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CHAPTER 6 : FLOW MEASUREMENT & MEASUREMENT OF LIQUID LEVEL (M/S 15…
CHAPTER 6 : FLOW MEASUREMENT &
MEASUREMENT OF LIQUID LEVEL
(M/S 15-32)
DIRECT LIQUID LEVEL MEASUREMENT
METHOD WHICH THE ACTUAL LIQUID LEVEL IS DIRECTLY INDICATED BY A SIMPLE MECHANICAL TYPE OF DEVICE
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
WORKING PRINIPLE
THE INSTRUMENT CONSISTS OF A FLOAT, COUNTER WEIGHT AND A FLEXIBLE CONNECTION THAT MAY BE A CHAIN OR 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: Capable of giving continuous
direct liquid level measurement
Drawbacks : Proper correction is required if there is a change in the liquid density
HOOK GAUGE METHOD
Advantages: Accuracy up to +-0.1 mm
Method
Determining the liquid level in a large tank
for accurate measure of a very small changes. A small change in a large tank would mean large volumetric changes
Drawbacks : Manually operated and no
automatic reading
Sight Glass Method ( Piezo-meter)
Determining the liquid level by a graduated glass tube mounted on the side of the liquid containing vessel for providing a visual indication of the liquid level.
Drawbacks
Not suitable for a dirty , viscous & corrosive liquids
Manually recording needed
Advantages: Simple & direct ( clean & coloured liquids )
Measurement of liquid level
Liquid level measurement
To measure the quantity of liquid held in a
container or vessel
Classification
Direct method
Indirect method
Why we needed liquid level measurement?
Liquid level affects both pressure and rate flow in and out of the container and therefore its measurement and control becomes important to a lot processes in modern manufacturing plant.
Dip - stick method
Advantages: Economyic.
Applications
a. Level of iol in car engine
b. Level of oil in petro station
Drawbacks
Not possible for real time observation
Not accurate ( moving fluid)
Method
Determining the liquid level by dipping a graduated rod in a liquid
Automatic level Cobtrol System
To provide automatic control, thus, machines, electronics or computer replace the operatios 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 signal, s
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
Ultrasonic level sensors
They are also widely used in water treatment applications for pump control and open channel flow measurement
Sound waves are directed towards the free surface of the liquid from an ultra sound transmitter
These waves get reflected from the surface of the liquid and are received by the receiver
In this technique, liquid variation are quite accurately determined by detecting the total time taken by the wave to travel to the liquid and then back to the receiver.
The longer these times travel, the father away is the liquid surface, which i a measure / indication of the liquid level
Requires high degree of experience and sklii to operate since the actual instrument is expensive.
Hydrostatic pressure level measurement Device
The hydrostatic pressure created by a liquid is directly related to the height of the liquid column (p=pgh)
Method
