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Tuesday, February 28, 2012

Level Measurement : Direct Methods






Mechanical or Direct Method:


Direct level measurement is simple, almost straightforward and economical. It uses a direct measurement of the distance (usually height) from the datum line, and used promarily for local indication. It is not easily adopted to signal transmission techniques for remote indication or control.


Dip Sticks and Lead Lines:


Flexible lines fitted with end weights called chains or lead lines have been used for centuries by seafarinng men to gauge the depth of water under their ships. Steel tape having plump bob- like weights and stored conveniently in a reel are still used extensively for measuring level in fuel oil bunkers and petroleum storage tanks.


Though crude as this methods seems, it is accurate to about 0.1% with ranges up to about 20 feet.


Although the dipstick and lead line method of level measurement are unrivalled in accuracy, reliability, and dependability, there are drawbacks to this technique. First, it requires an action to be performed, thus causing the operator to interrupt his duty to carry out this measurment. There cannot be a continuous representation of the process measurement. Aother limitation to this measuring principle is the inability to successfully and conveniently measures level values in pressurised vessels. These disadvantages limit the effectiveness of these means of visual level measurement.
Sight Glass:


Another simple method is called sight glass (or level glass). It is quite straightforward in use, the level in the glass seeks the same position as the level in the tanks. It provides a continuous visual indication of liquid level in a process vessel or a small tank and are more convenient than dip stick, dip rod and manual gauging tapes.
  


Sight glass A is more suitable for gauging an open tank. A metal ball normal used in the tube to prevent the fluid from flowing out of the gauge. Tubular glass of this sort is available in lenghts up to 70 inches and for pressure up to 600 psi. It is now seldom used.


The closed tank sight glass B, sometimes called a 'relfex glass', is used in many pressurized and atmospheric processes. The greatest use is in pressurised vessel such as boiler drums, evaporators, condensers, stills, tanks, distillation columns, and other such applications.The length of reflex glass gauges ranges from a few inches or eight feet, but like the tube type gauges, they can be gauge together to provide nearly any length of level measurement.


The simplicity and reliability of gauge type level measurement results in he use of such devices for local indication. When level transmitters fail or must be out of service for maintenance, or during times of power failure, this method allow the process be measured and controlled by manual means.


However, glass elements can get dirty and are susceptible to breakage thus presenting a safety hazard especially when hot, corrosive or flammable liquids are being handled.


Chain or Float Gauge:


The visual means of level measurement previously discussed are rivaled in simplicity and dependability by float type measurement devices. Many forms of float type instruments are available, but each uses the principle of a buoyant element that floats on the surface of the liquid and changes position as the liquid level varies. Many methods have been used to give an indication of level from a float position with the most common being a float and cable arrangement. The operational concept of a float and cable is shown in the following diagram.


The float is connected to a pulley by a chain or a flexible cable and the rotating member of the pulley is in turn  connected to an indicating devices with measurement graduation. As can be seen, as the float moves upward, the counterweight keeps the cable tight and the indicator moves along the circular scale.


As in the figure A, as the float moves, the weight also moves by means of a pulley arrangement as in the diagram above. The weight which moves along a board with calibrated graduations, will be at the extreme bottom position when the tank is full and at the top when the tank is empty. This type is more commonly used for closed tanks at atmospheric pressure.




When it is desired to add a bit of sophistication to the measured systems for ease of reading the level at one location near the bottom of the tank and to gain a bit more accuracy, the system in the Figure B can be used.


In the system shown in Figure B, a perforated stainless steel tape connects the float to a spring or weight loaded drum that rotates as floats moves up or down. The position of the tape on the drum can be read through a window. This type of level float can also be adapted to remote reading capabilities by installing a transmitter assembly.


Article Source: Level Measurement by N. Asyiddin (wwwpiyushpanchal2007.mynetworksolutions.com)

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Reasons for level measurement






The reasons for level measurement:


Safety: in boilers, a dangerous state can develop if the water level varies outside certain limits.


Economy : Good level control of solids is also desirable, excessive build up in hoppers can be expensive to clear.


