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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.

Pressure gauge temperature error is the change in a gauge's indication error associated with a change from its specified temperature conditions, while the pressure comparison is otherwise controlled. It belongs to the complete Bourdon tube pressure gauge response, not merely the number of scale divisions. Distinguish an instrument-temperature effect from a real change in the pressure being measured.

A concrete manufacturer specification is WIKA's June2025 US data sheet, which gives an additional error bound of $\pm 0.4\%$ of span per $10\mathrm{K}$ departure from the $20{}^{\circ }\mathrm{C}$ reference.^[ref] This is a version-specific bound; it is not a measured signed correction for an individual gauge.

For an original gauge with span $P_{\mathrm{span}}=1\mathrm{MPa}$ at $40{}^{\circ }\mathrm{C}$, the specified form givesAt a pressure reading of $100\mathrm{kPa}$, that bound is $8\%$ of the reading. The percentage is referenced to span, so the same absolute bound represents a larger relative contribution near the bottom of the range. Without the sign and an appropriate measurement calibration, subtracting $8\mathrm{kPa}$ would not be justified.

For an original controlled comparison, hold reference pressure at $400\mathrm{kPa}$ and allow the gauge assembly to equilibrate first at $20$ and then at $40{}^{\circ }\mathrm{C}$. Suppose indications are $401$ and $407\mathrm{kPa}$. Their errors relative to the reference are $1$ and $7\mathrm{kPa}$, so the observed temperature-associated change is $6\mathrm{kPa}$, or a local average $0.3\mathrm{kPa}{\mathrm{K}}^{-1}$. These are illustrative readings, not manufacturer test results. Repeat at other pressures and on return to the reference temperature to check reversibility and hysteresis.

Record gauge and reference conditions, equilibration and uncertainty in the pressure comparison. Heating a sealed gas can change its actual pressure through the ideal gas law; an uncontrolled change of that kind cannot be assigned entirely to instrument error. The bound or fitted correction remains one contribution to the final measurement uncertainty.

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.