What Is a Manometer?
A manometer is an instrument that uses a column of liquid to measure pressure. The simplest form is a U-tube partially filled with a liquid (often mercury or water). When one end is connected to a pressure source and the other is open to atmosphere, the liquid levels differ by a height proportional to the pressure difference. Manometers are among the oldest and most reliable pressure measurement devices, still used as calibration standards today.
The fundamental principle is hydrostatic: the pressure at the bottom of a liquid column equals the weight per unit area of the liquid above it. Mercury manometers are preferred for high pressures because mercury's high density (13,600 kg/m³) keeps the column height manageable. Water manometers are used for small pressure differences because water amplifies the height reading by 13.6 times compared to mercury.
Manometer Formula
Where P is the pressure, ρ is the manometer fluid density (kg/m³), g is gravitational acceleration (9.81 m/s²), and h is the height difference (m).
Types of Manometers
| Type | Fluid | Range | Application |
|---|---|---|---|
| U-tube (mercury) | Mercury | 0-200 kPa | Laboratory standard |
| U-tube (water) | Water | 0-5 kPa | HVAC, low pressure |
| Inclined | Oil/water | 0-500 Pa | Very low pressure |
| Differential | Various | Varies | Flow measurement |
Frequently Asked Questions
Why is mercury used in manometers?
Mercury has several advantages: very high density (13,600 kg/m³) keeps the instrument compact, it does not wet glass (making meniscus reading easy), it has low vapor pressure, and it does not absorb gases. A mercury barometer at sea level shows 760 mm, while a water barometer would need a 10.3 m tube. However, mercury toxicity has led to restrictions in many applications.
What is the difference between gauge and absolute pressure?
Gauge pressure is measured relative to atmospheric pressure (0 at atmosphere). Absolute pressure includes atmospheric pressure: P_abs = P_gauge + P_atm. An open-end manometer measures gauge pressure directly. A sealed-vacuum manometer (barometer) measures absolute pressure. Standard atmosphere is 101,325 Pa = 760 mmHg = 14.696 psi.
What is an inclined manometer?
An inclined manometer tilts the measuring tube at an angle to amplify the fluid movement for small pressures. If tilted at angle α, the fluid moves L = h/sin(α) along the tube for a vertical height h. At 10 degrees inclination, the amplification is about 5.8x, allowing measurement of pressures as small as a few pascals with readable scale divisions.