Monitoring : Monitoring of level in bulk storage tanks and process vessels is necessary in order that:



  1. Plant efficiency may be assessed and optimized.
  2. Stock records may be kept.
  3. Cost may be correctly allocated.



In the oil and natural gas industries, Liquid level measurment is necessary to achieve the following objectives:


1. Compute tank inventories of hydrocarbon liquid products and utility liquids.
2, Protect equipment such as columns, compressors, turbines and pumps from damage,
3. Protect operating and maintenance personnel against injury resulting from hydrocarbon, corrosive or toxic liquid spillage.
4. Protect the environment from the release of objectionable liquids into the rivers and the sea.
5. Control phase separation processes and product loading operations.




Unlike the pressure and temperature, liquid level has no absolute value and is always relative to some reference point such as the bottom of the tank. It is the height or depth of a liquid above a reference point and is specific to a particular vessel.


Article Source: Level Measurement by N. Asyiddin (wwwpiyushpanchal2007.mynetworksolutions.com)

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Thursday, February 23, 2012

Bubbler level measurement system :Indirect measurement of level








If the process liquid contains suspended solids or is chemically corrosive or radioactive, it is desirable to prevent it from coming into direct contact with the level transmitter. In these cases, a bubbler level measurement system, which utilize a purge gas , can be used.

Purge-type liquid-level measurement:

Usage: for corrosive liquids, liquids that contain suspended objects, high viscosity liquids, and underground tanks.

Principle of operation: A bubble tube is inserted into a tank, and a fixed flow of air is forced through the tube such that bubbles emerge from the end of the tube. If the air flow rate is not extremely large, the air pressure in the bubble tube can be considered to be the same as the pressure of the liquid at the end of the tube. The liquid level can be measured by measuring the air pressure.

Open tank application for bubbler system:

As shown, a bubbler tube is immersed to the bottom of the vessel in which the liquid level is to be measured. A gas (called purge gas) is allowed to pass through the bubbler tube.

Consider that the tank is empty. In this case, the gas will escape freely at the end of the tube and therefore the gas pressure inside the bubbler tube (called back pressure) will be at atmospheric pressure. However, as the liquid level inside the tank increases, pressure exerted by the liquid at the base of the tank (and at the opening of the bubbler tube) increases.

The hydrostatic pressure of the liquid in effect acts as a seal, which restricts the escape of, purge gas from the bubbler tube.

As a result, the gas pressure in the bubbler tube will continue to increase until it just balances the hydrostatic pressure (P=S.H) of the liquid. At this point the backpressure in the bubbler tube is exactly the same as the hydrostatic pressure of the liquid and it will remain constant until any changes in the liquid level occurs. Any excess supply will escape as bubbles through the liquid. As the liquid level rises, the backpressure in the bubbler tube increases proportionally, since the density of the liquid is constant. A level transmitter (DP cell) can be used to monitor this backpressure. In an open tank installation, the bubbler tube is connected to the high-pressure side of the transmitter, while the low pressure side is vented to atmosphere. The output of the transmitter will be proportional to the tank level.

A constant differential pressure relay is often used in the purge gas line to ensure that constant bubbling action occurs at all tank levels. The relay maintains a constant flow rate of purge gas in the bubbler tube regardless of tank variations or supply fluctuations. This ensures that bubbling will occur to maximum tank level and the flow rate does not increase at low tank level in such a way as to cause excessive disturbances at the surface of the liquid. Note that the bubbling action has to be continuous or the measurement signal will not be accurate.

An additional advantage of the bubbler system is that, since it measures only the backpressure of the purge gas, the exact location of the level transmitter is not important. The transmitter can be mounted some distance from the process.

Closed tank application for bubbler system:

If the bubbler system is to be applied to measure level in a closed tank, some pressure-regulating scheme must be provided for the gas space in the tank. Otherwise, the gas bubbling through the liquid will pressurize the gas space to a point where bubbler supply pressure cannot overcome the static pressure it acts against. The result would be no bubble flow and, therefore, inaccurate measurement signal. Also, as in the case of a closed tank inferential level measurement system, the low-pressure side of the level transmitter has to be connected to the gas space in order to compensate for the effect of gas pressure.

