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CHAPTER 6 : FLOW MEASUREMENT & MEASUREMENT OF LIQUID LEVEL, Untitled,…
CHAPTER 6 : FLOW MEASUREMENT &
MEASUREMENT OF LIQUID LEVEL
Direct liquid level measurement
Method which the actual liquid level is directly indicated by a simple mechanical type of device
Measurement of Liquid Level
Liquid level measurement
To measure the quantity of liquid held in a
container or vessel.
Classification
a. Direct method.
b. 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 of processes in modern manufacturing plant.
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
Advantages : Accuracy up to +-0.1 mm.
Drawbacks: Manually operated and no
automatic reading.
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.
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
1) The instrument consists of a float, counter weight and a flexible connection that may be a chain or a thin metallic perforated tape.
2) The counter weight keeps the chain / tape taut as the liquid rises or falls with any change in the liquid level.
3) The chain attached on a gear or sprocket
wheel to which the pointer is attached
4) 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.
Dip - stick Method
Advantages: economic
Drawbacks
Not accurate (moving fluid).
Not possible for real time observation.
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.
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 s called process control.
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.
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. (time travel technique)
•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.
WAN AHMAD FARISH DANIAL (2018280292)
