How Does a Bourdon Tube Pressure Gauge Work?

A Bourdon tube pressure gauge is a clever yet straightforward device used to measure pressure in liquid or gas systems. But how exactly does this widely trusted instrument work its magic?

Let’s unpack the inner workings step-by-step to uncover what gives the Bourdon tube its sturdy reputation across various industrial applications.

What is a Bourdon Tube?

The Bourdon tube is the star component that enables pressure gauges to function. It converts pressure into a measurable mechanical motion.

A Bourdon tube refers to a closed, curved tube that tends to straighten when pressurized.

The tube is bent into various shapes like a C, spiral, or helix. But the most common shape is the C-type frame.

Bourdon tubes are generally made up of:

• Phosphor bronze – the most popular because it offers high elasticity.
• Beryllium copper – for high accuracy.
• Stainless steel – for high temperatures and pressures.

The tube’s oval or elliptical cross-section gives it a flattened profile. This shape amplifies the level of motion produced when pressure is applied.

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How Does Pressure Straighten the Bourdon Tube?

The Bourdon tube implements a straightforward scientific principle – a flattened tube tends to regain its circular form when pressurized.

When pressure is applied to the Bourdon tube’s inner cavity, the oval tube tries to achieve a circular cross-section. This results in the tube unwinding and straightening somewhat.

The Bourdon tube’s tendency to regain its circular shape under pressure is known as the Bourdon effect.

Key Parts of a Bourdon Tube Pressure Gauge

A Bourdon tube pressure gauge consists of the following key components:

1. Bourdon Tube

The Bourdon tube is the core part that senses the applied pressure and moves in response.

2. Inlet

The inlet connects the Bourdon tube to the source of pressure.

3. Movement

It converts the motion of the Bourdon tube into the rotation of the pointer shaft. This is achieved through a series of levers and gears.

4. Pointer Shaft

The rotational motion of the pointer shaft moves the pointer.

5. Pointer

The pointer indicates the pressure reading on the gauge’s dial.

6. Dial

The dial is the round face that displays the pressure units. The pointer points to the reading.

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How Do the Internal Parts Produce the Pressure Reading?

When pressure enters the Bourdon tube through the inlet, the tube tends to straighten according to the intensity of the pressure.

The linear motion of the tip of the Bourdon tube is transmitted via levers to rotate the pointer shaft.

• The initial small displacement of the Bourdon tube tip is amplified through a series of levers.
• The lever system converts the linear motion into the rotational motion of the pointer shaft.
• The pointer attached to the pointer shaft now moves across the dial face to indicate the pressure value.
• The dial is calibrated to directly convert the pointer position into the pressure reading.

This clever linkage converts minute linear movements of the Bourdon tube into the readable rotation of the pointer on the dial.

The Math Behind a Bourdon Tube

Let’s geek out a bit and understand the math that governs the working of a Bourdon tube.

The displacement of the Bourdon tube tip is proportional to the applied pressure. Mathematically speaking:

Displacement α Pressure

The displacement also depends on the tube’s length, thickness, width, and modulus of elasticity.

Mathematically, the displacement (δ) of the tube tip can be expressed as:

This shows that the Bourdon tube’s dimensions directly impact its motion or displacement.

So the Bourdon tube can be designed according to the expected pressure range. This makes the Bourdon tube pressure gauge a versatile device that can measure a wide span of pressure levels.

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Key Characteristics of a Bourdon Gauge

What makes the Bourdon gauge so popular for pressure measurement across industries?

Here are some of its winning qualities:

• High sensitivity – can detect very low pressures due to amplification by the lever system.
• High accuracy – errors within 0.1 to 0.25% of full-scale reading.
• Mechanical reliability – works purely mechanically without any electronics.
• Robustness – withstands high pressures and temperatures.
• No calibration needed – factory calibrated to work out-of-the-box.
• Easy to read – provides direct pressure readout on dial without computations.