Article Source: Dr. Rosdiazli Ibrahim,Universiti Teknologi Petronas (EEB5223/EAB4223 Industrial Automation & Control Systems)

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Monday, February 20, 2012

Hydrostatic pressure type : Indirect measurement of level







Static Pressure Measurement of Level

A convenient means of measuring liquid level, where there is a considerable change in level employs conventional industrial instruments which is actuated by changes in hydrostatic pressure head of the liquid as the level changes.

This head is the “weight” of liquid above a reference level or datum line. Head is often expressed in terms of pressure or level height.

Measurement of pressure due to liquid head can be translated to level height above the datum line by the following basic relationship:
h=P/ρg
where h=height or level
P=pressure due to hydrostatic head
Ρ=density of the liquid
g=acceleration due to gravity

For  the readings to be accurate the density have to be constant. The accuracy will be affected for e.g., temperature variations is sufficient to cause changes in the density of the liquid.

The use of DP transmitter for liquid-level measurement:

 The DP transmitter must be positioned below the minimum liquid level. Corrections must be made for changes in the density ofthe liquid. If there is a pulsating motion in the liquid, the output of the transmitter will be unstable. The tapping tube should be as straight as possible so as not to trap air.

Inferential Level Measurement:

This technique obtains a level indication indirectly by monitoring the pressure exerted by the height of the liquid in the vessel. The pressure at the base of a vessel containing liquid is directly proportional to the height of the liquid in the vessel. This is termed hydrostatic pressure. As the level in the vessel rises, the pressure exerted by the liquid at the base of the vessel will increase linearly. Mathematically,
P=S.H
where
P=Pressure ( Pa),  S=Weight density of the liquid ( N/m3)= ρ g
H=Height of liquid column ( m),  ρ=Density (kg/m3)
g=acceleration due to gravity ( 9.81 m/s2)

DP capsules are the most commonly used devices to measure the pressure at the base of a tank. The level of liquid inside a tank can be determined from the pressure reading if the weight density of the liquid is constant. Use a pressure capsule that has a sensitivity range that closely matches the anticipated pressure of the measured liquid.


Three valve manifold:


A three-valve manifold is a device that is used to ensure that the capsule will not be over-ranged. It also allows isolation of the transmitter from the process loop. It consists of two block valves-high pressure and low-pressure block valve and an equalizing valve. During normal operation, the equalizing valve is closed and the two block valves are open. When the transmitter is put into or removed from service, the valves must be operated in such a manner that very high pressure is never applied to only one side of the DP capsule.

Open tank measurement:

The simplest application is the fluid level in an open tank. The figure shows a typical open tank level measurement installation using a pressure capsule transmitter. If the tank is open to atmosphere, the high pressure side of the level transmitter will be connected to the base of the tank while the low-pressure side will be vented to atmosphere. In this manner, the level transmitter acts as a simple pressure transmitter.

Phigh = Patm + S.H
Plow=Patm
Differential pressure,  ΔP = Phigh – Plow = S.H

Closed tank Measurement:

Should the tank be closed and a gas or vapour exists on top of the liquid, the gas pressure must be compensated for. A change in the gas pressure will cause a change in transmitter output. Moreover, the pressure exerted by the gas phase may be so high that the hydrostatic pressure of the liquid column becomes insignificant. For example, the measured hydrostatic head in a boiler may be only three meters (30kPa) or so, whereas the steam pressure is typically 5 MPa.

Compensation can be achieved by applying the gas pressure to both high and low-pressure sides of the level transmitter. This cover gas pressure is thus used as a back pressure (or reference pressure) on the LP side of the DP cell. One can immediately see the need for the threevalve manifold to protect the DP cell against these pressures.

Closed tank measurement- Dry leg system:

A full dry leg installation with three-valve manifold is as shown. If the gas phase is condensable, say steam, condensate will form in the low-pressure impulse line resulting in a column of liquid, which exerts extra pressure on the low-pressure side of the transmitter. A technique to solve this problem is to add a knockout pot below the transmitter in the low-pressure side. Periodic draining of the condensate in the knockout pot will ensure that the impulse line is free of liquid.


