A New Divers Guide To Dive Computers
Dive tables are used to determine how long you can safely stay underwater at a maximum depth. They provide you with information such as nitrogen saturation levels and surface time for both your initial and subsequent dives. Before the development of dive tables, it was not a question of “if” you would get decompression sickness (DCS), it was when and how bad. Dive tables and the research behind them lead to a better understanding of DCS and a means to control the risk.
When using a dive table to plan your first dive of the day, you would find the line that shows your planned depth. Then you will follow it over to find the maximum time you can stay down. Planning a second dive of the day required more steps. You use the time and depth of the first dive to find your pressure group. Using that pressure group you look for the amount of time of your surface interval to find your new pressure group. You then use that group information on the chart and the depth you plan on diving at to find your maximum time for the next dive.
Many people find dive tables confusing, mainly due to the type of assumed profile. Tables calculate based on a maximum depth and then heading to the surface which is not how we really dive. We might dive to 90 feet (27 meters) to a wreck and then come up to 70 feet (21 meters) as we start exploring the deck. A dive plan based on a dive table only sees the dive as 90 feet.
Dive tables can be complicated and intimidating for new divers, which is why most agencies now teach the use of Dive Computers as an alternative. A dive computer is constantly giving us remaining bottom time based on the current depth and adjusted for the time already underwater.
A dive computer will make it easier to develop your dive plan and once underwater will make it easier to follow your plan and allow for an overall better experience. If you vary from your plan, the computer will keep you updated with accurate information.
Basic Decompression Theory
When considering purchasing a dive computer, it may be helpful to have a little more information about basic decompression theory. The science behind dive tables is very complex and involves a number of different scientific fields. Often in our training, the knowledge of decompression is reduced to if you stay down longer than your no-decompression limit or come up too fast you can get the bends.
We are told that there are two types of dives: No decompression dives and decompression dives. A no-decompression is categorized as a dive where we can head directly to the surface without the need to decompress. Still, we should do a safety stop. When we dive past a No Decompression Limit (NDL), we are required to pause our ascent to allow additional time to off-gas built-up nitrogen. This is a decompression stop. We are also taught that decompression dives are beyond the knowledge of recreational divers and are reserved for technical divers.
This is a concept you should remember. However, from a scientific standpoint, every dive is a decompression dive. When we are at the surface and breathing that fresh ocean air, our bloodstream has a certain amount of nitrogen dissolved in it. This is our natural state, and the pressure of the gas in our system and that of the outside air is balanced. Also, our tissues will have a balanced amount of soluble nitrogen. When we dive, the air in our lungs has increased pressure. This in turn increases the partial pressure of the gases. The increased pressure allows the blood to accept more nitrogen into the solution until it is equal. As the blood circulates, tissue matter absorbs some additional nitrogen. When we stay at a stable depth long enough, the partial pressure in the lungs and blood will stabilize.
When we start to ascend, the pressure in the lungs becomes less and the process is reversed. Our body starts to release the gas. This is where we could possibly have problems. If the process is rapid, like shooting to the surface, the gas in the blood may come out of the solution before the lungs are able to remove it from the body. These gas bubbles can clog capillaries and cause pain or even block blood flow. This is the reason divers are taught to limit the rate of ascent to 30 feet (~9 meters) per minute.
Another factor that has to be accounted for is the release of nitrogen from tissue. The tissue in our bodies will absorb nitrogen from the bloodstream and it will release nitrogen back to the bloodstream when decompression has started. However, the rate that which this is done is not constant across different types of tissue. Some types of tissues will absorb and release the gas rapidly, while others could take hours before they are saturated. If the rate the tissue gives up its nitrogen is balanced with what the lungs can get rid of, we can slowly proceed to the surface. If there is an imbalance, we need to pause to allow the nitrogen to be released from the tissue. A decompression stop.
Dive Computer Algorithms
Recreational dive tables are similar as they were all developed with similar research. Algorithms were created from the research to predict acceptable dive parameters. The algorithms are mathematical formulas that include predicted rates of absorption of tissue types. Typical dive tables used 6 to 9 different tissue rates to create standards for different depths and times. Dive computers also use algorithms. They generally use between 9 and 15 tissue rates, with some using 20 or more.
The dive computer uses pressure and time as inputs for the algorithms. The results are shown as the NDL or if necessary, decompression stops.
You will hear experienced divers talking about how conservative or liberal their dive computers are. This is not a political statement, but one that concerns the margins of safety or sensitivity built into the algorithms. A conservative dive computer will give you less time for your NDL, and a greater margin of safety. Also, some algorithms are more appropriate for different types of diving, such as technical diving or closed circuit diving. There are several algorithms in use, and you will find that they are all similar.
More advanced dive computers may allow you to select levels of sensitivity, and some allow you to adjust a number of settings to personalize the risk factors to your desired settings. Oceanic’s dive computers go a step further by allowing divers to select from two different algorithms; DSAT and Z+. Currently, they are the only brand that offers this safety feature.
Should I Own a Dive Computer?
When you are just starting to scuba dive, there is often a desire to purchase your own equipment. Most divers will have a mask, fins, and snorkel as their first purchase, often while still training. This trio of equipment also allows you to go snorkeling. While some divers would rather rent equipment, other divers will move on to purchasing other items of gear and you may find yourself asking, do I really need to own a dive computer.
Once you start diving below 60 feet (ca. 18 m) or diving more than two dives in a day, the function of a dive computer becomes more important. Yes, you can safely dive deeper with just dive tables, divers did it for decades before dive computers were developed. However, as mentioned above, your dive is not as flexible, you need to follow your dive plan exactly and your dive will be shorter. Dive computers help calculate your dive times more precisely and can allow for a longer more enjoyable dive.
