A Bourdon pressure gauge is a mechanical instrument that measures fluid pressure by converting the elastic deflection of a curved metal tube into rotation of a pointer. The key component is the Bourdon tube itself — a flattened, curved tube sealed at one end that straightens slightly as internal pressure rises. That tip motion drives a link, sector gear, and pinion to swing the pointer across a calibrated dial. The result is a rugged, power-free readout used everywhere from 0-15 psi air regulators to 10,000 psi hydraulic test stands.
Bourdon Pressure Gauge Interactive Calculator
Vary pressure, gauge span, Bourdon tip travel, and gear ratio to see tube deflection amplified into pointer rotation.
Equation Used
The calculator uses the article diagram values for a Bourdon pressure gauge: a 0-100 psi dial, 3-8 mm full-scale tip travel, about 10:1 gear ratio, and 270 deg pointer sweep. Pressure fraction of span is applied linearly to estimate Bourdon tip travel and pointer rotation.
- Gauge response is treated as linear over the calibrated span.
- Full-scale pointer sweep is 270 deg as shown in the article diagram.
- Tip travel is the selected full-scale Bourdon tube travel.
- Overpressure is shown numerically but elastic behavior may not remain valid above full scale.
Operating Principle of the Bourdon Pressure Gauge
The Bourdon tube is the whole trick. Take a metal tube, flatten its cross-section to an oval, then bend it into a C-shape (about 250° of arc), weld one end to a fixed socket where the pressure enters, and seal the other end. When you pressurise the inside, the oval cross-section wants to become circular — and because the tube is curved, that small cross-section change forces the free tip to uncurl outward by a tiny amount, typically 3-8 mm of travel at full scale. That motion is what you're reading.
The tip connects through an adjustable link to a sector gear, which meshes with a small pinion on the pointer shaft. The link multiplies that few millimetres of tip motion into roughly 270° of pointer sweep. A hairspring takes up backlash in the sector-pinion mesh — without it, the pointer would jitter and read low on rising pressure, high on falling pressure. If the link length, sector pivot, or hairspring tension drift, you get non-linearity, hysteresis, or a zero shift, and the gauge falls outside its accuracy class (typically Grade A 1% of span for industrial gauges per ASME B40.100, or Grade 4A 0.1% for test gauges).
Failure modes are predictable. Pulsation from a piston pump fatigues the tube wall and the pointer linkage — within months the pointer wears a slop band and reads erratically. Overpressure beyond about 130% of full scale yields the tube permanently and shifts zero. Hydrogen embrittlement attacks brass tubes in hydrogen service, so you specify 316 stainless or Monel. Freeze a water-filled tube once and it ruptures the C-curve — that's why steam gauges always run with a siphon loop full of condensate to keep the tube itself below 65°C.
Key Components
- Bourdon Tube (C-tube, helical, or spiral): The elastic pressure element. A flattened oval cross-section bent into a C of roughly 250° arc for ranges up to 1,000 psi, or wound into a helix or spiral for higher sensitivity at low ranges. Wall thickness sits between 0.2 and 0.8 mm depending on range; tip travel at full scale is 3-8 mm.
- Socket and Process Connection: Holds the fixed end of the tube and routes pressure in through a 1/4 NPT or 1/2 NPT thread (G threads in Europe). The socket also carries the gauge body. Tube-to-socket joint is silver-brazed or TIG welded — a leak here vents to atmosphere and zeroes the gauge.
- Link: An adjustable rod connecting the tube tip to the sector gear. Its length sets span calibration — shortening the link increases pointer sweep per psi. Pivot pins must run with less than 0.05 mm of clearance or you introduce hysteresis.
- Sector Gear and Pinion: Converts the linear arc of the link into pointer rotation. Gear ratio typically 8:1 to 12:1. Tooth backlash directly creates pointer dead-band, so quality gauges use lapped brass sectors and hardened steel pinions.