Phigh=Pgas + S.H
Plow=Pgas
 ΔP=Phigh-Plow=S.H


The effect of the gas pressure is cancelled and only the pressure due to the hydrostatic head of the liquid is sensed. When the low-pressure impulse line is connected directly to the gas phase above the liquid level, it is called a dry leg. In practice, a dry leg is seldom used because frequent maintenance is required. One example of a dry leg application is the measurement of liquid poison level in the poison injection tank, where the gas phase is non-condensable medium. In most closed tank applications, a wet leg level measurement system in used.


Closed tank measurement - Wet Leg System:

In a wet leg system, the low pressure impulse line is completely filled with liquid (usually the same liquid as the process) and hence the name wet leg. A level transmitter, with the associated three-valve manifold, is used in an identical manner to the dry leg system. At the top the low pressure impulse line is a small catch tank. The gas phase or vapour will condense in the wet leg and the catch tank. The catch tank, with the inclined interconnecting line, maintains a constant hydrostatic pressure on the low pressure side of the level transmitter. This pressure, being a constant, can easily be compensated for by calibration. (Note that operating the three-valve manifold in the prescribed manner helps to preserve the wet leg.)


If the tank is located outdoors, trace heating of the wet leg might be necessary to prevent it from freezing. Steam lines or an electric heating element can be wound around the wet leg to keep to keep the temperature of the condensate above its freezing point. Note the two sets of drain valves. The transmitter drain valves would be used to drain (bleed) the transmitter only. The two drain valves located immediately above the three-valve manifold are used for impulse and wet leg draining and filling.

In addition to the three-valve manifold most transmitter installations have valves where the impulse lines connect to the process. These isolating valves, sometimes referred to as the root valves, are used to isolate the transmitter for maintenance.

Level Compensation:

It would be idealistic to say that the DP cell can always be located at the exact bottom of the vessel we are measuring fluid level in. Hence, the `measuring system’ has to consider the hydrostatic pressure of the fluid in the sensing lines themselves. This leads to two compensations required.

Zero Suppression:

In some cases, it is not possible to mount the level transmitter right at the base level of the tank. Say, for maintenance purposes, the level transmitter has to be mounted X meters below the base of an open tank.

The liquid in the tank exerts a varying pressure that is proportional to its level H on the high-pressure side of the transmitter. The liquid in the high pressure impulse line also exerts a pressure on the high-pressure side. However, this pressure is a constant (P=S.X) and is present at all times.

 When the liquid level is at H meters, pressure on the highpressure side of the transmitter will be:
Phigh=S.H + S.X + Patm
Plow=Patm
P=Phigh -Plow=S.H + S.X

That is, the pressure on the high-pressure side is always higher than the actual pressure exerted by the liquid column in the tank ( by a value of S.X). This constant pressure would cause an output signal that is higher than 4 mA when the tank is empty and above 20 mA when it is full. The transmitter has to be negatively biased by a value of -S.X, only. This procedure is called Zero Suppression and it can be done during calibration of the transmitter.

Zero Elevation:

When a wet leg installation is used, the low-pressure side of the leg transmitter will always experience a higher pressure than the high pressure side. This is due to the fact that the height of the wet leg (X) is always equal to or greater than the maximum height of the liquid column (H) inside the tank. When the liquid level is at H meters, we have:
Phigh = Pgas +S.H
Plow = P gas +S.X
P=Phigh –Plow = S.H – S.X - = -S(X-H)

The differential pressure,  Δ P a negative number ( ie., low pressure side is at a higher pressure than the high pressure side).   ΔP increases from P=-S.X to P=-S(X-H) as the tank level rises from 0% to 100%. If the transmitter were not calibrated for this constant negative error (-S.X), the transmitter output read low at all times. To properly calibrate the transmitter, a positive bias (+S.X) is needed to elevate the transmitter output. This positive biasing technique is called zero elevation.

Example: Zero Suppression in Level Measurements of open vessels ( tanks):

A d/p transmitter is connected to the tank by a pressure tapping tube.
The liquid is tapped for the high pressure side, and the open air is tapped for the low pressure side.
The following relationship exists:
P=ρ1g ( H+h1)
P = the pressure
ρ1=density of the liquid
H=distance between the surface and the minimum liquid level
h1=distance between the minimum liquid level and the pressure detector.

