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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)
Measurement of Liquid Level
Liquid level measurement
To measure the quantity of liquid held in a container or vessel.
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 of processes in modern manufacturing plant.
Classification
a. Direct method.
b. Indirect method.
Direct liquid level measurement
Method which the actual liquid level is directly indicated by a simple mechanical type of device.
Dip - stick Method
Method
Determining the liquid level by dipping a graduated rod in a liquid.
Application
a. Level of iol in car engine.
b. Level of oil in petrol station.
Advantages
: Economic.
Drawbacks
Not accurate (moving fluid).
Not possible for real time observation.
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.
Advantages
: Simple & direct (clean & coloured liquids).
Drawbacks
Not suitable for dirty, viscous & corrosive liquids.
Manually recording needed.
Hook Gauge Method
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.
Advantages
: Accuracy up to +-0.1 mm.
Drawbacks
: Manually operated and no automatic reading.
Float Gauge Method
Determining
the liquid level that always follow the varying liquid level.
The float usualy used are
hollow metal spheres, cylindrical ceramic float or disc shaped floats of synthetic material.
working Principle
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
: Capable of giving continuos, direct liquid level measurement.
Drawbacks
: Proper correction is required if there is a change in the liquid density.
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 applications 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.
In this technique, liquid level variations 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 farther away is the liquid surface, which is a measure / indication of the liquid level.
Requires high degree of experience and skill 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).
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 with solid suspensions, 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 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.
MUHAMMAD FARIS IRSYAD BIN MOHD ZAKI
2018280906
EM110 4A