This is especially useful for repetitive dives or multiple days of diving. Using the same dive computer each time allows you a history of your dive profiles and nitrogen loading to ensure a safe dive plan.
Dive computers allow you to create digital logs and in the case of Oceanic computers, they are Bluetooth compatible with the Diverlog+ app which means you can download your detailed dive information wirelessly to your smart device. Also, dive computers that are air integrated, provide your air consumption rates and keep your air pressure on a single display.
A dive computer of your own will rapidly pay for itself in planning, awareness, and enjoyment.
What to look for in a dive computer
When you are looking at purchasing a dive computer there are a few items that will guide your choices. Your budget will often narrow the choices you will have. A dive computer will last for years, so keep that in mind as you set your budget.
I feel that the first parameter you need to set is what type of diving you plan on doing in the next 5 years. I would recommend that you do get a dive computer that allows the use of Enriched Air Nitrox (EAN). There are many benefits of diving with EAN, however, your dive computer will need to be set for the mix you are using. Most dive computers on the market today are able to accept EAN diving.
There are two styles of dive computers, console, and wrist mount. The console-style replaces the depth gauge in the console that attaches to your first stage. Some console dive computers are air integrated, these computers will also replace the pressure gauge with a hose connection to the first stage. Many divers prefer the console-style as both the computer and cylinder pressure can be seen together. The wrist mount is worn on the wrist. Smaller wrist-worn dive computers are also called watch style. Air integrated wrist-style dive computers require a wireless transmitter to be attached to the first stage. These transmitters are normally sold separately. It is also recommended to dive with a backup computer in case your primary fails, so it is not uncommon to see a diver with both a wrist and console computer being used at the same time.
Should I get a Wrist or Console Computer?
Ask the question “Should I get a Wrist or Console Computer”, to a group of divers and you will find those advocating each. Each style may have some advantages over the other, you really can not say that there is any disadvantage in selecting one over the other. Here are some general comments on the two styles.
Many divers find that a wrist computer is more convenient to use. Most of the time when we are diving our hands are out in front of us and it does not take much effort to look at our wrists. The same can be said while we are doing a safety stop. If you are going to be using rental equipment, then a wrist computer has a definite plus. A console-style requires a hose connection to the first stage. Few dive shops will look favorably on you disconnecting their pressure gauge so that you can hook up your dive computer. A wrist dive computer, such as the Geo 4.0, can be worn as a watch as well.
Some divers find the smaller size of a wrist computer makes it a little more difficult to read. Also, the controls may be harder to use wearing gloves. Air integration is available on some wrist-style models, like the Oceanic OCI, with an optional wireless transmitter.
When we are discussing a console-style dive computer, its larger size is often the first point mentioned. This makes the display easier to read and the controls easier to use. Most console dive computers are air integrated being connected by a hose to the first stage. This eliminates the need for a pressure gauge. These computers will be showing us our remaining air. In our post dive review, they may also give us information on our air consumption rates. Many divers appreciate being able to look at the remaining air at the same time they are checking the dive time, NDL and depth information. The Proplus 4.0 console provides all the information a diver needs.
You will also find another type of dive computer often called a Hockey Puck. Hockey Puck dive computers, such as the Veo 4.0, are not air integrated and can be worn as a wrist computer or placed in a console. The computer is the same just used in different amounts. A wrist mount will have the module inserted into a wrist-worn boot. A two-gauge console will have the computer and a pressure gauge. The three gauge system adds a compass. They are larger than a wrist computer, however, they have a smaller display than a console. They are a good choice for a new diver who will be using their own equipment sometimes and using rental equipment at other times.
Dive Computer Displays
The display is one of the most important aspects of the dive computer for you to consider. The display can be dot matrix, segmented, or color. The choice is one of personal preference and budget and Oceanic has options for each. The most important part is that it is comfortable for you to read and operate. Before you purchase your own dive computer, try a few different screen types to see which is the most comfortable for you. The layout, display type, and size are all important aspects that need to fit you.
Generally, you will find these standard display features:
- No Decompression Limit (NDL) time left. This is likely the most commonly referred to a portion of the screen.
- Current Depth.
- Maximum Depth is often a small number off to the side of the display.
- Dive Time since the start of the dive.
- Ascent Rate is generally shown as bars. A peep will often sound if you ascend too fast.
- Battery Warning. This is usually segments and will flash when your battery gets low. Most computers will not let you start a dive with a low battery.
- Water Temperature.
- Tissue Loading Bar Graph. To me, this is one of the most important features of a dive computer which Oceanic innovated years ago. These bars are a graphic representation of the nitrogen in your body that the dive computer is making its presentations on. They are color-coded green, yellow, and red. A red bar will cause your computer to present you a decompression stop requirement. If your computer is set for EAN, there will also be bars for oxygen saturation. A slightly slower ascent or a few extra minutes at a safety stop may be required to bring a yellow reading down to green.
Earlier, the concept of algorithms was discussed. It is generally not something that most divers consider when purchasing a dive computer. Still, it should be something to consider. Since you will likely be using the same computer for years, I suggest you find one that allows you to change the settings. Oceanic’s Dual Algorithm is found on all Oceanic computers and will be able to meet your needs as your diving changes.
New Divers Are Welcomed
If you are a new diver or considering becoming one, welcome to a wondrous adventure. Scuba diving will cause you to look at the world differently. When you are purchasing a dive computer or any other major item of scuba equipment, you are really making an investment for the rest of your life. Soon you will be telling friends and family about your great experiences.