Liquid-level measurement: Open tank:


Example:
A pressure transmitter connected at a position 10 cm below the bottom of a tank sends 13.57 mA to a computer. The transmitter was calibrated for a range of 0-200kPa to produce 4-20 mA. If the liquid has a specific gravity of 1.26, calculate the level of the liquid in the tank.
P=hρwgRD
RD=1.26 
ρw=1000 kg/m2                                        g=9.81 m/s2
Reading 13.57 mA :(200 kPa *13.57mA/16mA)-50 kPa=119.62 kPa
h=P/  ρwgRD=119.62kPa/(1000 kg/m2 *9.81 m/s2*1.26)=967.75cm
hactual =h-10 cm =957.75 cm

Example: Zero Elevation in Level Measurements of closed vessels (tanks)

Dry leg method:

A DP transmitter is connected to the closed tank: the low pressure tap is the pressure of the gas above the liquid in the upper  art of the tank.

The pressure of the gas is also applied to the high pressure tap at the same time. Hence, when taking the pressure differential, it cancels out and so does not affect the transmitter output.

If condensation from the gas in the upper part of the tank collects inside the tapping tube, the low pressure tapping pressure in the tube will change and the output of the d/p transmitter will be affected. To avoid this, the condensation is collected in a drain pot.

Liquid-level measurement: Closed tank –- Dry Leg


 

The following relationship exists:
High pressure tap pressure, PH = ρ1g ( H+h1)+PG
Low pressure tap pressure, PL=PG
Pressure differential, PH-PL= ρ1g (H+h1)
Where,
PG = the pressure of the gas in the upper part of the tank
ρ1=density of the liquid
H=distance between the surface and the minimum liquid level
h1=distance between the minimum liquid level and the pressure detector.

Example: Zero Elevation in Level Measurements of closed vessels (tanks) -  Wet Leg

Wet leg method:

Similarly, a d/p transmitter is connected to the closed tank: the low pressure tap is the pressure of the gas above the liquid in the upper part of the tank.

A relatively heavy liquid (high density) that does not easily evaporate to fill the tube. The pressure of the gas in the tank is then applied to the pressure detector through this liquid.

Liquid-level measurement: Closed tank –- Wet Leg 

The following relationship exists:
High pressure tap pressure, PH =ρ1g (H+h1)+PG
Low pressure tap pressure, PL= ρ2gh2 +PG
Pressure differential, PH-PL= ρ1g (H+h1)- ρ2gh2
Here,
ρ2=density of the liquid in the wet leg, (kg/m3)
h2=the height of the liquid in the wet leg, (m) .


Article Source: Dr. Rosdiazli Ibrahim,Universiti Teknologi Petronas (EEB5223/EAB4223 Industrial Automation & Control Systems)

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Thursday, February 9, 2012

Level Measurement : Introduction






This post is the starting of a series of post which will give you detailed explanation about level measurement, its types, principles, etc. I have divided this section into a number of post so that it becomes easy for readers to understand and follow. To see all the post published related to level measurement i.e to view the whole topic in a single page click the label titled ' LEVEL'

Introduction:

Measurement of liquid level is important in a variety of industrial processes. The liquid level may be expressed in terms of the pressure column exerts over a datum level or in terms of the length of the liquid column.

Liquid level may be measured by two methods:

1. Direct method
2. Indirect method

In direct methods the hydrostatic pressure of the liquid column is measured.

Indirect methods of liquid level measurements generally used in industries are:

1. Hydrostatic pressure type
2. Electrical methods.

Hydrostatic pressure type:

A liquid in a tank at rest exerts a force on the wall of the tank. This force in a liquid at rest is known as hydrostatic pressure. This is proportional to the depth (or height) of liquid in the tank. 


Hydrostatic pressure methods used for liquid level measurement are listed below:

1. Pressure gauge method
2. Air Bellows
3. Air Purge System (or) bubbler system
4. Liquid purge system
5. Diaphragm box type
6. Force balance type


Electrical Methods:

In electrical methods, liquid levels are converted into electrical signals and it can be measured by electrical or electronic means.

