Types of Manometers Edited

In the name of Allah, the Most Beneficent, the Most Merciful

2 Types Of Manometer Syed Shah Jehan Gillani (09-CH-39) Assignment # 1

Chemical Engineering Session 2009-2013

[2010]Submitted To :-

Sir Engr. Munawar Hussain.

Fluid Dynamics[Types of Manometers]Assignment # 01

Roll # 09-CH-39 Syed Shah Jehan Gillani

3rd semester

Measurement of PressureThe pressure of a fluid is measured by following devices:i. Manometersii. Mechanical gauges

Manometers• A manometer is a device for measuring fluid pressure consisting of a bent tube containing one or more liquids of different densities.• A known pressure (which may be atmospheric) is applied to one end of the manometer tube and the unknown pressure (to be determined) is applied to the other end.• Differential pressure manometer measures only the difference between the two pressures.

Manometer Principles

The manometer has many advantages in this age of technology. Containing no mechanical moving parts, needing nothing but the simplest of measurements, the primary standard manometer is readily available at modest cost. The principle of the manometer has not changed since its inception, however great strides have been made in its arrangement and the application of the instrument to various industrial measurement requirements. Whereas formerly the manometer was considered a laboratory instrument, today we find the manometer commonly used to measure pressures ranging from as high as 600 inches of mercury to space vacuums.

The manometer utilizes the hydrostatic (standing liquid) balance principle wherein a pressure is measured by the height of the liquid it will support. For example, the weight of a column of mercury at 0 deg C that is one inch high and one inch in cross sectional area is .4892 pounds. Thus we can say that a column of mercury one inch high imposes a force of .4892 pounds per square inch or .4892 PSI.

Manometer UsesLiquid manometers, because of inherent accuracy and simplicity, have applications in every industry and laboratory. They are unique in being both basic pressure measurement instruments and standards for calibration of other instruments.

In addition to straight pressure and vacuum measurement, other process variables that are a function of pressure can be readily measured with a manometer. Common applications are flow, filter pressure drop, meter calibrations, leak testing and tank liquid level.Manometer types

• Simple U - tube manometer• Inverted U -Tube Manometer• U - tube with one leg enlarged• Two fluid U - tube manometer• Inclined U - tube manometer

[Chemical engineering] [Session 2009-2013]


Using a "U"-Tube enables the pressure of both liquids and gases to be measured with the same instrument. The "U" is connected as in the figure below and filled with a fluid called the manometric fluid. The fluid whose pressure is being measured should have a mass density less than that of the manometric fluid and the two fluids should not be able to mix readily - that is, they must be immiscible.

1. Simple U - tube manometer

Using a "U"-Tube enables the pressure of both liquids and gases to be measured with the same instrument. The "U" is connected as in the figure and filled with a fluid called the manometric fluid. The fluid whose pressure is being measured should have a mass density less than that of the manometric fluid

Characteristics of liquid used in U-tube Manometer:

1. Viscosity should be low.

2. Low surface tension is required.

3. The liquid should stick on the walls.

4. Should not get vaporized.

5. The two fluids should not be able to mix readily - that is, they must be immiscible.

Advantages of U-tube Manometer:

1. Simple in construction

2. Low cost hence easy to buy.

3. Very accurate and sensitive

4. It can be used to measure other process variables.

Disadvantage

1. Fragile in construction.

2. Very sensitive to temperature changes.

3. Error can happen while measuring the Rm.

*Limited pressure range (due to the length restriction).



*The densest fluid normally available in manometers of this type is mercury, with a density 13.6 times that of water. To measure a pressure difference about 4 atm, for example, a device height is about 10 ft¿ 3m and a ladder is required to read it. Equating the pressure at the level XX’ (pressure at the same level in a continuous body of fluid is equal),

The maximum value of P1 - P2 is limited by the height of the manometer. To measure larger pressure differences we can choose a manometer with higher density, and to measure smaller pressure differences with accuracy we can choose a manometer fluid which is having a density closer to the fluid density.

Applications of U-tube manometer:-

1. It is used for low-range pressure measurements. 2. Extensively used in laboratories.3. Is used in Orifice meter and Venturi meter for flow measurements.4. It is used for calibration of gauges and other instruments.5. It is used for measuring pressure drop in different joints and valves.



2. U-tube with one Leg Enlarged

To read the locations of the two interfaces of U-manometer is rather inconvenient. To overcome this problem, on can use the well with the cross-sectional area larger then the area of the transparent tube. When a pressure is applied, the change in surface elevation of the well is extremely small and only one reading is required:

Industrially, the simple U - tube manometer has the disadvantage that the movement of the liquid in both the limbs must be read. By making the diameter of one leg large as compared with the other, it is possible to make the movement

the large leg very small, so that it is only necessary to read the movement of the liquid in the narrow leg. In figure, OO’ represents the level of liquid surface when the pressure difference P1 - P2 is zero.

Advantages and Uses of Well-type manometer :-

1- The Well-type manometer is widely used because of the convenience in its requiring the reading of only a single leg.In a single-leg instrument, high accuracy is achieved by setting the zero level of the well at the zero level of the scale before each reading is taken.

2- For raised-well manometers, the well connection can be used for gauge and vacuum measurements.



3. Inverted U-tube Manometer

Inverted U-tube manometer is used for measuring pressure differences in liquids. The space above the liquid in the manometer is filled with air which can be admitted or expelled through the tap on the top, in order to adjust the level of the liquid in the manometer.

For inverted U - tube manometer the manometric fluid is usually air.

The inverted U-tube manometer is used with liquids that may be under pressure. It is shown diagrammatically in figure 2-8. The U-tube is now upside down and the two pressure connections are made to the lower ends of the tubes. A non-return valve is fitted to the top of the inverted tube. In order to start the manometer the liquid in the pipe is first allowed to flow through the manometer and then air is pumped in with a bicycle pump through the non-return valve to separate the liquid in the two limbs.

