Bourdon Tube, Diaphragm, and Bellows Gauges

Accurate pressure measurement is vital across industries, from heavy manufacturing and energy production to healthcare and environmental monitoring. Pressure is often a key variable that reflects the health and performance of a system. Whether in steam boilers, hydraulic presses, chemical reactors, HVAC units, or medical devices, maintaining the right pressure ensures safety, reliability, and efficiency.

Among the various instruments available, Bourdon tubes, diaphragm gauges, and bellows gauges have stood the test of time. These devices represent the backbone of mechanical pressure measurement. While they operate on different principles, they share a reputation for robustness, accuracy, and dependability in critical applications.

1. Bourdon Tube Pressure Gauges

Working Principle

Invented by Eugène Bourdon in 1849, the Bourdon tube remains one of the most widely used mechanical pressure sensors. It consists of a curved, flattened metal tube sealed at one end. When pressurized fluid enters the tube, the tube tends to straighten as the internal pressure acts against the elastic walls. This mechanical deformation is transferred through linkages and gears to a pointer, which indicates pressure on a calibrated dial.

Because the motion is purely mechanical, Bourdon gauges require no external power source, making them ideal for environments where electricity is unavailable or undesirable.

Applications

• Steam boilers: Ensures boilers do not exceed safe operating pressures.
• Hydraulic systems: Monitors pressure in presses, pumps, and hydraulic machinery.
• Compressors: Provides real-time readings in pneumatic systems.
• Process industries: Widely used in refineries, chemical plants, and power generation facilities.

Advantages

• Cost-effective and easily available worldwide.
• Rugged, simple design with minimal moving parts.
• Works across an extremely wide range of pressures (vacuum up to thousands of bar).
• No external power or batteries required.

Limitations: Bourdon tubes are not ideal for very low pressures or highly dynamic (fast-changing) pressures. They can also be affected by mechanical wear over time, leading to calibration drift.

2. Diaphragm Pressure Gauges

Working Principle

Diaphragm gauges use a thin, flexible membrane (metallic, rubber, or composite) that deflects when exposed to pressure. One side of the diaphragm is exposed to the medium being measured, while the other side is often at reference pressure (atmosphere or vacuum). The deflection is proportional to the applied pressure and is transmitted to a pointer or converted into an electronic signal in hybrid models.

This makes diaphragm gauges excellent for applications where pressure values are low, and accuracy is critical. Additionally, the diaphragm can be coated or manufactured from special materials to resist corrosion or contamination.

Applications

• Gas pressure measurement: Used for low-pressure gas pipelines and utilities.
• Chemical and pharmaceutical industries: Ideal for measuring corrosive or reactive media when combined with protective diaphragms.
• Water treatment plants: Tracks pressure in pumps, filters, and distribution networks.
• Food and beverage production: Ensures hygienic monitoring of process pressures.

Advantages

• High accuracy for low and medium pressures.
• Available in corrosion-resistant designs for harsh media.
• Less prone to clogging when measuring viscous or contaminated fluids.

Limitations: Diaphragm gauges are less suitable for very high pressures. Overpressure may damage the diaphragm permanently, requiring replacement.

3. Bellows Pressure Gauges

Working Principle

Bellows gauges employ a collapsible element resembling an accordion. The bellows expand and contract when pressure is applied. Because of their large effective area and elasticity, they are highly sensitive to even the smallest pressure changes. The movement of the bellows is transferred via linkages to an indicator or sensor.

Bellows can be used for both absolute pressure measurement (against a vacuum) and differential pressure measurement (difference between two inputs). They are widely valued in applications requiring extreme sensitivity.

Applications

• HVAC and refrigeration: Measures small pressure differences in cooling systems.
• Medical instruments: Used in anesthesia machines, ventilators, and monitoring equipment.
• Differential pressure measurement: Essential in filters, air ducts, and cleanroom monitoring.
• Environmental monitoring: Detects subtle atmospheric pressure changes for weather analysis.

Advantages

• High sensitivity and precision.
• Effective for very low differential pressure readings.
• Can measure both positive and negative pressures.
• Compact design suitable for portable instruments.

Limitations: Bellows can be mechanically delicate. They are not recommended for very high pressures or rugged environments where vibration and shock are common.

Comparative Summary

Choosing the Right Gauge

The decision to use a Bourdon tube, diaphragm, or bellows gauge depends on three main factors:

• Pressure Range: For very high pressures, Bourdon tubes dominate. For low to medium pressures, diaphragms excel. For extremely small differential pressures, bellows are ideal.
• Media Compatibility: Diaphragm gauges, with coatings and corrosion-resistant materials, are suited to aggressive chemicals. Bourdon tubes handle steam and hydraulic oils well. Bellows are best for clean gases and sensitive applications.
• Application Sensitivity: If fine resolution and precision are required (e.g., in medical or HVAC applications), bellows or diaphragm gauges are better choices. For rugged environments, Bourdon tubes remain the standard.

Modern Relevance

While digital sensors are increasingly common, mechanical gauges like Bourdon tubes, diaphragms, and bellows remain highly relevant. They offer fail-safe operation (no dependency on electricity), simplicity, and reliability. In fact, many industries use them as backup instruments alongside digital systems to ensure redundancy and safety.

Hybrid designs also exist today — for example, a diaphragm or Bourdon tube gauge linked to an electronic transducer. This allows digital data logging while retaining the robustness of mechanical sensing.

Conclusion

Bourdon tube, diaphragm, and bellows gauges each have unique strengths. Bourdon tubes are rugged workhorses suitable for high-pressure industrial systems. Diaphragm gauges excel in low-pressure and corrosive applications. Bellows gauges deliver unmatched sensitivity for precision and differential measurements.

By understanding their principles, applications, advantages, and limitations, engineers and technicians can select the right gauge for the job. This not only ensures safe operation but also improves efficiency and prolongs equipment life. Even in an age dominated by digital technologies, these classic mechanical instruments remain indispensable in the field of pressure measurement.