The types of electrical methods are:

1. Resistive (or) contact point type
2. Inductive method.
3. Capacitive method


The upcoming posts will cover the above mentioned types and methods in detail.

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Level Measurment : An Overview





Level measurement is to monitor and measure quantitatively the liquid content in vessels , reservoirs and tanks. The determination of the location of the interface between two fluids, separable by gravity, with respect to a fixed horizontal datum plane. The respective fluids may be any fluids, liquid or gaseous, which do not mix and have specific gravities significantly different from one another. Fluids include granular or particulate solids which are fluidized or handled like fluids. The most common level measurements are, however, between a liquid and a gas or vapour.
Different level measurement devices

Level measurement may be classified in the main categories: direct visual indication of interface location; remotely transmitted indication of interface location; interface location inferred from hydrostatic pressure; interface location inferred from fluid properties. Level measurements may be of an analog or on-off nature.

Direct visual measurement:

In many cases, fluid levels may be observed directly and consequently measured to obtain trends or magnitudes in volume.

Graduated scale:

Level is measured directly from a vertical graduated scale partially immersed in the liquid.

Glass window:

Level is observed through a transparent window in the side of a tank. The window may be graduated with a vertical scale.

Gauge glass:

Level is observed in a transparent vertical tube attached to a closed tank. The bottom of the tube is connected to the liquid space and the top of the tube to the gaseous space. The liquid level in the tube corresponds with the level in the tank and may be observed or measured against a graduated scale. Isolating valves usually are fitted in the upper and lower connecting pipes to allow for replacement of the transparent tube (which is usually of glass) without draining the tank (Fig. 1a).


Fig. 1  Direct visual and float-type measurements. (a) Gauge glass. (b) Float with electrical sensor. (c) Float with magnetic switches. (d) Float with buoyancy effect.

Closed tanks are often under some pressure, hence the use of external tubes able to withstand pressure rather than windows. In pressurized systems the upper and lower connecting tubes are fitted with ball check valves to avoid a dangerous discharge of fluid in the event of a tube rupture. For very high pressure systems, metallic ducts with thick glass windows are used. These windows are sometimes made refractive and artificially illuminated to show more clearly the difference between the two fluids (such as water and steam).

Remote measurement from float:

Where levels cannot be observed and hence measured directly, it is common to use a float and to indicate remotely the elevation of this float.

The float must be of an average density between that of the two fluids, the densities of which must be significantly different (such as water and air) to ensure that a sufficient buoyant force is generated on the float, with changing level, to activate the position-sensing mechanism.

The float may generate an analog signal which varies over the whole range of operating level, or may generate an on-off signal as the level rises above or falls below a predetermined elevation. A series of on-off sensors at different elevations can generate a digital type of level indication.

Float with mechanical indicator:

Several simple methods allow the float position and hence level to be observed indirectly from outside the vessel containing the fluids. A vertical rod attached to the float and protruding through a hole in the top of the tank can show the level by the amount of protrusion of the rod, which may be graduated. An external weight attached to the float by a rope or tape running over a pulley at the top of the tank can show the level on an inverse scale outside the tank. A rotating shaft passing through a sealed hole in the side of the tank can be connected to the float by a lever so that any rise or fall in the level rotates the shaft appropriately and so moves a pointer on an external graduated scale.

Float with electrical resistance sensor:

In a closed vessel a float having a lever connected to a variable-resistance sensor can cause a change in the electrical resistance as the level rises or falls. The change in electrical resistance can be used, via a suitable calibrated electrical instrument, to indicate the level or volume in the tank (Fig. 1b).

Float with magnetic switches:

In a closed tank a float pivoting about a fixed point can move a magnet close to the wall of the tank down or up as the level rises or falls about a selected level. A similar magnet just outside the tank is flipped by magnetic repulsion to operate electrical contacts to give a corresponding on-off signal. Magnetic repulsion rather than magnetic attraction is employed to create a definite toggle effect (Fig. 1c).

