Tank liquid sensors are an integral part of the ship

Describe with the aid of suitable sketches the working principle of cap activity type level sensor

Ultra level sensor

Float.

Level Detection and Measurement by Using a Float Sensor

Level Detection Using a Float Sensor

Principle of Operation: A liquid level control system by using a float sensor works on the principle of buoyancy, which states, “A float immersed in a liquid is buoyed towards upward direction by an applied equal force to the weight of the displaced liquid”. As a result, the body drives partially and gets submerged upon the liquid surface and covers the same distance the liquid level moves.

Construction: A level measurement float system consists of a float, a sensor stem, a magnet, a reed switch and a weight suspended on the outside of the open tank. A scale is fixed on the outside of the tank, and the contents of the tank’s level are indicated by the position of the weight along the scale.

Working: Level detection of liquids is often done with a float-type liquid level switch. The float transfers on a mechanical arm or sliding pole and activates a switch when the level moves towards upward direction. Sometimes the float itself contains a small magnet that varies the state of a switch when the liquid level gets moving up and moves into the original position. This type of level sensor comes with many advantages like it is very simple, highly accurate, and best suitable for various products.

The Disadvantages of this sensor are that it requires various mechanical equipment, especially the pressure vessels.

Primary Areas of Float Sensor Application: In view of the requirements pertaining to the increase in usage of sealed tanks, the current industrial systems make use of this type of float method for precise reading and accuracy, which is a good example of electronics and mechanical engineering, making it the most accurate level-measuring system for various applications in very large storage tanks.

2. Level Detection and Measurement by Using Capacitance Sensor

Level Detection Using Capacitance Sensor

Capacitance level sensors are made available for wide range of solids, aqueous, organic liquids and slurries. This technique is frequently stated as the radio-frequency signals applied to a capacitance circuit. The capacitive sensors are designed to sense material with dielectric constants as low as 1.1 for coke and fly ash, and as high as 88 for water or other liquids.

Principle of operation: The principle of capacitive level measurement is based on the change of capacitance. There are two plates in capacitive sensor: one plate acts as an insulated electrode and the other plate acts as a tank wall. The capacitance depends on the liquid level. An empty tank has low capacitance while a filled tank has higher capacitance. A simple capacitor consists of two electrode plates separated by a small thickness of an insulator such as solid, fluid, gas, or vacuum.

The Value of C depends on dielectric constant used, area of the plate and also on the distance between the plates.

C=E(KA/d) Where: C = Capacitance in Pico farads (pF) E = a constant known as the absolute permittivity of free space K = Relative dielectric constant of the insulating material A = Effective area of the conductors d = Distance between the conductors

This change in capacitance can be measured by using an AC Bridge.

Construction and Working

The measurement of liquid level is done by applying a Radio Frequency signal between the conductive probe and the vessel wall. The Radio Frequency signal results in a very-low current which flows through the dielectric process material in the tank from the probe to the vessel wall. If the liquid level in the tank drops, then the dielectric constant decreases, which leads to the drop in capacitance reading as well as minute drop in current flow.

This change can be detected by the liquid-level switch’s internal circuitry and translated into relay state changes of the level switch in case of a point level detection.

The main advantages of these capacitance systems include easy installation, broad application range, good accuracy suitable for variety of applications and highly recognized and well -proven technology.

The disadvantages include sensitivity to changes in the measurable properties such as dielectric constant and conductivity which creates an issue; furthermore, it is an intrusive system.

Primary Areas of Capacitance Sensor Application: Capacitance level sensor probes are used for measuring the levels of:

• Fluids
• Liquid metals at very-high temperature range
• Dissolved gases at very-low level of temperature
• Very-high density industrial processes.

Continuous Level Measurement and Detection Systems

A continuous level sensor is most sophisticated and also provides liquid level monitoring of an entire system. This liquid level sensor is used to measure the fluid level within a specified range, moderately than at a one point, which produces an analog output and directly correlates to the level in the vessel. To create a liquid level management system, the output signal is interconnected to process a control loop and linked as a visual liquid level indicator. This measurement system consists of variety of sensors such as

• Ultrasonic Sensor

1. Level Detection and Measurement by Using Ultrasonic Sensor

Level Detection Using Ultrasonic Sensor

Ultrasonic level instruments works on the basic time-of-flight principle which states that sending a sound wave from a peizo electric transducer to the contents of the vessel, which may contain liquid, solid or slurries level.

This liquid level sensor comprises two elements such as an associated electronic transceiver and a transducer with relatively high efficiency. In case of a liquid level controller, the fluid level can be determined by measuring the trip time difference between a transmitted ultrasonic pulse and a reflected echo.

The frequency range for ultrasonic methods varies from a range of 15-200 kHz for transmitting and receiving ultrasonic waves. The lower frequency instruments are used for the measurement of difficult applications such as longer distances and solid-level measurements, and the instruments with higher frequency are used for shorter liquid level measurements.

The ultrasonic sensors are advantageous as they are not in direct contact with the product, and the level measurement is done without involving any physical contact. They do not have any moving parts.

As a disadvantage, these types of sensors are not suitable for higher pressures in a vacuum and temperature limits of up to 170 degree centigrade.