PADI Divemaster Certification 2025 – 400 Free Practice Questions to Pass the Exam

The oil-filled Boudoir tube operates on the principle of pressure causing a coiled tube to coil tighter. In this type of depth gauge, as a diver descends and the surrounding water pressure increases, the pressure acts on the oil within the Bourdon tube. This pressure causes the coiled tube to straighten or expand, which is then connected to a needle that indicates the depth on a dial. This relationship between pressure and the coiling of the tube allows for an accurate reading of depth based on the environmental pressure experienced underwater.

While digital depth gauges utilize electronic components and sensors to measure depth, they do not rely on the mechanical properties of a coiled tube. Mechanical depth gauges also use moving parts, but they may not specifically use the coiled principle unless they are specifying Bourdon tubes. Anechoic depth gauges are not standard instruments used in scuba diving contexts and generally refer to sound measurement systems, which do not apply to depth measurement in diving. Thus, the oil-filled Bourdon tube is the correct answer as it directly employs the mechanics of pressure on a coiled structure to function.