Bourdon tube pressure gauge

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.

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.