A Bourdon tube is a curved, hollow metal tube that straightens slightly when pressure builds inside it, converting that tiny movement into a needle reading on a pressure gauge. It’s the mechanism inside most of the round, dial-face pressure gauges you see on boilers, air compressors, and industrial piping. Invented in 1849, it remains one of the most common and reliable ways to measure pressure in both everyday and industrial settings.
How a Bourdon Tube Works
The key to understanding a Bourdon tube is its cross-section. Rather than being perfectly round like a garden hose, the tube is flattened into an oval or rectangular shape. When pressurized fluid or gas enters the tube, the internal pressure pushes outward on the walls, trying to force that oval cross-section into a circle. As the cross-section rounds out, the overall curve of the tube straightens slightly, the same way a party blower uncoils when you blow into it.
One end of the tube is fixed and open, connected to whatever system you’re measuring. The other end is sealed and free to move. As pressure increases, the sealed tip moves outward. A small mechanical linkage connects that tip to a pointer needle on the gauge dial. When the pressure drops, the tube’s natural elasticity pulls it back to its original curved shape, and the needle returns to zero. No electricity, no batteries, no calibration software. The whole system is purely mechanical.
The Shape That Started It All
The Bourdon tube is named after Eugène Bourdon, a French engineer and instrument maker. The story of its invention is a happy accident. While overseeing the repair of a damaged tube, Bourdon used high-pressure water to reshape it. Instead of the cross-section simply rounding out as expected, the entire curved tube began to straighten. Intrigued, he repeated the experiment with a stiffer, elastic steel tube sealed at one end and confirmed the effect was predictable and repeatable.
His 1849 French patent included several designs, from a simple omega-shaped tube with a single link to a needle, to spirals connected by small toothed gears. The timing was significant. Before Bourdon’s gauge, the only way a steam boiler operator could judge internal pressure was by listening to the hissing of a valve and feeling how stiff its spring was. Boiler explosions were common, and many people viewed steam engines with genuine distrust. Bourdon’s invention gave operators a simple, robust way to see the pressure difference between the inside of a boiler and the outside air, making steam power dramatically safer.
Types of Bourdon Tubes
Not all Bourdon tubes look the same. The shape of the curve determines how much the tip moves for a given pressure change, which in turn determines the gauge’s sensitivity and pressure range.
- C-type: The most common design. The tube forms a simple arc, roughly shaped like the letter C. These are compact, inexpensive, and work well for moderate pressure ranges. You’ll find them in standard industrial gauges.
- Spiral: The tube is wound into a flat spiral, like a watch spring. This longer tube length amplifies the tip movement, making spiral designs more sensitive and useful for lower pressure ranges or where greater needle travel is needed.
- Helical: The tube is coiled into a helix, stacking the coils vertically. Helical designs are used for very high pressures where a C-type tube wouldn’t produce enough deflection on its own. The multiple coils multiply the movement of the free end.
Common Materials
The tube material matters because it needs to flex repeatedly without cracking, resist corrosion from whatever fluid it contacts, and spring back to its original shape consistently. The most common choices are phosphor bronze, stainless steel (typically 316 grade), and beryllium copper. Phosphor bronze offers good elasticity and corrosion resistance for general-purpose applications. Stainless steel handles aggressive chemicals and high temperatures. Beryllium copper provides excellent fatigue resistance, meaning it holds its accuracy over many pressure cycles.
For extremely corrosive environments, specialty alloys like Monel (a nickel-copper alloy) or Inconel are sometimes used, though at a higher cost.
Protective Accessories
A bare Bourdon tube gauge mounted directly on a process line can fail prematurely if the conditions are harsh. Two common accessories extend gauge life significantly.
In steam applications, a coil siphon (a looped tube, typically made of steel, brass, or stainless steel with a 180-degree coil) sits between the process line and the gauge. It traps a pocket of condensed water that prevents live steam from reaching the Bourdon tube directly, which would overheat and damage it.
For systems with rapid pressure pulses or vibration, a pressure snubber restricts the flow into the gauge, smoothing out the spikes. Without a snubber, the needle chatters constantly, making the gauge hard to read and wearing out the tube’s linkage mechanism far sooner than it should.
When to Use a Bourdon Gauge
Bourdon tubes are the default choice for a wide range of pressure applications, but they aren’t ideal for every situation. They handle moderate to very high pressures well, they’re resistant to shock and vibration, and they install easily with a standard threaded connection. Compared to diaphragm or bellows gauges, Bourdon gauges generally cost less and require simpler mounting.
Where they fall short is at low pressures. At very low readings, the tube doesn’t deflect enough to give an accurate or readable measurement. Bellows gauges, which use an accordion-like metal chamber, are better suited for those applications. Diaphragm gauges, which use a flexible membrane, are the better option when you need to measure pressure in thick, sticky, or highly corrosive fluids that would clog or damage the inside of a Bourdon tube.
For most applications between roughly 15 psi and several thousand psi, though, the Bourdon tube remains the simplest, most cost-effective, and most widely trusted pressure measurement technology available. Its purely mechanical design means it works without power, survives harsh environments, and keeps running for years with minimal maintenance.