Level Measurement
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Introduction
The determination of liquid level applications has been developed for many years especially for process industries. There are many ways or methods to determine the level of a liquid. Whatever method we chosen, much depend on the nature of the liquid itself and sometimes the environment also might limit our choice of the methods available.
Level Measurement Techniques
Fibre Optic
There are many works done using this method. Pekka Raatikainen and team, using this method in their project named "Fibre-Optic Liquid Level Sensor" was published in October 1996. The project was based on total reflection of light that is distributed by a contact of a sensor tip with a liquid. There are four different tip of shapes being fabricated which are conical, rounded conical, second order polynomial and third order polynomial.
The basic operation of this system, the light from the LED (transmitter) is transferred to the sensor through the fibre (first fibre - forward path). When the sensor in air, we know that, in air the light reflection is 100% or also called total internal reflection (TIR) 141 whereas when the sensor immersed in liquid, the light reflection is decreased because some of the light being refracted into a liquid. The light then passes into the second fibre (return path) that is connected to receiver (photodiode), which detects the optical power. The value of optical power is decreased when the sensor immersed in liquid as the light also being refracted into a liquid.
Ultrasonic (Ultrasound)
This method can be used in many applications. Using of acoustic impedance as based on measuring attenuation of longitudinal ultrasonic wave in waveguide sensing element to control the level of coolant industrial power-generating equipment. This ultrasonic attenuation is determined when the sensing element is immersed in liquid. The principle operation of the system, the impulses from the converter travel through the waveguide communication to the sensing element.
Therefore, it is very important to use a good waveguide communication to minimize losses of the signal when impulses travels through it to reach the sensing element, in order to get a desired output. In addition, it is important to ensure the waveguide communication is protected from ambient factor.
Capacitive
A capacitive instrument also can used to determine the level of liquid helium 171 . The sensor system consists of two concentric cylindrical plates that are an inner and an outer rod of stainless steel. The level of liquid helium can be determine by partially immerse the plates in the liquid helium. By doing this, as plates immerse in liquid, the capacitance of the plates is increased as the liquid level is increased. The capacitance of the plates can be measured using LC oscillator that is based on a tunnel diode.
Conclusion
Level sensors detect the level of liquids and other fluids and fluidized solids, including slurries, granular materials, and powders that exhibit an upper free surface. Substances that flow become essentially horizontal in their containers (or other physical boundaries) because of gravity whereas most bulk solids pile at an angle of repose to a peak. The substance to be measured can be inside a container or can be in its natural form (e.g., a river or a lake). The level measurement can be either continuous or point values. Continuous level sensors measure level within a specified range and determine the exact amount of substance in a certain place, while point-level sensors only indicate whether the substance is above or below the sensing point. Generally the latter detect levels that are excessively high or low. Accurate control of the level of material in a tank, reactor, or other vessel is important in many process applications especially, in bulk material handling industry. One of the main purposes of the level measurement system is to measure inventory. In order to achieve good control, accurate measurement is a must.
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