by Joe Stellini:
As an SDI Diver you most certainly have had some experience with a dive computer, but what is it trying to tell you and how do you use it to keep you safe on the dive? This article will help explain the most common features of dive computers and how to use them to improve your experience and maximize your bottom time within a safe recreational limit.
I think it is safe to say that we can skip an explanation of what depth, bottom time, temperature and air pressure are trying to tell you. These are standard features and will all remain active when you put a Personal Dive Computer (PDC) into gauge mode since this is the info you would get from an analog system.
Let’s start with one of the most important things your PDC is displaying. Most likely found on the main dive mode screen, you will find No Decompression Limit (NDL). What is No Decompression Limit? It is the amount of bottom time you have left at your current depth based on where you currently are in the dive, where you have been during the dive, and how much time you have spent there before you incur a mandatory decompression stop. It is always displayed in time units and is based primarily on tissue loading of nitrogen.
So how do we use it to enhance our dive? First you should know that every time you change your depth the NDL is recalculated to compensate for the change in pressure. We all know that diving deeper means more nitrogen loading, so that means less NDL or available bottom time before a mandatory deco stop. Using this to your advantage is easy as many PDC’s have audible and visual alarms built in which make it easier to track. Although it can be programmed for a lower number, as a safety margin set your alarm to let you know when you have at least five minutes of NDL time left and when you get there just ascend to a shallower depth. By changing your depth you will be loading less nitrogen allowing for the PDC to compensate with more NDL time. You can do this every time you reach five minutes of NDL time until you must ascend because of gas limitations or time limits set by your dive plan. Do not run your NDL clock down to zero or you will most definitely incur a mandatory decompression stop.
Tissue Loading Bar Graphs or Meters – something paid attention to far less than the actual NDL, but is also an important feature on your dive computer. Many PDC’s have some sort of meter or bar graph that gives you a representation of nitrogen in your system. The more bars, the more nitrogen. For the most part, these graphs go hand in hand with your NDL, however toward the end of the dive while you are ascending, your NDL may rise, but your nitrogen loading bar graph or meter are not quickly reduced. This is because the nitrogen absorbed by your tissues is being released slowly, and in shallower depths we usually have plenty of NDL time to spare.
But what do you use the bar graph for? For one, it is a quick way to look and see how much nitrogen you have loaded and how close to decompression you are. Toward the end of a dive you can use this graph to let you know if you’ve off-gassed a significant amount of nitrogen. So, on the deeper and longer dives you can use this information to determine if you should make a longer safety stop to off-gas. This feature can also be controlled by an alarm if your PDC is capable. Your computer can alert you when you reach a certain level on the graph if you are interested in running a more conservative profile. Unlike the NDL which adds more time just because you ascend a little, nitrogen loading continues to occur as increasing bars on the graph on the PDC. However, as long as you have NDL time to spare, your nitrogen levels and the bar graph on the PDC will keep you out of a mandatory decompression.
Let’s move on to other features like the Nitrox settings on your computer. If you have taken an SDI or TDI Nitrox Course, then you have learned how to set the FO2 to the gas in your tank. As a Nitrox Diver you should know that adding O2 removes nitrogen from your breathing gas, therefore increasing your NDL. You should have also learned that just because you have NDL time it does not mean that you have Oxygen Exposure Time left. Your FO2 setting will run in the background and calculate NDL’s accordingly, but there is another feature called CNS that will track your exposure to oxygen. PDC’s will commonly display your O2 exposure in minutes left before you have hit the maximum and risk oxygen toxicity, but many display this reading as a percentage of exposure used up. Because of limited space on a recreational dive computer, this feature can usually be found on the secondary dive mode screen during the dive.
Some dive computers will have a feature that will indicate Air Time Remaining (ATR). This requires that the PDC be air integrated so it can track gas usage. This calculation is similar to NDL, but uses the current Surface Air Consumption Rate (SAC) to calculate ATR taking into account your depth and breathable gas left in the tank. Most air integrated computers will have an end PSI setting which will subtract this value from the available gas when calculating Air Time Remaining; another added safety feature. You should be aware that computers that have this feature might replace your NDL time with ATR time if it is less, showing you the more critical of the two. Be sure to educate yourself on how your computer displays this information.
When we were first certified as divers, one of the things we practiced the most was the ascent. Even though this usually means our dive is coming to an end, controlling your buoyancy during an ascent is one of the most important safety skills you will learn. You guessed it; our dive computers will help us out with that too. In the basic scuba course we all learned that the ascent rate should not be faster than 30 feet per minute. By ascending faster than that we risk several maladies like DCS, lung overexpansion, or barotrauma. So using the ascent rate monitor makes your ascent much easier by showing you another bar graph or meter usually on the opposite side of the Nitrogen Loading Bar Graph. The faster you come up, the more your meter will indicate. Once you have reached the top or end of that meter, you are coming up too fast. Always remember that just because your computer is showing a safe ascent rate, it does not mean that a slower ascent may be necessary for various other reasons like sinus squeeze or reverse block.
Keep in mind that all dive computers have differences in features and function and this article is not a substitute for training on the use of your PDC. It is written only to give you a better understanding of what your computer is trying to tell you and how it can provide information to allow a much more controlled and safer dive. ***