Float with buoyancy effect:

A float constrained in its vertical movement will exert a varying force on the restraining mechanism. This force in turn can be measured or converted into an analog electrical signal which can be calibrated to indicate the liquid level. In such an application, level can be measured only over the height of the float, so such floats usually have slender dimensions (Fig. 1d).

Measurement by hydrostatic pressure:

The pressure p at any depth h in a liquid of density ρ is given by the following equation, p = ρgh, where g is acceleration due to gravity. Hence, if the density of liquid is known, the depth of liquid above a selected point can be determined by measuring the pressure in the liquid at that point.

Pressure gauge:

In an application with a simple pressure gauge, it may be calibrated to give a direct reading of the depth of liquid. If the tank is closed and there is pressure in the space above the liquid surface, the difference in pressure between this space and the measuring point must be used.

Pressure diaphragm:

In applications where the liquid may be contaminated with aggressive impurities or contain solids or sludge, pressure gauges or their pressure-measuring tappings may become blocked and unresponsive. In such cases, pressure diaphragms installed flush with the inside surface of the vessel may be used. Movement of the diaphragm against a spring may be measured directly to give an indication of the pressure or depth of liquid. However, since such movement is limited, it is better to apply a corresponding pressure to the back of the diaphragm so as to restore the diaphragm to its neutral position. Measurement of this external pressure then can be converted into a measure of the depth of liquid (Fig. 2a).


Fig. 2  Hydrostatic pressure and acoustic-wave measurements. (a) Pressure diaphragm. (b) Bubble tube. (c) Manometer. (d) Acoustic.

Bubble tube:

In order to discharge gas from a pipe or vessel into a liquid at some depth, the pressure of the gas must be at least equal to that of the liquid at that depth. Hence the depth of liquid at the point of discharge can be determined from the gas pressure, provided the gas flow rate is low enough to eliminate dynamic or frictional effects. A pipe supplied with a steady but low flow of gas (such as air) may be inserted into a tank of liquid and the gas pressure in the pipe measured. This pressure measurement can then be converted into a measurement of the depth of liquid above the point of discharge (Fig. 2b).

Manometer:

A manometer may be used instead of a pressure gauge for measuring pressure (more correctly pressure difference). A manometer using mercury as the reference liquid reduces the level variation by a factor of about 13, making direct measurement more convenient, and is more sensitive than a pressure gauge. Its location relative to that of the vessel in which the liquid level is being measured usually necessitates twin pipes from each side of the manometer extending back to the measuring points and filled with the same liquid as in the vessel (Fig. 2c).

Measurement by fluid properties:

Certain fluid properties can be readily measured or used to determine the presence or extent of a known fluid. The presence or absence of a fluid can provide an on-off signal indicating whether a certain level has been reached or not, whereas the extent of a fluid can be used to determine the depth.

Conductivity:

If a liquid is a conductor of electricity, its presence can be detected by a pair of electrodes subject to a potential difference. When immersed on rising level, they can generate an off-on electrical signal.

Capacitance:

If a liquid is a dielectric, probes can be inserted into a tank and the capacitance between them measured. This will vary with the degree of immersion and can be converted to a measurement of level. 

Acoustic:

Most liquids conduct sound waves readily, and these are reflected from any interfaces, including the liquid surface. If an acoustic transmitter-receiver located at the bottom of a tank directs sound waves vertically upward and senses their reflection from the surface, the depth, and hence level, can be determined by the time taken for the sound wave to travel up and be reflected down (Fig. 2d). 

Nuclear:

Since gamma rays are absorbed by many liquids, the presence of liquid can be sensed by the attenuation of gamma rays emitted from a gamma-ray source and measured by a detector a short distance away. 

Thermal:

If an electrically heated thermistor is subject to immersion in a liquid, it will be cooled more effectively. The resulting drop in temperature will be reflected as a change in resistance which will indicate the presence of the liquid.  

Article Source: www.accessscience.com

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Basics of Instrumentation & Control


To view more posts on BASICS : CLICK HERE

Pressure


To view more posts on PRESSURE : CLICK HERE

Flow


To view more posts on FLOW : CLICK HERE

Level


To view more posts on LEVEL : CLICK HERE

Temperature


To view more posts on TEMPERATURE : CLICK HERE
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