Short Answer:

The common devices for measuring low and high pressures depend on the pressure range and the type of fluid being measured. For low pressures, instruments such as manometers, piezometers, and diaphragm gauges are used because they are sensitive and accurate for small pressure variations. For high pressures, instruments like the Bourdon gauge, bellows gauge, and pressure transducers are commonly used due to their strength, durability, and ability to withstand large forces.

In simple words, low-pressure devices are designed for delicate and precise readings, while high-pressure devices are made strong enough to measure and withstand high fluid pressures in industrial systems safely.

Detailed Explanation :

Common Devices for Measuring Low and High Pressures

Pressure measurement plays a vital role in fluid mechanics and mechanical engineering because it helps in monitoring, controlling, and maintaining the safety and efficiency of machines, pipelines, and industrial systems. The choice of pressure-measuring device depends on whether the pressure is low, medium, or high, as each range requires different sensitivity and design.

Low pressures are usually found in gas systems, ventilation ducts, and vacuum systems, while high pressures occur in hydraulic, steam, and fuel systems. Therefore, selecting the right instrument ensures accuracy, safety, and reliability in pressure measurement.

Devices for Measuring Low Pressures

Low pressures are those pressures that are slightly above or below the atmospheric pressure (usually below 1 bar). Devices used for low pressures are sensitive and designed to detect small variations in pressure accurately.

Piezometer

The piezometer is the simplest instrument used to measure low pressures in liquids.
It consists of a vertical transparent tube connected to the point where pressure is to be measured.
The liquid rises in the tube due to the pressure at that point, and the height of the liquid column indicates the pressure head.

Limitations:

Can only measure gauge pressures of liquids.
Not suitable for gases or negative pressures.

Example:
If the water rises 3 m in the piezometer, the pressure at that point is:

U-Tube Manometer

A U-tube manometer consists of a U-shaped glass tube filled with a manometric liquid (mercury, water, or oil).
One limb is connected to the pipe or vessel, and the other is open to the atmosphere.
The difference in liquid levels in the two limbs indicates the pressure.
It can measure both positive and negative (vacuum) pressures.

Formula:

Where is the height difference of the liquid columns.

Applications:
Used in laboratories and fluid experiments for precise pressure measurement.

Inclined Tube Manometer

An inclined tube manometer is a modified version of the U-tube manometer used for very small pressure differences.
One limb is inclined at a small angle to increase the length of the liquid movement for better accuracy.
It is suitable for air ducts, gas flow systems, and ventilation measurements.

Advantages:

Highly sensitive.
Easy to read small pressure variations.

Diaphragm Gauge

A diaphragm gauge consists of a thin metallic diaphragm that deflects when pressure is applied.
The deflection is proportional to the pressure and is transferred to a pointer through a mechanical linkage.
It is suitable for measuring low and moderate pressures of gases and liquids.

Uses:

Commonly used in air compressors, vacuum systems, and laboratory applications.

Bell Gauge or Bell Manometer

A bell gauge measures low pressures by balancing the pressure of a gas against the weight of a bell floating in liquid.
The movement of the bell is proportional to the pressure difference.
It is very sensitive and accurate but mainly used in laboratories.

Devices for Measuring High Pressures

High pressures are found in systems such as steam boilers, hydraulic presses, compressors, and gas cylinders. Instruments used for these conditions are strong, durable, and designed to withstand high mechanical stress.

Bourdon Gauge

The Bourdon gauge is the most widely used instrument for measuring high pressures.
It consists of a curved, hollow metal tube that tends to straighten when pressurized.
This motion is transferred through gears to a pointer that moves over a calibrated scale.
It can measure pressures up to 1000 bar or more.

Advantages:

Simple, reliable, and accurate.
Suitable for both liquids and gases.
Used in boilers, gas cylinders, and industrial systems.

Bellows Gauge

A bellows gauge has one or more flexible metallic bellows that expand when pressure is applied.
The movement is amplified and shown on a dial through a linkage mechanism.
It is suitable for medium and high pressures, especially in gas systems.

Applications:
Used in industrial process control and hydraulic systems.

Hydraulic Pressure Gauge

Used for very high pressures (up to several thousand bars).
It works by transmitting the pressure through a liquid (usually oil) to a piston or diaphragm connected to a pointer.
These gauges are designed with heavy materials to resist deformation.

Uses:

Commonly used in hydraulic presses, injection molding machines, and high-pressure testing equipment.

Dead Weight Pressure Gauge

A dead weight tester is a primary standard instrument for calibrating other pressure gauges.
It works on the principle of balancing the unknown pressure with the known weight applied to a piston.
When the system is balanced, the pressure is given by:

where is the applied weight and is the piston area.

Advantages:

Highly accurate.
Used for calibration and testing of high-pressure instruments.

Pressure Transducer

A pressure transducer converts the mechanical pressure into an electrical signal.
It uses strain gauges, piezoelectric elements, or capacitive sensors.
It is suitable for both low and high pressures and is used in automation and process control systems.

Advantages:

High accuracy and fast response.
Provides digital readings for monitoring and data recording.

Selection of Devices

The selection of a suitable pressure measuring device depends on:

The range of pressure (low, medium, or high).
The type of fluid (liquid or gas).
The required accuracy and application.

For example:

For small pressure differences in air ducts → Inclined manometer.
For high-pressure steam lines → Bourdon gauge or pressure transducer.

Conclusion

In conclusion, different devices are used to measure low and high pressures depending on the application and pressure range. For low pressures, instruments like piezometers, manometers, and diaphragm gauges are ideal due to their high sensitivity. For high pressures, devices such as Bourdon gauges, bellows gauges, and pressure transducers are used for their strength and accuracy. The correct selection of the measuring device ensures safe operation, accuracy, and long service life in mechanical and industrial systems.