A Bourdon tube measures pressure by exploiting a simple physical principle: a curved, hollow tube with a flattened cross-section tends to straighten out when pressure builds inside it. That tiny movement at the tube’s tip gets translated through a mechanical linkage into a pointer rotation on a dial, giving you a pressure reading. It’s the mechanism behind most analog pressure gauges you’ll see in industrial settings, HVAC systems, and hydraulic equipment.
The Core Principle: Unequal Areas
A Bourdon tube starts as a metal tube that’s been bent into a C-shaped arc and sealed at one end. The key detail is its cross-section: instead of being round, the tube is deliberately flattened into an oval or elliptical shape. This flattening is what makes the whole thing work.
When pressurized fluid enters the open end, it pushes outward against every interior surface equally. But because the tube is curved, the outer wall of the curve has a larger surface area than the inner wall. Equal pressure acting on a larger area produces a greater force. So the outward-pushing force on the outer wall wins, and the tube tries to uncurl. It straightens slightly until the spring-like resistance of the metal balances out the pressure force. Remove the pressure, and the tube’s elasticity pulls it back to its original curved shape.
The flattened cross-section plays a critical role here. A perfectly round tube would resist deformation in all directions equally and barely move. The oval shape makes the tube more flexible along the axis of its curve, amplifying the straightening effect so the tip moves enough to drive a pointer.
From Tip Movement to Dial Reading
As pressure increases, the sealed tip of the tube sweeps outward in a small arc. This movement is tiny, often just a few millimeters across the full pressure range. To turn that into a full sweep of a pointer across a dial, the gauge uses a mechanical amplification system.
A thin connecting link attaches the tube’s tip to a sector gear, which is a small arc-shaped gear. That sector gear meshes with a tiny pinion gear mounted on the pointer shaft. When the tip moves, the link pulls the sector gear, which rotates the pinion, which spins the pointer. This gear ratio multiplies the small tip travel into roughly 270 degrees of pointer rotation across the dial face. A fine hairspring keeps tension on the gear train to eliminate slack and ensure the pointer returns smoothly when pressure drops.
Tube Shapes and Pressure Ranges
The classic C-shaped tube is the most common design, but it’s not the only one. The geometry of the tube determines how much pressure it can handle and how sensitive it is.
- C-type tubes are the workhorse design. They can measure pressures up to 6,000 bar (about 87,000 psi), covering everything from low-pressure gas lines to extreme hydraulic systems.
- Spiral tubes wind the tube into a flat coil, like a watch spring. This increases the effective length of the tube, which means more tip travel per unit of pressure and finer resolution. The tradeoff is a lower maximum range of about 700 bar.
- Helical tubes stack the coils vertically into a helix. Like spiral types, they offer greater sensitivity but cap out around the same 700-bar limit.
Across all designs, Bourdon tube gauges typically cover pressures from about 10 kPa (just above atmospheric pressure) up to 100 MPa, with accuracy around ±1% of full scale. That’s precise enough for most industrial monitoring but not for laboratory-grade measurement.
What the Tube Is Made Of
The tube material matters because it needs to flex thousands of times without losing its shape. Common choices include phosphor bronze (a copper-tin alloy with good springiness), beryllium copper, and silicon bronze. For corrosive environments or high temperatures, manufacturers use nickel-based superalloys or stainless steel alloys containing chromium, nickel, and molybdenum.
The French engineer Eugène Bourdon patented this design in 1849, and the core concept hasn’t changed. What has changed is material science. Fused quartz, for instance, is now used in some high-precision helical tubes because it exhibits less hysteresis, creep, and relaxation than any metal. That means it returns to its original shape more faithfully after being deformed, which translates to more consistent readings over time.
What Affects Accuracy
Bourdon tubes have a few built-in limitations worth understanding. Hysteresis is inherent to the design: the tube doesn’t return to exactly the same position after being pressurized. If you pressurize a gauge to 50 bar and release it, then approach 50 bar again from a higher pressure, the two readings won’t be identical. The difference is small but real.
Temperature changes also introduce error. For every 25°C shift in ambient temperature, readings can drift by about ±0.25% of the full span. Some gauges compensate for this with a bimetallic strip that adjusts the linkage as temperature changes, similar to how an old thermostat works.
Vibration and pressure pulsations are the most practical concern. In applications with pumps, compressors, or engines, rapid pressure swings can rattle the gear train and make the pointer unreadable. Over time, this mechanical stress also accelerates wear. The solution is a liquid-filled gauge: the case is filled with glycerin or silicone oil, which dampens pointer oscillation and cushions the internal components against shock. If you’ve ever seen a pressure gauge where the dial appears to be submerged in clear fluid, that’s what it’s for.
Why Bourdon Tubes Are Still Everywhere
Despite electronic pressure sensors being widely available, Bourdon tube gauges remain one of the most common pressure instruments in the world. They need no power source, work reliably across a huge pressure range, handle both liquids and gases, and are simple to install. A well-made gauge in a stable environment will give consistent readings for years with no calibration. They’re also easy to read at a glance, which matters in environments where operators are monitoring dozens of systems at once. For applications requiring remote data logging or sub-0.1% accuracy, electronic transducers are the better choice. For everything else, the 175-year-old curved tube still does the job.