Manometers Explained: A Guide to Pressure Measurement in Engineering

Manometers are the simplest devices that are used for measuring pressure differences.

Pressure measurement is essential in monitoring and controlling fluid flows within pipelines, vessels, and equipment. It ensures that processes like chemical reactions, material processing, and mixing occur under the desired conditions.

Types of Manometers

1. U-Tube Manometer
2. Inclined Manometer
3. Manometer with Multiple Fluids

U-Tube Manometer

A U-tube manometer is the simplest form of manometer. A device used to measure pressure differences.

Construction

It consists of a small diameter U-shaped tube of glass. A scale fixes between the two arms or legs of the manometer on the same board.

A manometric fluid, which is heavier than the process fluid, partially fills the U-tube.

The tubing connects the two limbs of the manometer. We attach these to the taps between which we measure the pressure drop.

The design includes air vent valves at the end of each arm to remove trapped air.

The manometric fluid is immiscible with the process fluid. The common manometric fluid is mercury.

We have to fill a U-tube manometer with a given manometric fluid up to a certain height. The process fluid fills the remaining portion of the U-tube, including the tubing.

One side of the manometer connects to the upstream tap in a pipeline. The other side connects to the downstream tap. We need to measure the pressure difference \(p_a – p_b\) between these two points.

At steady state, we assess a given flow rate. We measure the difference in the level of the manometric fluid in the two arms. This difference gives the value of pressure difference using manometric fluid across the taps (stations).

Working

Assume that the shaded portion of the U-tube is filled with liquid ‘A’ having density ρA. The arms of the U-tube above the liquid are filled with fluid ‘B’ having a density ρB.

Fluid ‘B’ is immiscible with fluid ‘A’, and the density of fluid ‘B’ is less than that of fluid ‘A’. Fluid ‘B’ is often a gas such as air or nitrogen.

Pressure (Pa) is applied to one arm of the U-tube. The other arm of the U-tube manometer exerts a pressure (pb).

The difference in pressure pa – pb influences the meniscus position. One branch of the U tube has a higher meniscus than the other. The vertical distance between the two meniscuses is Rm. You can use this distance to measure the difference in pressure.

Let’s derive a relationship between pa – pb and Rm,

Start at the point 1, where the pressure is pa

The pressure at point 2 is pa + g (Zm + Rm) ρB.

By the principles of hydrostatics, the pressure at point 3 is the same as the pressure at point 2.

The pressure at point 4 is less than that at point 3 by the amount g Rm ρA.

The pressure at point 5, denoted as Pb​. This is still lower by the amount g Zm ρB.

These statements can be summarized by the equation

Simplifying of equation (1) gives

Here, the relationship is independent of the distance Zm and the dimension of the tube. This independence holds if you measure pressure pa and pb in the same horizontal plane.

If fluid B is a gas, its density ρB is usually extremely small. This means we can ignore it in equation (2). Therefore, equation (2) simplifies to

Equation (2) is valid only for low-density fluid compared to manometric fluid.

Applications

Pressure Measurement: U-tube manometers are commonly used to measure pressure differences between two points in a system. Industries like HVAC (Heating, Ventilation, and Air Conditioning) frequently use them. They measure pressure drops across filters, coils, and other components.

Flow Rate Measurement: Place a U-tube manometer in a pipe or duct. Measure the pressure difference across the manometer. You can relate this difference to the flow rate of the fluid passing through the pipe. This is useful for measuring fluid flow in systems like water supply networks.

Hydrostatic Pressure Measurement: U-tube manometers can measure hydrostatic pressure in tanks, vessels, and other containers filled with liquids. This is particularly useful in monitoring liquid levels and controlling fluid levels in tanks.

Calibration: Other instruments sometimes use U-tube manometers as a reference. They help calibrate other pressure-measuring instruments, such as pressure gauges and transducers.

Inclined Manometer

We use Inclined Manometer for measuring small pressure differences.

Construction and Working

In this type of manometer, one leg is inclined. Hence, it is referred to as an inclined manometer. Used to measure small differences in pressure.

The angle of the inclination α is around 5 to 10o with the horizontal. With a small magnitude of Rm, the meniscus must move a long distance. This movement is along the inclined tube.

This distance is R1 can be given by the following equation.

By making α small, the size of Rm increases to a long distance R1. This means a large reading equals a small pressure difference. So,

In this type of pressure gauge, you must provide an enlargement in the vertical leg. This ensures that the movement of the meniscus in the enlargement is negligible within the instrument’s operating range.

Applications

Low-Pressure Measurements: Inclined manometers are often used for measuring low pressures and vacuum pressures. They can measure small pressure differences more accurately than some other pressure-measuring devices.

