Which pressure-sensing instrument uses a curved metal tube that straightens as pressure increases and drives a dial?

• A. Bimetal Tube
• B. Flat-Tube Gage
• C. Whiskey Tube
• D. Bourdon Tube

Answer: (D)

Pressure gauges often use a Bourdon tube to convert pressure into motion. The curved metal tube is designed so that as internal pressure rises, the tube tends to straighten. That slight straightening moves the tube’s end, which is attached to a linkage and gearing that translate that small displacement into rotation of a dial pointer. The geometry—a hollow, curved tube with an oval cross-section—makes the tube want to straighten under pressure, and that mechanical movement is what the gauge reads as a pressure value. Other options describe devices that respond to temperature or use less effective configurations for measuring pressure, but only the Bourdon tube’s curved shape produces that straightening action that drives the dial.