Working and Types of Manometer

• Unlike dead weight gauges, these manometers are self-balancing, deflection type rather than null type instruments.
• Manometers offer continuous rather than stepwise readouts.
• The manometer is the simplest instrument used to measure pressure by balancing the pressure against the weight of a column of liquid.
• Manometers are usually found only in laboratories as local indicators.
• Depending upon the reference pressure used, they can indicate absolute gauge pressure and Differential Pressures.
• Vernier scale can be used to determine the mercury column height more accurately.
• Manometers are of comparable accuracy to dead weight gauges at lower pressures.
• But higher-pressure measurements with a manometer are impractical because of the length of liquid columns involved.
• The manometric fluids used in practice are mercury, water, Benzene, Kerosene, and so on.
• Here only Water or Mercury is used as manometric fluid because their specific weight for various temperatures is known.
• These fluids do not stick or wet to the tube walls of the manometer.
• When a manometer is used for Low-pressure measurement Water is used.
• When a manometer is used for High-pressure measurement Mercury is used
• Weight of Mercury is almost 14 times as heavy as Water.

What is the working principle of a manometer?

• The working principle of the manometer is based on the hydrostatic pressure exerted inside the liquid column.
• Hydrostatic equilibrium defines that when a liquid is at rest the pressure is equal at any point.
• If both ends of the manometer are vented or kept open to the atmosphere, then the pressure on each side will be equal.
• So that the liquid level on the lefthand side marks an equal level on the righthand side.
• In case, if one end of the manometer is vented or kept open to the atmosphere and the other end is connected to an additional gas or liquid supply, the differential pressure is created.
• If the applied pressure from the additional gas or liquid supply is greater than the atmospheric pressure this will exert downward pressure on the measuring liquid so that the liquid with greater pressure is pushed downwards on one side causing the liquid with lesser pressure to rise on the other side.
• If the applied pressure from the additional gas or liquid supply is lesser than the atmospheric pressure this will exert upward pressure on the measuring liquid so that the liquid falls on the open side of the tubeand rises on the additional gas or liquid supply connected side.
• It will balance perfectly, if there is the same weight on both sides and additional pressure is not exerted.
• However, if some weight or some amount of pressure is exerted on one end, the heavier side will lower and the lighter side will rise.

Describe various types of manometers used to measure pressure:

Several types of manometers are used to measure pressure as laboratory or local indicating equipment

1. U-Tube Manometer:

• The U-Tube manometer is a traditional device.
• This U-tube manometer is a transparent glass tube constructed in the form of an elongated-U partially filled with manometric fluid such as Mercury or Water.
• One end of the Utube manometer is connected to the pressure tap, and another end of the manometer is connected to another pressure tap or may be vented out to the atmosphere.
•  When there is a pressure difference between two ends of the tube the liquid levels do not match each other.
• By measuring the heights of liquid both on the left and the right hand side of the U-tube, it is possible to calculate the pressure in relation to atmospheric pressure.
• This level represents the differential pressure of ∂P= P1- P2.
• Then this differential pressure ∂P= P1- P2 is obtained by

P₁-P₂ = (ρ-ρ₁)*(h₁-h₂)*g

 In general

P₁-P₂=ρ*h*g

Where

ρ= Fluid Density

h= Height

g= Gravitational Constant = 9.8 ms^-2

• If the fluid over manometer liquid has appreciable density, then the static balance equation is given by the equation

P2-P1 = h*(Pm-Pt)*g

2. Well Type Manometer:

• The vessel of this manometer is a well-type so called by name.
• This is modified by replacing one leg with a large diameter well usually 300 times greater than the tube area.
• Due to this change in area and change in pressure P2 has a negligible effect on manometric fluid may be mercury or water.
• The most important advantage of a well-type manometer is that the reading of a single leg gives desired pressure value.
• When P1 and P2 have been applied the height of the fluid is measured from the zero line.

