NAUI Master Scuba Diver 16 Diving Equipment Several gauges are required for diving, such as a submersible pressure gauge, a depth gauge, and a timing device (figure 1-13). This section describes the function and care of these gauges. Analog Versus Digital An analog instrument has one or more hands that move on a dial, while digital instruments provide a numeric information display. Usually, mechanical instruments give analog data and electronic instruments give digital information. Analog gauges are reasonably accurate and reliable. Digital gauges are very accurate but require batteries and are more subject to failure than mechanical instruments. Cylinder Pressure Gauges One of the most common types of pressure gauges is known as a Bourdon movement gauge. A Bourdon movement is a flattened helical tube that is sealed at one end. The tube tends to uncurl when pressurized internally, due to the pressure difference between the inner arc of the tube and the larger outer arc. The free end of the tube is attached to a linkage that moves an indicator needle. If both ends of a Bourdon tube are sealed, it may be used to measure external pressure, which tends to curl the tube inward when a pressure greater than the internal pressure is surrounding the tube. There are two types of pressure gauges for determining the amount of air in a scuba cylinder. A surface cylinder pressure gauge is used to check the amount of air in a cylinder above water. It attaches to the cylinder valve in the same manner as a regulator to provide a one-time check of cylinder pressure. A bleed valve relieves trapped pressure so the gauge can be removed when the cylinder valve has been closed. A submersible pressure gauge (SPG) is typically attached to the first stage of a regulator with a high pressure hose (figure 1-12). Some units use a console to incorporate other gauges along with the SPG. The SPG provides a continual readout of the air pressure in your cylinder. It contains a spiral form of Bourdon movement, FIGURE 1-12. BOURDON TUBE SUBMERSIBLE PRESSURE GAUGE consisting of numerous coils with the movable end connected directly to the indicator needle. As pressure uncoils the spiral tubing, the needle moves. SPG gauge movements are usually designed with an accuracy range of about ±2 to 7 bar (35 to 100 psi) at a reading of approximately 35 bar (500 psi). At full scale the accuracy is about ±5%. The SPG is attached to the high pressure port of your regulator. The high pressure regulator port is larger than the low pressure ports on modern regulators. This prevents attachments with low pressure hoses from being installed in the high pressure port, creating a dangerous situation in which the hose can rupture. Ports are generally labeled as HP (high pressure) and LP (low pressure). Older regulators still in use have ports of equal size for both high and low pressure, so care is required when making first stage attachments. Adapters are available for connecting old-style attachments to newer regulators and vice-versa. See your NAUI Dive Center if you have questions or need advice. Regulator high pressure ports have a restricted orifice (opening) to limit the flow of air in case of a rupture of the SPG or its hose. The tiny hole (0.127 mm 0.005 in in diameter) prevents rapid depletion of the air supply. A high pressure leak does not constitute an emergency situation. Should one occur, simply make a normal ascent and close the cylinder valve once at the surface. Some pressure gauges are “hose-less.” They receive a signal that is transmitted from a unit mounted on the
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