Air may be pumped through a valve V at the top of the manometer until the liquid menisci are at a suitable level.

Application:-

1. The inverted U-tube manometer is used especially for flow of slurries where solids tend to settle out.

2.If difference of densities is sufficiently small, a large value of “h” may be obtained for a small value of differential pressure.. Air is used as the manometric fluid.

Draw backs

It is very sensitive type of manometer used for high accuracy and measurement. It is often used because of its operating cast. So, it is used necessarily where highly accuracy and precision required for minute pressure difference.

4. Two fluid U-tube Manometer



In accordance with the manometer principle, it is obvious that the higher the measured pressure, the longer must be the indicating fluid tube. To provide greatest ease in reading high range manometers, Small differences in pressure in gases are often measured with a manometer of the form shown in the figure. Most of the pressure is absorbed by the fluids used in the instrument. There are two reservoir for manometric liquids. One of the manometric liquid which has relatively lower density than other changes its heads with reference to differential pressure. Thus in this way the range of differential pressure measured is increased.

Advantages and Applications

1- The instrument thus combines the differential manometer's advantage of compensation with the simple manometer's simplicity of reading and calculation.

2- This type of manometer is used for more accurate readings of small pressure differences.

3- It is used for measuring small gas heads.4- It has relatively large range of measuring the differential pressure.

5. Inclined U-tube manometer



Many applications require accurate measurement of low pressure such as drafts and very low differentials, primarily in air and gas installations. In these applications the manometer is arranged with the indicating tube inclined, as in Figure , therefore providing an expanded scale. This arrangement can allow 12" of scale length to represent 1" of vertical liquid height. With scale subdivisions to .01 inches of liquid height, the equivalent pressure of .000360 PSI per division can be read using water as the indicating fluid.

1- This type of manometer is also used for more accurate readings of small pressure



differences.

2- The minimum pressure measured by U manometer (of 0.5 in. of water ) can be reduced to 0.1 in. (0.19mmHg) by the use an inclined manometer.

6. The Piezometer Tube Manometer:

The simplest manometer is a tube, open at the top, which is attached to the top of a vessel containing liquid at a pressure (higher than atmospheric) to be measured. An example can be seen in the figure below. This simple device is known as a Piezometer tube. As the tube is open to the atmosphere the pressure measured is relative to atmospheric so is gauge pressure.

EQUATION:

DISADVANTAGE:

This method can only be used for liquids (i.e. not for gases) and only when the liquid height is convenient to measure. It must not be too small or too large and pressure changes must be detectable.

7. Sealed Tube or Absolute Manometer

The term "absolute pressure" is derived from the fact that a perfect vacuum, the complete absence of any gas, is called "absolute" zero. In an absolute pressure manometer the pressure being measured is compared to the vacuum or absolute zero pressure in the sealed tube above the mercury column.The most common form of sealed tube manometer is the conventional mercury barometer used to measure atmospheric pressure. Such a barometer is a mercury filled tube over 30 inches high immersed in a container of mercury which is exposed to the atmosphere. The mercury column is supported by atmospheric, or barometric pressure.

Applications:-



Some processes, tests and calibrations are based on pressures near or below atmospheric pressure and are most conveniently measured on a sealed tube manometer, referred to as an absolute pressure manometer. These are available in U-type or well type configurations.

Calibration of gauges:-A new standard mercury manometer has been developed to calibrate low vacuum gauges in the range from atmospheric pressure to 1 mbar. It consists of a cistern that is a small stainless steel container used as mercury reservoir and also as the first Hg column connected to a long glass tube used as the second Hg column. Manometer scale covers the difference in Hg heights in two columns up to the length equivalent to 1000 mbars. This is a novel low cost manometer with simple design, compact fabrication, better accuracy, easy operation, low vibration, and thermal stability. In order to evaluate the performance of the equipment, its generated pressures are compared with those of secondary standard, i.e., calibrated capacitance diaphragm gauge, giving the average correction factor 0.998. Different uncertainties of the generated pressures are discussed in detail along with the evaluation of correction factors. The relative uncertainty in the higher pressure side is found to be in the range of 10−4 which is within the limit ( ≈ 10−4).

© 2007 American Institute of Physics





Applications of Digital manometers:-





8. A Fringe Counting laser Interferometer ManometerA prototype standard mercury manometer using a fringe counting laser interferometer to measure the differential height of mercury columns has been built and successfully operated. The 10.6 micron wavelength radiations from a CO2 laser and specially treated manometer tubes have been used to reduce the effect of disturbances on the mercury surface so that reliable operation of manometer is possible.

REFERENCES

1. Mercury Barometers and Manometers U.S National bureau of standards 2. Chemical Engineering Volume 1 Richard Colsin 3. Unit operation McCabe Smith4. Perry's Chemical Engineers' Handbook (7th Edition)5. http://www.cartage.org.lb/en/themes/Sciences/Physics/Mechanics/

FluidMechanics/Statics/Measurement/Measurement.htm6. www.wilipedia.org

7. LESSON 2: PRESSURE MEASUREMENT AND MANOMETERS Rai University

8. www.transcat.com/technical.../Manometer_Merium.htm9. www.epa.state.oh.us/opp

10. http://www.meriam.com/


http://www.epa.state.oh.us/opp

http://www.wilipedia.org/

http://www.cartage.org.lb/en/themes/Sciences/Physics/Mechanics/FluidMechanics/Statics/Measurement/Measurement.htm

http://www.cartage.org.lb/en/themes/Sciences/Physics/Mechanics/FluidMechanics/Statics/Measurement/Measurement.htm

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Types of Manometers Edited