5 Things You Should Know Before Purchasing Your First Dive Computer

Whether you are new to the undersea world of diving or an old hand who has finally accepted that a dive computer is not just a fad, buying your first dive computer can be a taunting task. Dive computers range in price from a few hundred dollars to a few thousand. The basic function of the dive computer is to give us a real time guidelines to how much longer we can stay at the current depth while staying within the no decompression limit.

The dive computer will also give us information on how to safely recover if we exceeded that limit by providing decompression stop information. Beyond that primary function dive computers have a range of options such as dive planning, multiple gases, exporting data, breathing gas supply and consumption, time to fly calculations and surface intervals.

1. Technical boring stuff

The modern dive computer has its beginnings in the recreational dive tables that most divers were introduced to in their initial training. These tables, which were themselves based on US Navy dive tables, provided a framework to reduce the risk of decompression sickness (DCS) by giving us a maximum amount of time, referred to as No Decompression Limit (NDL), we can stay at a specified depth. Decompression sickness, under various names such as caissons disease or the bends, have been studied since the mid 1800s.

Workers building footers for bridges were frequently afflicted. Caissons were lowered into the water surrounding the work site. The water was pumped out and the caisson was pressurized to keep water from seeping in. Workers would enter into the work space by a airlock and proceed to the bottom by ramps. At the end of their shift they would proceed up the ramp and exit the airlock. Many of the workers would experience the effects of what we now call DCS. It was discovered that workers who rested a part of the way to the surface either did not get the symptoms or if they did they were mild. These were the first concepts that we now use in decompression diving.

Just a fast review of the concepts you learned or will learn in your open water training:

Our bodies has nitrogen in the blood stream, tissues and organs. Under normal atmospheric conditions this nitrogen poses no problems within our body. When we dive to depths, the pressure increases and the nitrogen molecules become smaller. This is expressed as partial pressure commonly shown as pp. The gases will flow from the higher pressure to the lower. The body will absorb this into the bodies cells. However, the rate of that absorption varies greatly between the different types of muscles and other tissues.

Scientist calculate the time for half of the nitrogen to move from the higher pp to the lower, they use the term halftime. Those tissues or body fluid with the lowest halftime will quickly absorb it and pass it on to other parts of the body with a lower pp or out the respirator system if the outside air has a lower pp. The halftime, the pressure difference and the amount of time the body is under pressure will determine how much nitrogen is in any given part of the body. For simplification, body components are looked at as groups with the similar halftimes.

The dive tables’ NDL represents the the time a diver can return from the depth without the need to stop going towards the surface to allow excess nitrogen to leave the body. The diver will have more nitrogen than normal in their body, however the extra should not cause the diver any problems as it works it way out of the body. The dive tables are based on the tissue group that is the most at risk. If diver exceeds the NDL and returns to the surface with nitrogen beyond a safe level, the trapped nitrogen and cause damage to cells and nerve and even block the proper flow of blood. This results in DCS.

The dive tables are static and assumes that the diver will descend to the planned depth directly, stay at the depth for the prescribed time and slowly but directly ascend to the surface. Divers seldom do that and as a result relying on dive tables can significantly reduce the amount of time allowed for the dive. A dive computer on the other hand is adaptive using real time depth data to constantly calculate the amount of nitrogen in the body. They use algorithms that are very complicated to apply the data to a number of different tissue groups and displays the remaining NDL for the controlling tissue group. Mares a leading manufacture has the following half times in minutes as the tissue groups they use: 2.5, 5, 10, 20, 30, 40, 60, 80, 120 and 240.

2. Algorithms are not all equal

While the concepts and science behind the algorithms are the same, there are a number of different algorithms that the manufactures of dive computers use. The result are similar, however, there are some variations. For about 17 years I have been diving with a Genesis Nitrox Resource. To save a number of negative comments, yes I know that the current Genesis Resource does not adjust for Nitrox, however, the early models did. It uses a modified Haldanean algorithm with 12 tissue groups and I have been very happy with it. I decided to replace it only because it did not export data.

I recently acquired a Aqwary Smart Console which uses a reduced gradient bubble model (RGBM) algorithm. On my first dive with the new computer, I also took my old one to compare the readings. When I dived to 90 feet the Aqwary gave me 10 minutes less NDL than my Genesis. When I returned to 60 feet there was still a 4 minute difference. The difference was shocking, so on my next day of diving I added a third dive computer a Suunto Zoop. The Zoop also uses a RGBM Algorithm and is considered a conservative profile. Diving at 80 feet, the Aqwary gave me 8 minutes less NDL than the Genesis and the Zoop 12 minutes less. The Aqwary and the Zoop are two computers among many that also has the ability to adjust the profile to be even more conservative with two additional settings for divers who feel they are at a higher risk than the average diver. When considering dive computer how conservative a dive computer is and whether it can be adjusted should be considered.

3. Multiple Gases

Our Open Water Diver Training has us diving on compressed air that contains 21% oxygen and about 79% nitrogen. Our dive tables base the nitrogen intake on these percentages. In certain circumstances, properly trained divers might use a different concentration of gases. One very common practice is the use of Enrich Air Nitrox (EAN). Simply put, EAN has additional oxygen added to the compressed air so that the nitrogen percentage is lower. The lower nitrogen level means it takes longer for the nitrogen to store in our body. To provide the correct NDL, the computer needs to know if you are using more oxygen. Some computers allow for this adjustment allowing the diver the rely on the adjusted readings. Diving on EAN is becoming very common so it is recommended to insure that your first computer contains this feature even if you are not currently trained.

Technical divers often use a mixture of three gases to overcome problems of high concentrations of oxygen. Their dive computers should be able to be programmed for multiple gases and even different concentrations during a single dive. Very few diver progress to becoming a technical diver and this feature can be very expensive, so it generally not a purchase for a first computer.

4. Breathing Gas Monitoring

One feature that many divers like is the integration of their breathing gas monitoring into their dive computer. Instead of using a Submerged Pressure Gauge (SPG) to track how much breathing gas is in your cylinder, a dive computer with this feature will display the information on your dive computer screen. It will also allow you to analysis your air usage after a dive.

5. Exporting Data

Most dive computers now have the ability to export data to your computer and even to online storage. The question for a purchaser is mostly on the method that it is done. Some computers will require a wired interface, some will use a Bluetooth connection and other a WIFI connection. This is mostly which one you are comfortable with. Also note that some models will sell a connection module separately from the computer, so that needs to be considered when looking at the prices.

This should give you a good overview to get you started as you look at different models. Just remember to consider the future. The dive computer you purchase now should meet what you think you will be diving a few years from now.

Buying Guides

5 Things You Should Know Before Purchasing Your First Dive Computer

1. Technical boring stuff

2. Algorithms are not all equal

3. Multiple Gases

4. Breathing Gas Monitoring

5. Exporting Data

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