Short Answer:

A manometer is a simple instrument used to measure the pressure of fluids (liquids or gases). It works based on the principle that the pressure at any point in a continuous fluid at rest is proportional to its height or column difference. Manometers use a column of liquid, such as mercury or water, to balance the pressure being measured.

In simple words, a manometer compares the unknown pressure with the known pressure (usually atmospheric pressure) by showing the height difference of the fluid column. Common types include simple manometers (piezometer, U-tube) and differential manometers (inverted U-tube, inclined manometer).

Detailed Explanation :

Manometer

A manometer is a device that measures the pressure of fluids by balancing the fluid column of known density against the pressure to be measured. It operates on the principle of hydrostatic equilibrium, which states that the pressure difference between two points in a stationary fluid is proportional to the vertical distance between them and the density of the fluid.

Manometers are one of the most basic and accurate instruments for measuring pressure differences. They are commonly used in laboratories, fluid mechanics experiments, and industrial applications where high accuracy is required.

Principle of Manometer

The working principle of a manometer is based on the hydrostatic law, which states:

“The pressure in a static fluid increases with depth according to the relationship .”

Where,

= Pressure difference (N/m²),
= Density of the manometric fluid (kg/m³),
= Acceleration due to gravity (9.81 m/s²),
= Height difference of the fluid column (m).

Thus, the difference in height between the liquid columns in a manometer gives the pressure difference between the two points being compared.

Construction of Manometer

A manometer generally consists of:

Glass Tube: A transparent U-shaped or straight glass tube partially filled with a liquid (mercury, water, or oil).
Manometric Fluid: A liquid of known density used inside the tube to measure pressure differences.
- Mercury is used for high-pressure measurements.
- Water or light oils are used for low-pressure measurements.
Connection Points: One or both ends of the tube are connected to the pressure source (pipe, vessel, or system) whose pressure is to be measured.
Scale: A graduated scale is attached to measure the height difference (h) of the fluid columns accurately.

Types of Manometers

Manometers are mainly classified into two broad types:

Simple Manometers
Differential Manometers

Simple Manometers

A simple manometer measures the pressure at a single point relative to the atmosphere. Common types include:

(a) Piezometer:
A piezometer is the simplest form of a manometer. It consists of a vertical glass tube connected to a point in a fluid system. The height of the fluid column in the tube indicates the pressure at that point.

It measures gauge pressure only for liquids (not gases).
Not suitable for negative or very high pressures.

(b) U-tube Manometer:
A U-tube manometer consists of a U-shaped glass tube partially filled with a manometric liquid.

One limb is connected to the point where pressure is to be measured, and the other limb is open to the atmosphere.
The difference in the liquid levels in the two limbs indicates the gauge pressure.

For a U-tube manometer:

where is the difference in heights of the fluid columns.

(c) Inclined Tube Manometer:
This is a modified version of a U-tube manometer, where one limb is inclined at an angle to the horizontal.

It increases the accuracy of measurement for small pressure differences.
The height difference is measured along the inclined tube for better sensitivity.

Differential Manometers

Differential manometers are used to measure the pressure difference between two points in a fluid system, such as across an orifice, venturi, or pipe section.

(a) U-tube Differential Manometer:
It consists of a U-shaped tube filled with a manometric fluid, with each limb connected to different points of the same or different pipes.

The pressure difference between the two points is indicated by the difference in height of the manometric fluid.
Formula:

where is the density of manometric fluid and is the density of the fluid in the pipes.

(b) Inverted U-tube Manometer:
In this type, a light liquid (such as oil) is used as the manometric fluid. It is used when the pressure difference between two points is very small.

The tube is filled with a lighter liquid, and both ends are connected to two points having different pressures.
It provides accurate readings for low-pressure differences in gas systems.

Working of a Manometer

The working of a manometer is simple and depends on balancing the fluid column:

When connected to a pressure source, the pressure difference causes the liquid in the tube to move.
The difference in fluid column heights represents the pressure difference between the measured point and the reference (usually atmospheric pressure).
Using the hydrostatic law, the pressure difference can be calculated as:

For absolute pressure measurement, the atmospheric pressure is added to the gauge pressure obtained from the manometer reading.

Advantages of Manometers

High Accuracy: Gives precise measurements due to the direct relationship between height and pressure.
Simple Design: Easy to construct and use without any complex mechanisms.
No Calibration Required: The measurements depend only on physical constants.
Wide Range: Can measure both small and large pressure differences by using suitable fluids.
Low Cost: Inexpensive compared to electronic pressure sensors.

Limitations of Manometers

Slow Response: Not suitable for measuring rapidly changing pressures.
Inconvenient for Large Pressures: Requires large tubes and fluids for very high pressures.
Temperature Sensitive: Fluid density may change with temperature, affecting accuracy.
Cannot Measure Gas Pressure with Piezometer: Only suitable for liquids in certain cases.
Fragility: Glass tubes are delicate and must be handled carefully.

Applications of Manometers

Used in laboratories for fluid mechanics experiments.
Used in industries to measure gas and liquid pressures in pipelines.
Applied in venturi meters and orifice meters for pressure difference measurements.
Used in HVAC systems to monitor air pressure differences.
Used for calibration of pressure gauges and other instruments.

Conclusion

In conclusion, a manometer is a simple and effective device used to measure pressure in fluids using a column of liquid. It operates on the principle of balancing pressure differences through the height of a manometric fluid column. The difference in heights of the liquid levels directly represents the pressure difference. There are two main types of manometers — simple and differential — which are used for measuring single-point and pressure differences respectively. Despite some limitations, manometers remain one of the most accurate and reliable instruments in fluid mechanics and engineering applications.