Pressure measurement elements – Bourdon tube geometries Bourdon spring tubes are manufactured in several curved forms (C-shaped, helical, spiral). Which of the following configurations is never used as a practical Bourdon tube shape?

Introduction / Context:
Bourdon tubes are classic elastic elements that convert pressure into tip motion via cross-section ovalisation and curvature change. Their mechanical geometry strongly affects sensitivity, linearity, and manufacturability. Knowing which geometries are used (and which are not) is a standard instrumentation topic.

Given Data / Assumptions:

• Typical shapes: C-shaped (semi-circular), helical, and spiral tubes are well-known.
• Material is an elastic metal with an initially curved form.
• We identify a configuration that is essentially never employed.

Concept / Approach:
All working Bourdon tubes require an open-ended arc so that pressure-induced ovalisation causes a measurable free-end displacement. A completely circular, closed-ring tube would constrain deformation symmetrically and prevent the free-tip motion required for pointer amplification, offering no practical displacement for measurement. Hence “circular (closed ring)” is not used, whereas semi-circular (C-shaped), helical, and spiral forms are standard.

Step-by-Step Solution:

Verification / Alternative check:
Instrumentation handbooks illustrate C-type for low/medium pressures and helical/spiral for higher ranges, with no circular-ring designs.

Why Other Options Are Wrong:

• Semi-circular: The archetypal Bourdon tube.
• Helical / Spiral: Used to increase sensitivity or pressure range.

Common Pitfalls:
Assuming any curved tube qualifies; without a free end, pointer motion cannot be generated effectively.

Final Answer:
Circular (closed ring)