Pressure is defined as the force exerted over a unit area. Force may be exerted by liquids, gases, and solids. Pressure is governed by the following equation:
P = F/A
where:
P = Pressure , F= force and A = area
Measurement of Pressure
The pressure of a fluid is measured by following devices:
- Manometers
- 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.
Manometer operates on hydrostatic balance principle. A basic manometer includes a reservoir,filled with a liquid. The reservoir is usually enclosed with a connection point that can be attached to a source in order to measure its pressure. A transparent tube, or column, is attached to the reservoir. The top of the column may be open, exposing it to atmospheric pressure. Or, the column may be sealed and evacuated. Manometers that have open columns are usually used to measure gage pressure, or pressure in reference to atmospheric pressure. Manometers with sealed columns are used to measure absolute pressure, or pressure in reference to absolute zero. Manometers with sealed columns are also used to measure vacuum.
When a manometer is connected to a process, the liquid in the column will rise or fall according to the pressure of the source it is measuring. In order to determine the amount of pressure, it is necessary to know the type of liquid in the column, and the height of the liquid. The type of liquid in the column of a manometer will affect how much it rises or falls in response to pressure, its specific gravity must be known in order to accurately measure pressure.
Manometers are accurate, they are often used as calibration standards. The shape of the liquid at the interface between the liquid and air in the column affects the accuracy of the manometer. This level is called the meniscus. The shape of the meniscus is determined by the type of liquid used. In order to minimize the errors that result from the shape of the meniscus, the reading must be taken at the surface of the liquid in the center of the column.
The quality of the fill liquid will also affect the accuracy of pressure measurements. The fill liquid must be clean and have a known specific gravity.
Classification of Manometer
Broadly manometers are classified into two classes
- Simple manometers ; simple manometers are those which measure pressure at a point in a fluid containing in the pipe or a vessel.
Simple manometer is of many types
- Piezometer
- U- tube manometer
- Single column manometer
- Differential manometers ; Differential manometers measure the difference of pressure between any two points in a fluid contained in a pipe or vessel. Differential manometer is of following types
- U-tube differential manometer
- Inverted U- tube differential manometer ; This type of manometer is used for measuring the difference of two pressures (where accuracy is the major consideration)
Piezometer
Piezometer is one of the simplest forms of manometers. It can be used for measuring moderate pressures of liquids. The setup of piezometer consists of a glass tube, inserted in the wall of a vessel or of a pipe. The tube extends vertically upward to such a height that liquid can freely rise in it without overflowing. The pressure at any point in the liquid is indicated by the height of the liquid in the tube above that point.
Pressure at point A can be computed by measuring the height to which the liquid rises in the glass tube. The pressure at point A is given by p = wh, where w is the specific weight of the liquid
Limitations of Piezometer
- Piezometers can measure gauge pressures only. It is not suitable for measuring negative pressures.
- Piezometers cannot be employed when large pressures in the lighter liquids are to be measured since this would require very long tubes, which cannot be handled conveniently.
- Gas pressures cannot be measured with piezometers, because a gas forms
U – tube manometer
The piezometer cannot be employed when large pressures in the lighter liquids are to be measured, since this would require very long tubes , which cannot be handled conveniently.Further more, gas pressures cannot be measured by the piezometers because a gas forms no free atmospheric surface. These limitations can be overcome by the use of U-tube manometers. A U-tube manometer consists of a glass tube bent in U-shape, one end of which is connected to a point at which pressure is to be measured and other end remains open to the atmosphere .Using a “U’ Tube enables the pressure of both liquids and gases to be measured with the same instrument. The “U” is 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:
- Viscosity should be low.
- Low surface tension is required.
- The liquid should stick on the walls.
- Should not get vaporized.
- The two fluids should not be able to mix readily that is, they must be immiscible.
Advantages of U-tube Manometer:
- Simple in construction
- Low cost hence easy to buy.
- Very accurate and sensitive
- It can be used to measure other process variables.
Disadvantage
- Fragile in construction.
- Very sensitive to temperature changes
Applications of U-tube manometer:
- It is used for low range pressure measurements.
- Extensively used in laboratories.
- Is used in Orifice meter and Venturi meter for flow measurements.
- It is used for calibration of gauges and other instruments.
- It is used for measuring pressure drop in different joints and valves.
Single column manometer (micromanometer)
The U-tube manometer described above usually requires the reading of fluid levels at two ormore points since a change in pressure causes a rise of the liquid in one limb of the manometer and adrop in the other. This difficulty is however overcome by using single column manometers. A singlecolumn manometer is a modified form of a U-tube manometer in which a shallow reservoir having alarge cross-sectional area (about 100 times) as compared to the area of the tube is connected to one limb of the manometer,
Factors affecting manometer
Basic manometer principle of hydrostatic balance is inherently 100% accurate but there are some factors that can affect the pressure measurement
- Characteristics of indicating fluid ; Fluid must have good ‘Wetting’ characteristics and should be capable of forming a consistent well shaped meniscus in the indicating tube to facilitate accurate and repeatable reading.
- Readability; Scales must be clear, sharp, accurate and easy to read.
Advantages of Manometers
- Easy to fabricate and relatively inexpensive;
- Good accuracy;
- High sensitivity;
- Require little maintenance;
- Not affected by vibrations;
- Specially suitable for low pressure and low differential pressures; and
- It is easy to change the sensitivity by affecting a change in the quantity of manometric liquid in the manometer
Limitations of manometer
- Usually bulky and large in size.
- Being fragile, get broken easily.
- Readings of the manometers are affected by changes in temperature, altitude and gravity.
- A capillary effect is created due to surface tension of manometric fluid, and
- For better accuracy meniscus has to be measured by accurate means.
Applications
- specific pressure monitoring applications
- Visual monitoring of air and gas pressure for compressors.
- Vacuum equipment and specialty tank applications such as medical gas cylinders, fire extinguishers.
- In power plants,mercury absolute manometer have been used to check condenser efficiency by monitoring vacuum at several points of the condenser
- Used for the research of atmosphere of other planets.
- And many more applications such as in whether studies, research labs, gas analysis and in medical equipments.
- Some types of liquids used in manometers are toxic and can be damaging to the environment. Therefore, when using manometers to measure or indicate pressure, do not connect any manometer to a pressure that has the potential to exceed the range of the manometer. This could cause the liquid to be forced out of the tube.
- In addition, since the tubes in many manometers are made of glass and can be easily broken, it is important to use care in handling these manometers.
- Never operate damage equipment.
- Meter and its tubing should be free from any breaking and blockage.
- Electronic manometers do not measure water pressures; under these conditions they will fail. Do not exceed 10 PSI input pressure.
Safety
- Some types of liquids used in manometers are toxic and can be damaging to the environment. Therefore, when using manometers to measure or indicate pressure, do not connect any manometer to a pressure that has the potential to exceed the range of the manometer. This could cause the liquid to be forced out of the tube.
- In addition, since the tubes in many manometers are made of glass and can be easily broken, it is important to use care in handling these manometers.
- Never operate damage equipment.
- Meter and its tubing should be free from any breaking and blockage.
- Electronic manometers do not measure water pressures; under these conditions they will fail. Do not exceed 10 PSI input pressure.