- Hairspring: A fine spiral spring on the pointer shaft that loads the sector-pinion mesh in one direction, eliminating backlash. Without it, rising and falling pressure readings differ by 1-2% of span.
- Movement Plate and Pointer: The brass or stainless plate carrying the gear train. The pointer is a balanced aluminium needle pressed onto the pinion shaft — a friction fit that lets you re-zero by lifting and re-seating the pointer.
- Dial and Window: Printed aluminium or fibre dial calibrated against a deadweight tester. Window is glass for chemical service, polycarbonate or laminated safety glass for high-pressure gauges where tube rupture must be contained.
Who Uses the Bourdon Pressure Gauge
Bourdon gauges turn up wherever fluid pressure needs a local readout without batteries, transmitters, or signal wiring. They survive vibration, freezing, washdown, and decades of service when sized correctly. You see them on hydraulic power packs, steam boilers, fire suppression cylinders, scuba regulators, and oil refineries — anywhere a process operator wants a glance-and-go reading.
- Hydraulic Power: Enerpac P-392 hand pump test stands run a 10,000 psi glycerine-filled Bourdon gauge to verify cylinder pressure during structural jacking work.
- Steam and Power Generation: Cleaver-Brooks firetube boilers carry a 0-300 psi Ashcroft 1279 gauge mounted on a pigtail siphon to keep the tube cool below 65°C.
- Oil and Gas: Wellhead Christmas trees on Permian Basin completions use 0-15,000 psi WIKA 232.50 stainless Bourdon gauges to monitor casing and tubing pressure.
- Fire Protection: Ansul R-102 kitchen fire suppression cylinders carry a small 0-400 psi Bourdon gauge that confirms nitrogen charge during monthly inspection.
- Diving and Compressed Gas: Scubapro MK25 first-stage regulators feed a 0-5,000 psi miniature Bourdon submersible pressure gauge so divers can read tank pressure at depth.
- Process Chemical Plants: Dow ethylene oxide reactors run sealed-diaphragm-protected Bourdon gauges with Monel tubes because the C-tube alone would corrode in hours.
- Pneumatic Tools and HVAC: Yellow Jacket 49967 manifold sets use paired 0-500 psi Bourdon gauges for refrigerant charging on R-410A split systems.
The Formula Behind the Bourdon Pressure Gauge
Tip deflection of a Bourdon C-tube depends on internal pressure, tube geometry, and material stiffness. Designers use the simplified Wuest relation to size a tube for a target full-scale tip travel — typically 3-8 mm. Below 2 mm of travel the gear train can't develop full pointer sweep without amplifying backlash and noise. Above 10 mm you start running the tube into its yield zone and lose linearity. The sweet spot sits around 5 mm of tip travel at full scale, which gives clean 270° pointer rotation with a comfortable safety margin against overpressure.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| δ | Tip deflection of the Bourdon tube at the free end | mm | in |
| P | Internal gauge pressure | Pa | psi |
| R | Mean radius of curvature of the C-tube | mm | in |
| E | Young's modulus of tube material | Pa | psi |
| t | Tube wall thickness | mm | in |
| a | Major semi-axis of the oval cross-section | mm | in |
| b | Minor semi-axis of the oval cross-section | mm | in |
| α | Subtended arc angle of the C-tube (in radians) | rad | rad |
| χ | Geometry correction factor (typical 0.05-0.15) | — | — |
Worked Example: Bourdon Pressure Gauge in a craft distillery's copper still vapour-line gauge
A small Kentucky bourbon distillery is specifying a 316 stainless Bourdon gauge for the vapour line on a 500-gallon Vendome copper pot still. Operating pressure runs 0-30 psi gauge, with relief lifting at 35 psi. They want a 100 mm dial gauge with full-scale span of 60 psi (so nominal sits at half-scale), and they need to confirm the C-tube tip will travel enough to drive a clean 270° pointer sweep at full scale. Tube spec: R = 25 mm, t = 0.4 mm, a = 4 mm, b = 1 mm, α = 4.36 rad (250°), χ = 0.10, E = 193 GPa for 316SS.
