NAUI Scuba Diver 144 Decompression, Dive Tables, and Dive Computers you spend part of your dive in water shallower than your maximum depth, a dive computer considers this. The computer calculates only the amount of nitrogen that its mathematical model predicts you ingas or offgas at each depth. A dive computer allows a dive with time spent at shallower depths (called a multilevel dive profile) to have longer dive time limits. Also, there is a lesser penalty for residual nitrogen than when you use the fixed-calculation dive tables. A dive computer is an electronic device that can fail without warning. Most dive computers are highly reliable, but you must always consider this possibility. When a dive computer fails, your only option is to surface at the rate of ascent specified by the manufacturer and make a 3- to 5-minute precautionary decompression stop at 5 meters (15 feet). Then you must stop diving for the period recommended by the manufacturer. You must do this because there is usually no way to recalculate your previous dive or dives using tables. Also, you cannot start with a fresh computer when you have residual nitrogen in your system. There are dive tables available for computing multilevel dives. The manual planning and execution of these dives is complex and not recommended for recreational diving. If you want to receive credit for reduced nitrogen absorption during multilevel dives, use a dive computer. Dive Computer Terms You must be familiar with the terms associated with dive computers. A ceiling is the shallowest depth to which you can ascend without risk of forming bubbles in your body (figure 5-23). You must not pass or violate this ceiling. If your computer displays a ceiling, you have entered a decompression situation. To help avoid suffering DCS, you should use your computer to prevent a ceiling from being established. Scrolling occurs when your computer continuously flashes the Maximum Dive Times in sequence for progressively deeper depths. This feature is an aid for dive planning and for chamber treatment, should you develop DCS. Scrolling can also show your current dive profile history. FIGURE 5-23. THE CEILING DISPLAY INDICATES THAT YOU MUST Displays MAKE A DECOMPRESSION STOP. Although many different dive computers are available, many share the same functions. The layouts of the display screens differ among computers, but they all display much of the same information. Many dive computers will also interface with a personal computer so that you can download and visually display, or even log, your dives on your computer. This section describes a typical dive computer and explains the most common screens and functions. Your computer might be different and not function in the same way. It might also use the same terms with different meanings. You must read and understand the manual that is supplied with your computer. Initialization When a dive computer is first turned on, it must initialize itself. This is a process where the operating program is checked and loaded into the computer’s memory. When this happens, the computer typically runs through a self-diagnostic test and checks all of its functions. The computer display fills during this sequence to ensure that the entire display is working (figure 5-24). Also, any alarms that the computer has will display or sound. You can turn on all computers manually to check their operation. Some computers must be turned on with a switch before you enter the water. Other computers turn on automatically when you enter the water. You must initialize your computer to check its oper-
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