Fluid Pressure and Its Measurements - Lecture 02

Absolute, Gauge, Atmospheric, and Vacuum Pressure

• Absolute Pressure: The total pressure at a location measured from a complete vacuum.
• Gauge Pressure: Pressure relative to atmospheric pressure, indicating how much the pressure deviates from atmospheric conditions.
• Vacuum Pressure: Refers to pressures below atmospheric pressure, measured with vacuum gauges that quantify the difference from absolute pressure.

Measurement of Fluid Pressure

• Pressure can be measured using various devices, including:
  • Piezometers
  • Manometers
  • Mechanical Gauges
  • Pressure Transmitters

Piezometer

• A device with one open end to atmospheric pressure and another connected to the point where pressure is measured.
• The liquid column's height in the piezometer correlates directly with the pressure at the measured point.
• The pressure at point A is calculated using the formula:
• Pressure = ρ × g × h*, where ρ is the fluid density, g is the gravitational acceleration, and h is the height of the liquid column.

Example of Piezometer Calculation

• For oil with a density of 640 kg/m³ rising to a height of 1.2 m, gauge pressure can be calculated using:
• Pressure = 640 kg/m³ × 9.81 m/s² × 1.2 m*.

Manometers

• Instruments that measure pressure by balancing columns of liquid.
• Typically constructed as U-shaped tubes filled with a liquid like water or mercury.
• Do not require calibration and are cost-effective.

Applications of Manometers

• Used in HVAC systems, low-pressure gas systems, construction, climate forecasting, and clinical settings to measure blood pressure.

Principle of Manometers

• The relationship between pressures in a manometer is given by:
• (p1 - p2) = hρg*
  • where p1 is the pressure to be measured and p2 is atmospheric pressure.
  • It leads to the rearranged expression:
• p1 = p2 + hρg*.

Types of Manometers

• Simple and Differential Manometers.
• Simple Manometers: One end connected to the pressure source, the other open to the atmosphere.
• Common types include:
  • U-tube Manometer
  • Well type Manometer
  • Inclined Manometer
  • Float type Manometer

Mechanical Gauges

• Devices that measure pressure by using an elastic element that deflects under pressure.
• The deflection operates a pointer that indicates pressure on a graduated scale.