Looking for some bragging rights? Or just to get an official “badass” award? Diving the Doria can give you both. This wreck is impressive and it’s definitely NOT for newbie divers. Some might wonder if it’s worth to dive considering the number of casualties it’s taken over the years.
Do you know why it’s important to follow the rules? Or stick to your dive plan? These are both things that not only affect you but the people around you diving as well as the boat captain and the mates, too.
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. ***
by Joe Stellini:
You just finished your Open Water Scuba Diver Course and your head is spinning with all the knowledge and skills you have learned. At the top of your list is purchasing what your instructor may have said was the most important piece of dive gear you could own – a dive computer. Your question is, “Why? What is so important about a personal dive computer (PDC) that I should own one?” Most likely that question was answered for you, but here is a little reinforcement to what your instructor may have told you.
First, not everyone wants a fancy, all the bells and whistles PDC, and there are a lot of options out there. Sometimes simplicity means more enjoyment on your dive instead of trying to figure out exactly what you are supposed to be paying attention to on the screen. So getting down to the basics means that there are three things you absolutely need to know during your dive and how to access them on your PDC. They are: Where are you now? How long have you been there? How much longer can you safely stay? This translates into depth, elapsed dive time (EDT), and no decompression limit (NDL). All dive computers have these features, but it’s ease of use and readability that are most important. Everything else is just extra.
To break it down even further, here is why these things are important. Depth obviously comes first because when we plan a dive, depth is one of the first things we set a limit on. Diving within the agreed upon depth limit, whether it be with the Divemaster, your dive buddy, or with yourself on a solo dive, will keep things organized. Not sticking to your planned depth can be confusing and dangerous to all involved. The easiest way to monitor your depth is to learn how to process that information with a quick glance at your computer, often, throughout your dive. If the PDC happens to have an alarm to remind you, even better.
Second on the list, is elapsed dive time. You ask, “Won’t the Divemaster be leading us in and out of the water?” The answer is, “In a perfect world, yes. However in the slightly imperfect world we live in, that does not always work out.” What happens if the group doesn’t want to see what you and your buddy want to see? Or what if you get separated? Or even more common, what if you become too experienced to hang out with a bunch of newbies? You will have to monitor your own time during the dives. Again, an audible alarm for this feature helps. Most dive operations set a maximum dive time and part of being a good diver is following the dive plan whether you or the dive operation set it.
Finally, we have our no decompression limit; last, but far from least. Some computers have audible alarms for this feature as well. When it comes down to it, not following a good dive plan with regard to our two previous features, depth and EDT, could result in decompression illness. Going too deep, coming up too fast, and staying too long, will eventually and most certainly catch up with you. Yes, DCS has been drilled into your head during your open water class and will be addressed even further during any advanced or continued education courses you may take.
So why is NDL important? Because it takes your depths and times during each dive or repetitive dives and calculates how much longer you can safely stay at your current depth. Breaking these rules could cause the loading of too much nitrogen resulting in a mandatory decompression stop. As a new diver, we want to avoid a deco stop at all costs. Your PDC can tell you when to move to a shallower depth, will recalculate your NDL for the new depth, and will do this every single time. Not only does this keep you safely within your nitrogen limit, but it will significantly extend your dive times allowing you to multi-level dive. You can’t get that with dive tables.
The personal dive computer you used in your SDI course was probably attached to the regulator system. If you are not interested in the whole package then consider a wrist-mounted computer. This makes traveling with a PDC lightweight and easy.
On a final note, always remember to monitor your air. Although some PDC’s may be air integrated, divers that do not use one will have to check their pressure gauge every few minutes.
For more answers on personal dive computers please consult with your local SDI Dive Center. They are there to help and provide you with the best customer service possible and should be able to answer all of your questions on personal dive computers.