Additionally, Bourdon gauges do not need external power. This makes them suitable for operation at remote sites with no power supply.

Pressure Range and Engineering Units

Bourdon tube gauges can measure pressure over a wide range:

• From 10 kPa (100 mbar) up to 1000 MPa (10,000 bar)
• Most common range – 0 to 10 bar or 0 to 160 psi

The dial face displays the pressure units, typically:

• PSI (pounds per square inch)
• Bar
• kPa
• MPa
• kg/cm2
• Atmospheric pressure

So a single Bourdon tube gauge can show pressure in different units by changing the dial face labels.

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Applications of Bourdon Tube Pressure Gauges

Bourdon gauges are extensively used to measure pressure in:

1. Hydraulic Systems

Industrial hydraulic equipment relies on Bourdon gauges to monitor system pressure for smooth functioning. This includes hydraulic cylinders, jacks, pumps, and motors.

Bourdon gauge installed on a hydraulic press

2. Pumps

The outlet pressure of pumps needs to be continually monitored. Bourdon gauges are commonly installed near pump outlets.

3. Compressed Air Systems

Compressed air systems in factories use Bourdon gauges to measure air pressure at compressor outlets.

4. Process Piping

Bourdon tube gauges help monitor pressurized gases and liquids flowing through pipes in chemical plants, refineries, and offshore rigs.

5. Gas Cylinders

High pressure gas cylinders like oxygen, nitrogen, and CO2 cylinders have Bourdon tube gauges to show the remaining gas pressure.

6. Pressure Vessels

Tanks, boilers, and other pressure vessels are equipped with Bourdon gauges to monitor internal pressure.

7. Tire Pressure

Bourdon gauges allow monitoring tire pressure in automobiles and aircraft. They indicate if the pressure needs correction.

8. Meteorology

Weather stations use specialized Bourdon gauges to measure atmospheric pressure.

Pros and Cons of Bourdon Gauges

Advantages

• Simple working principle based on the Bourdon tube
• No external power source needed
• Withstand high pressures and temperatures
• Accurate and reliable measurements
• Low maintenance

Disadvantages

• Cannot provide remote pressure readout
• Prone to vibration errors
• Not suitable for very low pressure measurement
• Requires good lighting to read dial

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Evolution of Bourdon Gauges

Although Bourdon gauges still dominate pressure measurement, modern technological advancements have enhanced their functionality:

• Digital bourdon gauges show the pressure readout on an LCD or LED display instead of a pointer dial. This gives an easy digital pressure readout.

• Electronic bourdon gauges combine a Bourdon tube sensor with electronic signal conditioning. This allows transmission of pressure data to remote readouts.
• MEMS bourdon gauges replace the Bourdon tube with a silicon MEMS sensor but retain the mechanical dial movement. This shrinks the gauge size for portability.

The Future of Bourdon Gauges

Despite being a century old device, the Bourdon gauge remains irreplaceable for reliable and local pressure measurement.

With its simple working and high sensitivity, Bourdon tube gauges will continue to be indispensable for pressure monitoring across industrial and scientific applications.

While electronic pressure transmitters may slowly gain ground, mechanical Bourdon gauges will remain popular for their simplicity, cost-effectiveness, and robustness.

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Conclusion

The Bourdon pressure gauge is an ingenious device that uses the simple principle of a flattened tube regaining its circular shape. This enables continuous monitoring of system pressure to an accuracy of 0.25% full-scale.

With advances in materials and electronics, Bourdon gauges have evolved to meet modern demands. But the fundamental working remains unchanged since Eugène Bourdon invented the first gauge in 1849.

Overcoming limitations like fragility and bulkiness, the Bourdon tube stands strong as one of the most trusted and ubiquitous pressure measuring devices even after 170 years!

So the next time you see a Bourdon gauge, take a moment to admire the neat lever mechanism that converts tiny tube motions into readable pressure values.

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