAUGE IS CALIBRATED IN THE UNITS OF LEVEL MEASUREMENT

THESE GAUGES FUNCTION SMOOTHLY WHEN THE LIQUIDS ARE CLEAN AND NON CORROSIVE. FOR CORROSIVE LIQUIDS WWITH SOLID SUSPENSION, DIAPHRAGM SEALS BETWEEN THE FLUID AND THE PRESSURE GAUGE ARE GENERALLY EMPLOYED

AUTOMATIC LEVEL CONTROL SYSTEM 5

TO PROVIDE AUTOMATIC CONTROL, THUS MACHINES, ELECTRONICS OR COMPUTER REPLACE THE OPERATIONS OF THE HUMAN

AN INSTRUMENT CALLED A SENSOR IS ADDED THAT IS ABLE TO MEASURE THE VALUE OF THE LEVEL AND CONVERT IT INTO A PROPORTIONAL SIGNALS

THIS SIGNAL IS PROVIDED AS INPUT TO THE MACHINES, ELECTRONICS CIRCUIT OR COMPUTER CALLED THE CONTROLLER

THE CONTROLLER PERFORMS THE FUNCTION OF THE HUMAN IN EVALUATING THE MEASUREMENT AND PROVIDING THE OUTPUT SIGNALS, U AND V, TO CHANGE THE VALVE SETTING VIA AN ACTUATOR CONNECTED TO THE VALVE BY MECHANICAL LINKAGE

WHEN AUTOMATIC CONTROL IS APPLIED TO SYSTEMS, WHICH ARE DESIGNED TO REGULATE THE VALUE OF SOME VARIABLE TO A SET POINT, IT IS CALLED PROCESS CONTROL