What makes a needle in a gauge move?

• A. Capillary tube
• B. Bourdon Tube
• C. Diaphragm
• D. Capsule

Answer: (B)

Measuring gauges rely on a sensing element that deflects under pressure to move the needle. In a Bourdon tube gauge, the sensing element is a hollow curved tube. When pressure inside increases, the tube tends to straighten, and that mechanical change is transferred through a linkage to rotate the gear train that moves the needle. The geometry of the curved tube converts pressure into a measurable displacement, which is why this element is the classic mover in pressure gauges.

The other options aren’t the primary motion source in the typical gauge. A capillary tube is just a very thin conduit that transmits pressure to the sensing element rather than providing the direct movement of the pointer. A diaphragm gauge uses a flexible membrane that deflects under pressure and then the deflection is translated to motion, while a capsule (two diaphragms) also relies on membrane deformation to drive the indicator. However, the direct conversion of pressure into needle movement in many standard gauges is achieved by the Bourdon tube.