Calibration and Testing: They are commonly used in laboratory settings for calibrating other pressure-measuring instruments. Their accuracy and sensitivity make them suitable for this purpose.

Medical Equipment: Inclined manometers are used in medical devices. They help measure pressure differences in respiratory equipment. This measurement controls airflow and oxygen delivery.

Fluid Dynamics Research: Inclined manometers can be used in fluid dynamics experiments. They measure pressure differences across various parts of fluid flow systems.

Manometer with Multiple Fluids

Here, we are going to discuss Two Liquid Manometer. Also called a Differential Manometer or Multiplying Gauge.

A differential manometer is used for the measurement of minuscule pressure differences. Used for the measurement of pressure differences with exceptional precision.

It may often be used for gases.

Construction

It consists of a U-tube made of glass. The ends of the tube are connected to two enlarged transparent chambers/reservoirs.

The reservoirs at the ends of each arm are of a larger cross-section than that of the tube.

The manometer contains two manometric liquids of different densities. These liquids are immiscible with each other. They are also immiscible with the fluid for which the pressure difference is to be measured.

The densities of the manometric fluids are nearly equal to have a high sensitivity of the manometer.

Liquids that give sharp interfaces are commonly used as manometric fluid, for example, paraffin oil, industrial alcohol, etc.

Working

Consider the flowing fluid ‘A’ with density ρA. Let the manometric fluids be ‘B’ and ‘C’, with densities ρB and ρC respectively.

For the densities of the liquid ρA < ρB < ρC

To find the pressure difference between points 1 and 7, write down the pressures at points 1, 2, 3, 4, 5, 6, and 7. The difference is given by

If the liquid levels in two reservoirs are about the same, then h’ is close to 0. This means Equation (6) simplifies to

Where h is the difference in level in the two arms/limbs of the manometer.

When the densities ρB and ρC are nearly equal, the difference ρC – ρB becomes small. Then very large values of h can be obtained for small pressure differences.

Alternately, the pressure at the level a – a in Figure 3 must be the same in each of the limbs, and therefore,

Simplify above equation

Applications

HVAC Systems: Differential manometers are frequently used in HVAC systems. They measure pressure differences across filters, coils, dampers, and other components. This helps in monitoring and optimizing the airflow and efficiency of the system.

Fluid Flow Measurement: In fluid dynamics and engineering, differential manometers measure pressure differences. They are used across different sections of a pipeline or duct. This information is crucial for calculating flow rates using Bernoulli’s equation or other flow equations.

Aerodynamics Testing: In aerodynamics research and wind tunnel testing, differential manometers measure pressure differences around models. These models or prototypes include aircraft, vehicles, and other objects. This information helps in understanding aerodynamic forces and designing efficient shapes.

Hydraulic Systems: Differential manometers are employed in hydraulic systems to monitor pressure differences in various parts of the system. This is essential for ensuring proper functioning and safety of hydraulic equipment used in industrial and mobile applications.

Medical Equipment: In medical devices like ventilators, differential manometers are used to monitor and control airway pressure. They help medical professionals ensure that the air pressure delivered to patients is appropriate and safe.

Boiler Systems: In steam boiler systems, differential manometers can measure the pressure difference across critical points. This measurement helps to maintain safe and efficient operations.

Water Supply and Sewage Systems: Differential manometers can be used to monitor pressure differences in water supply networks. This ensures a steady flow of water to consumers. In sewage systems, they help maintain proper pressure levels for efficient sewage transportation.

Laboratory Experiments: Differential manometers are commonly used in educational and research laboratories for fluid mechanics experiments. They allow students and researchers to visualize pressure differences. This aids in understanding various scenarios.

Industrial Processes: In industrial processes that involve fluids, such as chemical processing, differential manometers can be used. They monitor pressure differences. They help optimize process efficiency.

Quality Control and Testing: In manufacturing, differential manometers can be used for quality control purposes. They help ensure that products meet pressure specifications and operate within safe ranges.

Environmental Monitoring: Differential manometers can play a role in environmental monitoring. They measure pressure differences in air pollution monitoring systems.

FAQ’s

What are manometers and what is their purpose?

Manometers are used to measure fluid pressure differences. They are used to monitor and control fluid flows in pipelines, vessels, and equipment. This ensures that processes happen under the right conditions.

What are the different types of manometers?

There are several types of manometers, including a U-tube Manometer, Inclined manometers, and a Manometer with Multiple Fluids.

What is the purpose of an inclined manometer?

It used for measuring small pressure differences. They have a slanted tube that makes the fluid level difference easier to see. This design helps measure small pressures more accurately.

FAQ’s

Read Also:

Hydrostatic Equilibrium: Principles and Applications in Engineering

Fluid Statics and Its Application

Fluid Flow Operations