Bourdon Pressure Gauge Mechanism Explained: How It Works, Parts, Diagram and Uses

From tube deflection to a pointer reading

Pressure enters the fixed socket and acts inside a curved tube with a flattened cross-section. Elastic deformation moves its sealed free end. A link carries that small movement to a sector gear; the sector turns the smaller pointer pinion. The parts remain connected through the illustrated motion.

The cutaway follows the arrangement shown in WIKA’s pressure-measurement handbook, Figure 1.70. The adjacent front dial displays the same shaft angle. The brass color and magnified tube movement aid recognition; they are not specifications for a particular commercial gauge.

What this calculator compares

Use the inputs to compare pressure as a fraction of an entered gauge span, an assumed full-scale tip travel, and the sector motion required for a chosen gear ratio. It is an educational calibration model. It does not select wetted materials, assess temperature or pulsation effects, or establish a pressure-containing component rating.

Linear calibration equations

Let q=P/Pfs, where P is applied pressure and Pfs is the gauge span. For a specified full-scale tip travel xfs, the linear tip estimate is x=xfs q. A 270-degree full-scale dial gives pointer sweep θp=270q degrees. A sector/pinion angular ratio G then requires sector sweep θs=θp/G.

The pointer sweep is a prescribed calibration target. Increasing G reduces the sector angle required to achieve it. Predicting a real gauge after changing its gearing would also require tube compliance and linkage geometry; this calculator does not claim to solve that problem. Nor does it estimate tube stress or derive compliance from material properties.

Worked calibration example

At 50 psi on a 100 psi span, q=0.5. With assumed full-scale tip travel 8 mm and a 10:1 sector/pinion ratio, the estimated tip travel is 4 mm, pointer sweep 135 degrees, sector sweep 13.5 degrees and span used 50%.

Changing the ratio to 12:1 makes the required sector sweep 11.25 degrees for the same calibrated pointer angle. Changing full-scale tip travel to 3 mm gives 1.5 mm at half span. These adjustments specify different calibration assumptions; they are not a recipe for modifying a pressure gauge.

Calibration versus construction

The linear equations are useful for comparing scale fractions. A real movement must also account for its actual link geometry, backlash, elastic response, temperature and calibration procedure. WIKA’s handbook describes the tube, movement, connection and dial as distinct elements of the instrument.

The illustration uses a constant-length link and meshing pitch radii, with magnified tube movement. Its intermediate tube deflection is schematic. Exact elastic behavior and gauge accuracy are not inferred from the drawing.

Questions about the gauge model

Why does gear ratio change sector angle but not pointer sweep?

The calculator starts from a specified 270-degree full-scale calibration. The ratio determines the sector motion needed to meet that target. It is not simulating a gear replacement in an otherwise fixed gauge.

What happens above the gauge span?

The linear calculation reports more than 100% span when the entered pressure exceeds the gauge span. The illustration stops at full scale and identifies the extrapolation. Neither the numbers nor the capped drawing establish overload capability.

Does the full-scale tip-travel input calculate the tube’s stiffness?

No. It is an entered calibration assumption. Tube stiffness would require geometry, material and an appropriate elastic model or measurement.

Why does the pressure rise and fall while the result cards stay fixed?

Playback illustrates a cycle from zero to the selected pressure, capped at full scale. The result cards always describe the selected maximum pressure. Pause and scrub to inspect the connected parts.