Given
- PFS = 60 psi (414,000 Pa)
- R = 25 mm
- t = 0.4 mm
- a = 4 mm
- b = 1 mm
- α = 4.36 rad
- E = 193,000 MPa
- χ = 0.10 —
Solution
Step 1 — compute the cross-section flatness term, which captures how much the oval wants to round out under pressure:
Step 2 — at the nominal half-scale operating point (30 psi = 207,000 Pa), compute tip deflection:
That's right at the lower edge of acceptable tip travel — enough to drive the pointer to roughly 135° (half scale) cleanly, but you'd feel any sector-gear backlash at this travel.
Step 3 — at the low end of normal operation, 10 psi (about 17% of span), tip travel scales linearly to roughly 0.83 mm. The pointer sits around 45° from zero. At this travel a 0.05 mm pivot slop in the link translates to a visible 1.5 psi reading error — which is why low-pressure readings on a poorly-built gauge always look noisier than high-pressure readings.
Step 4 — at full scale (60 psi), tip travel reaches:
5.0 mm at full scale lands squarely in the design sweet spot. The pointer sweeps a full 270°, the gear ratio doesn't have to be cranked up, and the tube is nowhere near its yield strain. Push the design to a 30 psi full-scale span and tip travel drops to 2.5 mm at FS — workable but tight. Drop the span to 15 psi and you'd need a helical or spiral element instead, because a C-tube can't generate enough motion at low pressure without going to a very thin wall that won't survive normal handling.
Result
Full-scale tip travel works out to about 5. 0 mm, with 2.5 mm at the nominal 30 psi operating point. That's a healthy design — the pointer covers the full dial without forcing the gear train, and the tube has plenty of margin against the 35 psi relief setting and the typical 130% overpressure proof requirement. At 10 psi you'll see roughly 0.83 mm of travel, which feels jumpy on a cheap gauge but reads cleanly on a quality one with tight pivot fits. At full scale 5.0 mm is the sweet spot Ashcroft and WIKA both target on their 100 mm industrial gauges. If your installed gauge reads 5-10% low at full pressure, the most likely causes are: (1) link length drifted outward from vibration loosening the calibration screw, (2) the hairspring lost tension from repeated overpressure events and isn't fully eliminating sector-gear backlash, or (3) glycerine fill leaked out of a case-fill gauge and the pointer is now damped by air, which shifts apparent readings on rapidly fluctuating pressure.
Bourdon Pressure Gauge vs Alternatives
Bourdon gauges aren't the only way to read pressure locally. Diaphragm gauges, capsule gauges, and electronic transducers each take a different cut at the same problem. Pick based on pressure range, accuracy, media compatibility, and whether you need a remote signal.
| Property | Bourdon Gauge | Diaphragm Gauge | Electronic Pressure Transducer |
|---|---|---|---|
| Pressure range | 0.6 to 100,000 psi | 0 to 600 psi (low range strength) | Vacuum to 100,000+ psi |
| Accuracy class (typical industrial) | ±1.0% of span (Grade A) | ±1.6% of span | ±0.1% to ±0.25% of span |
| Cost (100 mm dial, 0-1000 psi) | $25-$120 USD | $80-$250 USD | $200-$800 USD plus display |
| Power required | None — purely mechanical | None | 10-30 VDC, 4-20 mA loop |
| Lifespan under pulsating load | 6 months to 2 years untreated; 10+ years with snubber and glycerine fill | 3-7 years (diaphragm fatigue limited) | 5-15 years (no moving elastic element) |
| Best application fit | Hydraulic, steam, compressed gas, general process | Low pressure, viscous/clogging media, slurries | Remote monitoring, data logging, control loops |
| Overpressure tolerance | 1.3× FS without permanent shift | 1.5-2× FS with stop | 2-5× FS with burst protection |
| Local readability without power | Excellent — analog dial | Excellent — analog dial | None unless paired with display |
Frequently Asked Questions About Bourdon Pressure Gauge
That's almost always pulsation damage to the tube or linkage, not a manufacturing defect. Piston pumps generate pressure spikes well above the gauge's nominal reading — a 3,000 psi pump can throw 5,000 psi spikes that work-harden the tube and stretch the link pivots. After a few hundred thousand cycles the tube takes a permanent set and the pointer no longer returns to zero.
