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A Bourdon tube is a curved, hollow elastic element that converts a pressure difference into movement of its free end. A common form has an oval tube cross-section and a roughly C-shaped centreline. Fluid pressure is applied through the fixed end, while the other end is closed and free to move. WIKA describes the cross-section tending toward a more circular shape under internal pressure and the resulting end motion being transmitted to a pointer.^[ref]

In a Bourdon tube pressure gauge, the tube, linkage, gearing and pointer form a mechanical measuring chain. The sensing element undergoes elastic deformation; the movement converts a small displacement into a readable measurement indication. A steady pressure reading does not require continuous flow through the closed tube.

For an original small-range model, let free-end displacement be $x=K{p}_g$, with gauge pressure $p_g$ and stipulated $K=0.004\mathrm{mm}\mathrm{kP}{\mathrm{a}}^{-1}$. Let the linkage produce pointer angle $\theta =Gx$ with $G=500{}^{\circ }\mathrm{m}{\mathrm{m}}^{-1}$. At $p_g=100\mathrm{kPa}$,The combined response is $2{}^{\circ }\mathrm{kP}{\mathrm{a}}^{-1}$. These are illustrative coefficients, not a specification for a WIKA instrument. The actual response depends on tube geometry, material and movement.

The reference pressure belongs in the interpretation. With the outside of the element exposed to local atmosphere, the response measures gauge pressure, rather than absolute pressure. WIKA separately describes sealed reference arrangements for absolute-pressure instruments.^[ref]

For instrument calibration, connect the gauge and a suitable measurement standard to a common pressure source, allow equilibration, and compare several increasing and decreasing pressures within the intended range. Record the relative height of fluid-filled pressure ports because hydrostatic pressure can add a difference between them. Differences between increasing and decreasing readings can reveal hysteresis or backlash; a zero shift after loading can reveal a changed mechanical state. The original linear model alone does not establish the gauge's measurement uncertainty.

A Bourdon tube pressure gauge combines a pressure-sensitive Bourdon tube with a motion-transmission mechanism, pointer and calibrated scale. Pressure-driven elastic deformation moves the free end of the tube; a linkage and geared movement convert that motion into an indication on the dial. WIKA's technical diagram identifies the tube, end piece, link, movement, pointer and pressure connection as distinct parts.^[ref]

The complete gauge is a measuring system. Its measurement indication depends on the sensing tube and the motion transmission, including possible backlash and hysteresis. For a vented case, the tube responds to pressure relative to the surrounding atmosphere, so the intended quantity is gauge pressure. The dial alone does not define an absolute pressure reference.

For an original instrument calibration comparison, connect the gauge and a suitable pressure reference to a common fluid source. Apply several increasing pressure steps, wait for stable readings, and return through the same pressures. Record each discrepancy $e=p_{\mathrm{gauge}}-p_{\mathrm{ref}}$ and distinguish the two loading directions. This tests the assembled readout rather than only the elastic element.

The locations of the pressure ports can matter as much as their shared supply. If the test gauge's port is $0.5\mathrm{m}$ below the reference port in a stationary water-filled connection, take $\rho =1000\mathrm{kg}{\mathrm{m}}^{-3}$ and $g=9.81\mathrm{m}{\mathrm{s}}^{-2}$. Hydrostatic pressure givesWithout that correction, a correct lower gauge can appear to overread the upper reference. These are original apparatus values, not manufacturer test data.

Read the error specification for the actual model and size. WIKA's June2025 US sheet, for example, states an accuracy of $\pm 1.0\%$ of span for the specified four- and six-inch versions, while listing a different specification for the smaller version.^[ref] A percentage of span is not the same percentage of the instantaneous reading. Retain range, conditions, corrections and measurement uncertainty with the reported result.

WIKA is a manufacturer of measuring instruments whose technical publications describe pressure measurements using elastic sensing elements. Its technical information distinguishes pressure-driven deformation from the linkage and pointer mechanism that turns it into a measurement indication.^[ref] Its documentation is useful for connecting a physical measurement principle to the specifications of a complete instrument.

A concrete specification also illustrates the difference between an error limit and measurement uncertainty. WIKA's June2025 US sheet for specified four- and six-inch gauges gives a base limit of $\pm 1.0\%$ of span and an additional temperature error of $\pm 0.4\%$ of span per $10\mathrm{K}$ away from the $20{}^{\circ }\mathrm{C}$ reference.^[ref]

For an original instrument example with span $1\mathrm{MPa}$, the base bound is $10\mathrm{kPa}$. A $20\mathrm{K}$ temperature departure contributes a further bound $8\mathrm{kPa}$. Adding those magnitudes gives a conservative $18\mathrm{kPa}$ combined bound under this simplified interpretation; it is not a statistical root-sum-square uncertainty calculation. At a reading of $100\mathrm{kPa}$, even the base bound is $10\%$ of the reading, despite being $1\%$ of span.

The example concerns the stated version and conditions, not a brand-wide accuracy claim. Use the actual model, range and pressure gauge temperature error specification together with instrument calibration evidence when interpreting a particular Bourdon tube pressure gauge.