P2- P1 = ρ*h*[1+(A1/A2)]

Where

ρ= Fluid Density

h= Height represents differential pressure (P2-P1)

A1= Area of tube

A2= Area of well

3. Enlarged Leg Manometer:

• In this type, both legs of the manometer are enlarged and connected by separate tubing and a float is placed in one of the chambers.
• When P1 and P2 are applied the float gets displaced, this float displacement measured from the zero line represents the differential pressure (P1-P2).
• The manometer can be made recording type by connecting the pointer mechanism to the float that makes the pointer deflect the static balance equation can be written as

P1-P2 = ρ.d [1= (A1/A2)]                

where A1>A2.

4. Inclined Leg Manometer:

• An inclined manometer consists of a gradually inclined tube in the design.
• This allows for the pressure measurement for high levels of accuracy.
• This type is used for liquids having similar manometric properties.
• If one leg or limb of the manometer is inclined, then its sensitivity increases.
• In such cases, with no pressure difference DP is applied between the limbs of the manometer, the meniscus in each leg is at the same level.
• When P1 & P2 are applied then liquid rises in the tube.
• The level of manometric liquid inside the tube is measured from zero level that represents differential pressure (P1-P2).
• The static balance equation is

P2-P1 = ρ*d*sinα*[1+ (A1/A2)]

Where

α= inclination angle

d= Height difference measured along the tube.

Characteristics of Manometric fluid:

• These fluids should not absorb gas or chemically react with it.
• It should be high-density so that the pressure balancing column stays within the desired limit.
• It must have low vapor pressure at operating temperatures.
• It should freely move in the limbs of a manometer.
• It should not be compressible.

Properties of good Manometric Fluid:

• Low freezing point -38 Degree F for Hg.
• High boiling point   675 Degree F for Hg.
• Non-sticky in nature.
• Non-Wetting Characteristics.
• Low surface tension
• Clearly visible interface.

Sources of Error for Manometer:

• The density of fluid decreases with a rise in temperature which may affect the calibration and leads to an error.
• Thermal expansion of the fluid and the readout scale may cause an error.
• Compressible fluid  may change the calibration
• Evaporated fluid at low pressure and high temperature can interfere with measurements.
• To reduce the effect of capillary rise large bore tubes of over 10 mm diameter is used.
• Effect of the variable meniscus
• The high surface tension of Hg makes the meniscus slightly convex
• The low surface tension of water makes the meniscus slightly concave.

Advantages of Manometers:

• Construction  is simple
• High accuracy
• Highly sensitive
• Used to measure low pressure
• Span can be changed by using different size tubes as desired.
• The pressure range is  3KPa to 100KPa
• Reducing angle sensitivity and scale length can be increased easily.

Disadvantages of Manometers:

• No over-range protection
• Requires large space
• Non-Portable
• Leveling is required.
• Error is introduced due to non-infirmity of tube
• Zero level is set to get high accuracy.

Some Useful Questions related to manometer

1. What is manometer used for?

A manometer is a device used to measure the pressure of a gas or liquid. It consists of a tube or U-shaped glass tube filled with a liquid, such as mercury or water, and a measurement scale.

The pressure of the gas or liquid being measured is indicated by the height of the liquid in the tube.

Manometers are commonly used in the laboratory and in various industries to measure the pressure of gases and liquids in tanks, pipes, and other containers.

They are also used in the field of medicine to measure the pressure of gases in the lungs, and in the automotive industry to measure the pressure of gases in the tires.

2. Is manometer a pressure gauge?

Yes, a manometer is a type of pressure gauge. It is a device used to measure the pressure of a gas or liquid.

Manometers work by measuring the force that the gas or liquid exerts on a column of liquid contained in a tube or U-shaped glass tube. The pressure of the gas or liquid being measured is indicated by the height of the liquid in the tube.

Manometers can be used to measure both absolute pressure and gauge pressure. Absolute pressure is the pressure of a gas or liquid relative to a perfect vacuum, while gauge pressure is the pressure of a gas or liquid relative to atmospheric pressure.

Manometers are commonly used in various industries to measure the pressure of gases and liquids in tanks, pipes, and other containers.