Submersible Pressure Gauge The submersible pressure gauge (SPG) is a required piece of equipment for scuba diving. The SPG displays the amount of air pressure remaining in your scuba cylinder in the same way a fuel gauge shows how much gas you have left in your car’s gas tank. By looking at your SPG frequently during your dive, you will know when your air supply is getting low and it is time to end your dive before you run out of air. All mechanical SPGs perform the function of measuring the air pressure in your scuba cylinder (figure 2-25). SPGs give a reading in bar or pounds per square inch (psi). For a typical aluminum cylinder, the SPG reads 200 bar (3000 psi) when the cylinder is full, 100 bar (1500 psi) when the pressure is reduced to half, and so on. SPGs are sensitive instruments and you should not subject them to shocks or abuse. If you notice a small leak from the hose or connection between the hose and SPG, it is not necessary to end your dive immediately. However, you should have the SPG and hose serviced as soon as possible to your NAUI Pro Center or dive-equipment repair facility. If you see water in your SPG, have it serviced as soon as possible and do not rely on it for diving until you do. SPGs can also be integrated in dive computers and measure pressure electronically. In addition, some air-integrated dive computers can also monitor your breathing rate and predict how long the air in your cylinder will last based on your breathing rate (figure 2-27). Depth Gauge Your depth and the duration of your dive at any particular depth are limited by a number of factors, so you need to monitor your depth when diving. A depth gauge gives you a way to measure your depth when you are under water (figure 2-28). There are four types of depth gauges: • Capillary tube • Bourdon tube • Diaphragm mechanism • Electronic gauge The capillary tube is a simple, inexpensive instrument that consists of a clear piece of tubing sealed at NAUI Scuba Diver 28 Diving Equipment FIGURE 2-25. THE SUBMERSIBLE PRESSURE GAUGE IS A REQUIRED PIECE OF SCUBA EQUIPMENT. FIGURE 2-24. YOUR ALTERNATE AIR SOURCE MOUTHPIECE SHOULD BE LOCATED ON THE FRONT OF YOUR BODY AND BE CLEARLY VISIBLE.
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