Fix it by installing a piston-type snubber (a porous metal plug in the inlet) and switching to a glycerine-filled gauge. The glycerine damps pointer oscillation and isolates the linkage from high-frequency spikes. Expect 5-10× the service life.
The siphon loop has lost its water charge. A Bourdon tube exposed directly to saturated steam runs at 150-200°C, and Young's modulus of brass or steel drops measurably with temperature — the tube becomes more compliant, deflects further per psi, and the pointer reads high.
The siphon pigtail is supposed to trap a permanent column of condensate so the tube itself never sees steam, only warm water below about 65°C. If the loop dried out (common after a boiler blowdown) or was installed without a fill plug, refill it with water before reconnecting and the reading will return to spec.
You can hit 0.5% with a Grade 2A Bourdon test gauge (160 mm or 250 mm dial, mirrored scale, knife-edge pointer), but only if you calibrate it against a deadweight tester every 6 months and avoid pulsation. For a process line that sees flow surges or temperature swings, a piezoresistive transducer with 0.1% accuracy and digital compensation is more honest — the Bourdon will drift faster than you can recalibrate it.
Rule of thumb: if accuracy below 1% matters AND the environment is uncontrolled, go electronic. If accuracy below 1% matters AND you can control vibration, temperature, and overpressure, a quality test gauge is cheaper and lasts longer.
At 15 psi full scale, a standard C-tube can only generate 1-2 mm of tip travel — not enough to drive the gear train past its own internal friction smoothly. You're seeing stick-slip in the sector-pinion mesh.
The fix is to specify a spiral or helical Bourdon element instead of a C-tube. A spiral wound through 4-6 turns multiplies tip travel by the same factor without needing a higher gear ratio, so the pointer moves continuously at low pressure. Most quality manufacturers use spiral elements automatically below about 30 psi full scale — if your gauge has a C-tube at 15 psi, it's the wrong design for the range.
No. Hydrogen embrittlement attacks brass and copper alloys regardless of pressure — it's a chemical effect, not a mechanical one. Atomic hydrogen diffuses into the grain boundaries and the tube cracks, often suddenly and without warning.
Specify 316L stainless or Monel 400 for any hydrogen service, even at low pressure. The same rule applies to ammonia (attacks copper and brass) and chlorine (attacks most stainless grades — use Monel or Hastelloy). Match the wetted material to the media before worrying about range or accuracy.
Glycerine fill submerges the entire movement in viscous fluid, which damps pointer oscillation, lubricates the gear train, and keeps moisture out of the case. On pulsating systems (pumps, compressors, hydraulic) it extends service life by 5-10×.
On a clean, steady air system — a building instrument-air header at 90 psi, for example — glycerine fill buys you very little. The pointer isn't bouncing, the gear train isn't being shocked, and the case is already sealed. Save the money and use a dry gauge unless the gauge will live outdoors where temperature cycling drives moisture into a dry case.
That's classic hysteresis from a worn or undersized hairspring. The hairspring's job is to load the sector gear against the pinion in one direction so backlash doesn't show up as a dead band. When the spring weakens or the sector gear teeth wear, the pointer responds crisply when pressure pushes the tube outward but lags when the tube relaxes back, because nothing is pulling the gear mesh tight.
You can sometimes tap the gauge case lightly on falling pressure and watch the pointer jump down to the correct reading — that confirms the diagnosis. The repair is a movement replacement, not a recalibration; once the gear teeth wear, no amount of zero adjustment fixes it.
References & Further Reading
- Wikipedia contributors. Pressure measurement. Wikipedia
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