Answer MET Question 18

Question: Tank liquid level sensors are an integral part of ships. Describe with aid of suitable sketches the working principle of, (a) capacitive type level sensor; (b) Ultrasonic level sensor; (c) Float.

Answer:

A. The liquid is known as the dielectric. The key property of dielectric materials is the dielectric constant or relative permittivity. This property is the amount of charge the liquid can absorb.
As the liquid is introduced near the electrodes of the capacitor the capacitance changes progressively and liquid level can be determined. To measure variations in capacitance electric energy flowing into and out of the electrodes is measured as the circuit potential is varied. A regular flow of energy is established by connecting the electrodes to an alternating current measurement circuit. The more energy flow to the electrodes, the greater the capacitance, meaning more liquid between the electrodes.
For sensor calibration reference measurements at empty and full must be taken. Generally, we need to know the dielectric constant of the liquid being measured to calibrate the sensor at its full condition. With the empty and full outputs are established, liquid level sensing comes to relating sensor output to these values.
Advantages: No mechanical parts,
High lifetime, no wear
Fluid is not contaminated by sensor
Simple replacement in case of defect
Suitable with plastic and glass containers
Adhesion can be recognized.
Disadvantage : comparitively expensive
   

B. Ultrasonic sensors work by sending sound waves that echo off of a target and return to the transmitter. The term ultrasonic means above human hearing, or any sound wave above 20 kHz. This method is quite accurate, with a line of sensors with an accuracy of 0.25% of detected range. Because the speed of sound is a constant, under fixed atmospheric conditions, the time from the sound burst to the return is measured and translated into a distance. The sensor’s microprocessor calculates the distance and converts it to a level indication, volume measurement, or a rate of flow. It also compensates for temperature, and filters the signal.
Advantages:
Ultrasonic level sensors are typically quite small, low maintenance, and easy to ship and install.
It allows for more advanced control.
They do not come in contact with the target substance, which avoids build-up and damage.
Disadvantages:
Ultrasonic sensors require an unobstructed air column between the sensor and the target. Anything that deflects or absorbs the signal, or acts as a false surface, may cause erroneous readings. This can result from physical obstructions, excessive foam, heavy vapors, thick dust and light powders.

C. The principle of float type level meter is that a float moves up and down due to buoyancy. A reed switch in a stem is actuated by a magnet in the float, and outputs detection signal.
The advantage is simple construction, cheaper and easy installation.
Disadvantage:
1. If float gets stuck it can't be known from outside.
2. Liquid with high viscosity cannot be measured.
3. If the pressure inside of the tank is high or the liquid’s specific gravity is low, the float will not operate.