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
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
HOOK GAUGE METHOD
IN A LARGE TANK FOR ACCURATE MEASURE OF A VERY SMALL CHANGES
ADVATANGES : ACCURACY UP TO + - 0.1MM
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
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
ULTRASONIC LEVEL SENSORS
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
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
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