What is a piezometer?

Short Answer:

A piezometer is the simplest type of manometer used to measure the pressure of a liquid at a point in a system. It consists of a vertical transparent glass tube connected to the point where pressure is to be measured. The height of the liquid column in the tube shows the pressure head at that point.

In simple words, a piezometer measures static pressure in liquids by balancing the liquid column against atmospheric pressure. It is used only for measuring gauge pressure in liquids (not gases) and cannot measure negative or very high pressures.

Detailed Explanation :

Piezometer

A piezometer is a simple pressure-measuring device that works on the basic principle of hydrostatics. It is mainly used to measure the pressure head of a liquid at a specific point in a container or pipe. It is one of the most fundamental and inexpensive instruments for pressure measurement, widely used in hydraulic and civil engineering applications.

The word piezometer comes from the Greek word “piezein”, which means “to press” or “to squeeze.” The device operates based on the concept that the height of a liquid column in a vertical tube is proportional to the pressure at the base of the tube.

Construction of a Piezometer

A piezometer is a simple glass tube that is open at the top and connected at its lower end to the point where the pressure is to be measured. It generally consists of:

  1. Transparent Vertical Tube: A straight glass or plastic tube, open to the atmosphere at the top.
  2. Connection Point: The lower end is connected to a pipe or vessel containing the liquid under pressure.
  3. Support Frame: The tube is supported vertically to measure the liquid rise accurately.

The liquid from the container or pipeline rises in the tube due to pressure. The height of the liquid column (h) represents the pressure head at that point.

Working Principle of Piezometer

The working of a piezometer is based on Pascal’s law and the hydrostatic law, which state that:

The pressure at a point in a static fluid is proportional to the vertical height of the fluid column.

Mathematically,

Where,

  •  = Pressure at the point (N/m²)
  •  = Density of the liquid (kg/m³)
  •  = Acceleration due to gravity (9.81 m/s²)
  •  = Height of liquid column in the piezometer (m)

Thus, the pressure at a point is equal to the weight of the liquid column per unit area above that point.

When the liquid under pressure enters the tube, it rises until the hydrostatic pressure of the column balances the pressure at the point of connection. The height of the rise (h) gives the gauge pressure head.

Measurement of Pressure Using Piezometer

When the piezometer is connected to a point in a fluid system:

  • The liquid rises in the tube up to a height (h) due to the pressure at the connection point.
  • This height is directly proportional to the pressure at that point.
  • The pressure head (in meters of liquid) is given by the vertical rise of the column.

If the pressure at a point is  , then:

If we wish to express pressure as head (h) instead of force per area:

This equation shows that higher pressure causes a higher column of liquid in the piezometer.

Example Calculation

Suppose a piezometer is connected to a water pipeline, and the water rises to a height of 5 m in the tube.
Given:

Then the pressure at that point is:

Hence, the pressure at that point is 49.05 kPa above atmospheric pressure.

Advantages of Piezometer

  1. Simple Construction:
    Very easy to make and operate.
  2. Low Cost:
    It is an inexpensive device compared to other pressure measurement tools.
  3. Accurate for Low Pressures:
    Provides precise results for low and moderate static pressures.
  4. No Calibration Needed:
    Works directly based on the physical property of liquids and gravity.
  5. Useful for Liquids:
    Ideal for measuring pressure in static or slow-moving liquid systems.

Limitations of Piezometer

  1. Cannot Measure Negative Pressures:
    It cannot be used to measure suction or vacuum pressures.
  2. Not Suitable for Gases:
    Gases have low density and compressibility, so the height of rise becomes negligible.
  3. Unsuitable for High Pressures:
    For high pressures, the height of the column becomes too large to handle practically.
  4. Cannot Measure Pressure Difference:
    It can only measure pressure at one point, not the difference between two points.
  5. Evaporation and Capillary Effects:
    The liquid in the tube may evaporate or stick to the walls, causing slight errors in measurement.

Applications of Piezometer

  1. Water Supply Systems:
    To measure static water pressure in pipes or tanks.
  2. Dams and Reservoirs:
    Used to measure water pressure at different depths in dam structures.
  3. Soil and Groundwater Studies:
    Used to measure pore water pressure in soils during foundation and slope stability analysis.
  4. Hydraulic Research:
    Commonly used in laboratories to determine pressure head in experimental setups.
  5. Civil and Mechanical Engineering Projects:
    Used in pressure head calculations and flow energy distribution analysis.

Precautions While Using a Piezometer

  1. The tube must be perfectly vertical to avoid errors in height measurement.
  2. The open end should always be exposed to the atmosphere.
  3. The manometric liquid should not evaporate or react with the fluid being measured.
  4. For accuracy, the height should be measured from the centerline of the pipe connection.
  5. Avoid air bubbles in the liquid column to prevent incorrect readings.

Advantages Over Other Manometers

Compared to U-tube and differential manometers, the piezometer is:

  • Simpler and easier to use.
  • More suitable for static and low-pressure liquid systems.
    However, it is not as versatile as other manometers when measuring variable or gas pressures.
Conclusion

In conclusion, a piezometer is a simple and efficient instrument used to measure the pressure of a liquid at a specific point. It works on the principle that the height of the liquid column is proportional to the pressure at that point. Although it is limited to measuring gauge pressure in liquids and not suitable for gases or high-pressure systems, it remains one of the most basic and reliable tools for hydraulic pressure measurement. Piezometers are widely used in laboratories, water systems, and geotechnical engineering to determine fluid pressure and head.