📌 Quick Answer
A manometer is a pressure-measuring instrument that finds the pressure of a fluid by balancing it against a column of liquid of known density.
Its working is based on the hydrostatic relation P = ρgh, where ρ is the liquid density, g the acceleration due to gravity and h the height of the liquid column.
🔹 Key Takeaways
- A manometer measures fluid pressure using a liquid column; the governing equation is P = ρgh.
- It is most accurate for low to moderate pressures.
- Main types: piezometer, U-tube (simple), differential, inclined and micromanometer.
- Common manometric liquids are mercury (high pressure) and water or oil (low pressure).
What Is a Manometer?
A manometer is a device used to measure the pressure of gases or liquids by comparing it with the pressure exerted by a column of liquid. It is one of the simplest and most accurate pressure-measuring instruments for low and moderate ranges, and it needs no external power.
Working Principle of a Manometer (P = ρgh)
The manometer works on the hydrostatic principle: the pressure at the bottom of a static liquid column depends only on the column height, the liquid density and gravity:
P = ρ g h
where P = pressure (Pa), ρ = density of the manometric liquid (kg/m³), g = 9.81 m/s² and h = height difference of the liquid column (m). When the unknown fluid pressure pushes the liquid up by a height h, measuring h gives the pressure directly.
Types of Manometers
| Type | Description | Best for |
|---|---|---|
| Piezometer (simple) | A single vertical tube open to atmosphere | Low gauge pressures of liquids |
| U-tube manometer | U-shaped tube with a manometric liquid | Gas and liquid pressure, vacuum |
| Differential manometer | Measures pressure difference between two points | Flow measurement across orifices/venturi |
| Inclined manometer | Limb inclined to magnify small readings | Very low pressures (draught) |
| Micromanometer | High-sensitivity design | Extremely small pressure differences |
Advantages and Limitations
Advantages: simple, inexpensive, accurate for low pressures, no power needed, easy to read.
Limitations: bulky for high pressures, fragile glass tubes, temperature affects liquid density, not suitable for fast-changing pressures.
Applications of Manometers
- Measuring gas supply and draught pressure in boilers and furnaces.
- Differential pressure across orifice plates and venturimeters for flow measurement.
- Calibrating other pressure gauges.
- HVAC and laboratory pressure measurement.
Frequently Asked Questions
What is a manometer and what does it measure?
A manometer is an instrument that measures the pressure of a fluid (gas or liquid) by balancing it against a column of liquid of known density.
What is the working principle of a manometer?
It works on the hydrostatic equation P = ρgh — the pressure equals liquid density times gravity times the height of the liquid column.
What are the main types of manometers?
Piezometer (simple), U-tube, differential, inclined and micromanometer.
Which liquid is used in a manometer?
Mercury is used for higher pressures because of its high density; water or light oil is used for low pressures.
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