What is Bourdon gauge?

Short Answer:

A Bourdon gauge is a mechanical device used to measure pressure of fluids (liquids or gases). It consists of a curved, hollow metallic tube called the Bourdon tube, which tends to straighten when pressure is applied inside it. The motion of the tube is transferred through mechanical linkages to a pointer that indicates the pressure on a calibrated dial.

In simple words, a Bourdon gauge converts the pressure of a fluid into the movement of a needle on a dial. It is widely used in industries, laboratories, and mechanical systems due to its simple design, accuracy, and reliability.

Detailed Explanation :

Bourdon Gauge

A Bourdon gauge is one of the most common mechanical instruments used for measuring gauge pressure. It works on the principle that when a curved, hollow, and flexible metallic tube is subjected to internal pressure, it tends to straighten. This deformation is proportional to the pressure applied.

The device is named after its inventor Eugène Bourdon, who developed it in 1849. Bourdon gauges are used for measuring pressure in various applications such as hydraulic systems, steam boilers, gas cylinders, compressors, and process industries. The gauge is known for its durability, precision, and ability to measure both high and low pressures accurately.

Construction of Bourdon Gauge

A Bourdon gauge mainly consists of the following parts:

  1. Bourdon Tube:
    • It is a thin-walled, elastic metallic tube with an oval cross-section bent into a circular or spiral shape.
    • The tube is usually made of brass, bronze, stainless steel, or phosphor bronze depending on the pressure range and fluid type.
    • One end of the tube is fixed and connected to the pressure inlet, while the other end is free to move.
  2. Linkage and Gear Mechanism:
    • The free end of the Bourdon tube is connected to a series of levers, links, and gears that amplify and transmit its movement.
    • The gear mechanism converts the small linear movement of the tube into a rotary motion.
  3. Pointer and Dial:
    • A pointer (needle) is attached to the gear system and moves over a circular calibrated dial to indicate the pressure.
    • The dial is usually graduated in Pascal (Pa), bar, or psi (pounds per square inch).
  4. Casing or Housing:
    • All the components are enclosed in a protective metallic case to shield them from dust, mechanical damage, and environmental effects.
  5. Connection Socket:
    • The pressure of the fluid to be measured enters through this socket and reaches the Bourdon tube.

Working Principle of Bourdon Gauge

The working principle of a Bourdon gauge is based on the elastic deformation of the Bourdon tube when subjected to internal pressure.

When fluid pressure is applied inside the tube:

  1. The cross-section of the tube tends to become more circular.
  2. This change in shape causes the curved tube to straighten slightly.
  3. The free end of the tube moves due to this straightening action.
  4. The motion of the free end is transferred to the pointer through the linkage and gear mechanism.
  5. The pointer moves over the dial to indicate the corresponding pressure.

The amount of tube movement is proportional to the pressure applied, allowing accurate pressure measurement.

Mathematically,

where,

  •  = Applied pressure,
  •  = Deflection of the Bourdon tube,
  •  = Constant of proportionality (depends on tube material and geometry).

When the applied pressure is released, the elasticity of the tube brings it back to its original shape, returning the pointer to zero.

Types of Bourdon Gauge

Based on the shape of the Bourdon tube, these gauges are classified into different types:

  1. C-Type Bourdon Gauge:
    • The tube is C-shaped (curved over about 270°).
    • Commonly used for general pressure measurements up to about 700 bar.
    • Simple and most widely used type.
  2. Helical Type Bourdon Gauge:
    • The tube is coiled into a helical (spring-like) shape.
    • Suitable for measuring very high pressures up to 1000 bar or more.
  3. Spiral Type Bourdon Gauge:
    • The tube is wound into a flat spiral shape.
    • Used where a longer tube length is required for better sensitivity and high accuracy.

Advantages of Bourdon Gauge

  1. Simple and Robust Construction:
    The design is straightforward, durable, and can withstand rough handling.
  2. High Accuracy:
    Provides precise pressure readings over a wide range.
  3. No External Power Required:
    Operates mechanically, without the need for electricity.
  4. Wide Pressure Range:
    Can measure from very low pressures to extremely high pressures (vacuum to 1000 bar).
  5. Long Life:
    Minimal wear and tear ensure long service life.

Limitations of Bourdon Gauge

  1. Not Suitable for Rapid Pressure Fluctuations:
    Frequent changes in pressure may cause mechanical fatigue.
  2. Temperature Effects:
    Extreme temperatures can alter the elasticity of the tube, affecting accuracy.
  3. Friction and Hysteresis Errors:
    Mechanical linkage may cause minor lag in pointer movement.
  4. Limited for Corrosive Fluids:
    Requires special materials or coatings for aggressive or corrosive fluids.
  5. Calibration Required:
    Needs periodic calibration for maintaining precision.

Applications of Bourdon Gauge

  1. Industrial Pressure Measurement:
    Used in factories, refineries, and power plants to measure fluid and gas pressures.
  2. Hydraulic and Pneumatic Systems:
    Measures system pressure in pumps, compressors, and hydraulic machines.
  3. Boilers and Steam Lines:
    Commonly used to monitor steam pressure in boilers.
  4. Gas Cylinders:
    Measures internal pressure in gas tanks and cylinders.
  5. Process Industries:
    Used in chemical, petrochemical, and food processing plants for pressure control.

Example

If a Bourdon gauge shows a deflection of 25° when connected to a pressure line, and the calibration constant   is  , then the applied pressure is:

Hence, the applied pressure on the gauge is 10 bar.

Conclusion

In conclusion, a Bourdon gauge is a simple and reliable mechanical device that converts fluid pressure into pointer movement to indicate pressure on a dial. It operates on the principle of elastic deformation of a curved tube when pressurized. Because of its high accuracy, robustness, and wide measuring range, the Bourdon gauge remains one of the most widely used pressure measuring instruments in mechanical, industrial, and laboratory applications. Proper calibration and material selection further enhance its performance and durability.