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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.

A measuring transducer is a device whose output quantity has a specified relation to its input quantity and is used in measurement. VIM examples include a thermocouple, a strain gauge and a Bourdon tube.^[ref] A measuring transducer provides a link in a measuring chain; it need not itself display a finished measurement result.

For an original pressure readout, choose a linear modelwith pressure $p$, output electric potential difference $V$, transducer sensitivity $S=2\mathrm{mV}\mathrm{kP}{\mathrm{a}}^{-1}$, and offset $V_0=1\mathrm{mV}$. An output $V=201\mathrm{mV}$ then corresponds to $p=100\mathrm{kPa}$. An amplifier, analog-to-digital converter and display can follow the transducer, each contributing to the whole measuring system.

The inverse calculation $p=(V-V_0)/S$ exposes distinct uncertainty contributions. Holding the other inputs fixed, a small output change $\delta V$ changes inferred pressure by $\delta V/S$; a small offset error changes it by $-\delta V_0/S$. An output increment $0.2\mathrm{mV}$ corresponds to $0.1\mathrm{kPa}$ for the stipulated sensitivity, but does not establish a total measurement uncertainty of that size.

Characterize the relevant range, temperature conditions, response time and possible instrumental drift during measurement calibration. The relation can be nonlinear, in which case local transducer sensitivity is the appropriate derivative rather than one gain valid everywhere. The definition also allows input and output quantities of the same kind; it does not require changing from a mechanical quantity to an electrical one. A sensor denotes the part directly affected by the phenomenon being sensed, whereas a complete measuring chain can contain further measuring transducers.