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The need to store liquids has led us to design and build large containers which are widely used in industry with enormous volumetric capacities. The design of each tank depends on the liquid that it must store, having open, closed and vented tanks among others, but what they all have in common for any process is that we must know the level of liquid present to avoid spills or unnecessary activation of pumps as required by the process. In today's delivery I would like to mention the level measurement.

Measuring Liquid Levels

It consists of measuring directly the height of the liquid on a reference line, the hydrostatic pressure in the measuring tank, the displacement produced in a float by the liquid itself, or taking advantage of the electrical characteristics of the liquid.

TYPES By Floating It consists of a float located inside the liquid and connected to the outside of the tank directly indicating the level. The connection can be direct, magnetic or hydraulic. The directly connected float is connected by a cable that slides on a set of pulleys to an external index that points on a graduated scale. It is the oldest and most widely used model in large capacity tanks such as fuel oil and diesel oil. It has the disadvantage that the moving parts are exposed to the fluid and can break and that the tank cannot be under pressure. The operation of the magnetically coupled float slides externally along a sealed guide tube, located vertically inside the tank. Inside the tube, a magnetic piece follows the float in its movement and uses a cable and a set of pulleys to drag the index finger of an instrument located at the top of the tank. The instrument can also be a pneumatic or electric transmitter. A variant of the magnetic connection consists of a tube containing a float, equipped with a magnet that guides a series of magnetic strips arranged outside and along the tube. As the level rises or falls, the belts rotate and as they have different colors on their front and back, they directly display the level of the tank. The instrument may have a built-in alarm switch and transmitter. In small tanks, the float can be adapted to act magnetically on a pneumatic or electric transmitter located on the outside of the tank to allow level control.



By Bubbling

It consists of a tube immersed in the liquid through which air is bubbled by a rotameter with a built-in flow regulator.
The air pressure in the pipeline is equal to the hydrostatic pressure exerted by the liquid column, i.e. the level. The flow regulator allows a constant flow of air through the liquid regardless of the level (a flow rate of 150 N1/h is normal); if this did not exist, there would be a large difference in the required air flows from the minimum to the maximum level, with the disadvantage of undue air consumption. The bevelled tube allows for easy formation of air bubbles.


By Diaphragm

It consists of a diaphragm in contact with the tank liquid, which measures the hydrostatic pressure at a point on the bottom of the tank.
As far as operation is concerned, in an open tank the pressure is proportional to the height of the liquid at a point on the bottom of the tank and its specific weight. That is: P = H.y.g in which:
P = pressure
H = height of liquid above the instrument
y = density of the liquid
g = 9,8 m/s2

The diaphragm is part of a pneumatic, electronic or digital differential pressure transmitter similar to diaphragm flow transmitters.
In the most commonly used type, the diaphragm is fixed to a flange that is mounted flush with the tank to allow for the measurement of fluid levels, such as paper pulp and liquids with suspended solids, without difficulty. It can even be mounted overhung so that the diaphragm completely flush with the interior walls of the tank, as is the case with extremely viscous liquids in which no bends are allowed.


Hydrostatic Pressure


It is a fundamental characteristic of any fluid at rest, since the force exerted on any particle of the fluid is the same in all directions. If the forces were unequal, the particle would move in the direction of the resulting force.

Pressure is usually measured in atmospheres (atm); in the International System of Units (SI), pressure is expressed in newton per square metre; one newton per square metre is a pascal (Pa). The atmosphere is defined as 101,325 Pa and is equivalent to 760 mm of mercury in a conventional barometer. It is also measured in psi pounds/inches squared (lbs/in2) and in bar.

This concludes another installment that I leave for you with much affection hoping that I can be of use to you and sincerely thanking you for the time you have taken to read my article.

All images used in this article are my own, photographs taken during my professional work. I allow the use of these images as long as they are mentioned to me in the article and the source is cited.

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