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Gear Meister answers your gear questions about DIVE COMPUTERS...

Dive Computers Maximize your Dive Opportunities

Don't get to dive as often as you’d like?. And when you do “get wet”, never enough bottom time? The first problem is a function of your work and school schedule - we can’t help you with that. But more bottom time? Hey, it’s all about nitrogen absorption and safety. We’ve got some ideas about that and dive computers are a BIG part of the solution.

Dive Table Theory

Remember your initial certification class, where you learned to use “dive tables?”  The tables are a tool to plan and manage our nitrogen loading and dive safety. The rate and amount of nitrogen loading is a function of depth and the time at a given depth.  The greater your depth, the faster the loading. And the shorter your allowable bottom time. Otherwise, you’re absorbing more nitrogen than you can safely off-gas on re-surfacing. Dive tables use an algorithm (a mathematic model) of nitrogen loading to come up with the depth and time profiles that keep us safely within the no-decompression dive limits.

Table rules (and why they’re a problem)  

Rule 1: Calculate using the deepest depth attained during the dive.

Problem: We don’t dive in a flat line along the bottom for the entire dive, so calculating this way means that we lose bottom time.

Rule 2: Always round up depth and time to the next highest value when your exact profile is not listed in the table.

Problem: There’s a big difference between each 10 foot (3m) increment of depth in the dive tables. As an example, rounding 51 feet (17m) to 60 feet (20m), costs us bottom time.

Rule 3: Calculate carefully.

Problem: Dive tables can be a little complicated, especially if you don’t dive frequently (of course  you remember how to calculate a repetitive dive) and we all make math errors from time to time. These procedural and math errors can cause a diver to inadvertently exceed their no-decompression limit.

Dive Computer Theory

Dive computers aren’t magic. Actually, they function much like dive tables and use the same types of nitrogen loading algorithms. But wearing a dive computer gives the diver a couple of BIG advantages. Since it’s attached to the individual diver, the dive computer can very closely monitor the actual depth of that diver at any given moment. Plus, a computer’s super speedy microprocessor can do repeated calculations of the diver’s actual depth and elapsed dive time to create a safe dive profile “on the fly.”

How do these advantages give us more bottom time? Let's count the ways!

- Using it’s built-in depth gauge, the dive computer can calculate your profile based on your actual depth and time at that depth. No more rounding everything up. Voila! More bottom time.

- A dive computer calculates nitrogen loading at the actual depth of the diver throughout the dive, no matter how often the diver changes depth or time spent at that depth. In essence, the dive computer calculates a repetitive dive at intervals of every one to 3o seconds (depending on the computer). No penalty by having to calculate based on your maximum depth. And - you guessed it - more bottom time for you.

- Computers don’t usually make mistakes, so their calculations will be more error-free than our manual calculations.

Basic Operation and Function

Although there are many makes and models of dive computers, most modern computers have common basic features and functions, or “modes”.

Pre-Dive Mode (also called the Planning Mode), allows the diver to scroll through depths in pre-set increments (usually 10 feet or 3m) and see the allowable no-decompression time at that depth if the diver jumped in the water right then. These adjusted limits are continuously updated as the computer tracks the diver’s surface interval.

Dive Mode begins operation as the diver descends below a pre-set depth (usually around 3 feet or 1m) and continuously tracks depth and time throughout the dive. This information is displayed on the default information screen and includes current depth, current dive time, and remaining no-decompression time at that depth. The Dive Mode  usually includes an “ascent rate monitor” that provides a graphic and/or numerical display of your ascent rate to help you ascend at a safe rate. As you ascend, most basic dive computers have an “automatic safety stop” feature that goes into effect from 20 feet to 10 feet (7m to 3m) and counts down your safety stop for you. One downside of the automatic safety stop feature is that, if you skip the safety stop, most computers will slightly penalize you on the subsequent repetitive dive.

The computer enters Surface Mode upon surfacing. It  begins tracking surface interval and makes calculations for the Planning Mode discussed earlier.

History Mode When it’s time to fill out your logbook, the diver can access this mode (sometimes called the Logbook Mode) to get information -  date and time of the dive, dive duration, maximum depth, and average depth. Computer models differ in the maximum numbers of dives that remain in storage before being overwritten with new dives.

Set Mode allows the diver to set basic computer parameters such as date and time, salt or fresh water, units of measurement (metric or imperial), alarms, backlight, and gas type and mix if applicable.

Most computers also offer a choice of a console or wrist model, depending on your preference.

Advanced Operation and Function

The range of functions a dive computer could have is almost limitless (given enough money...) For example, you can currently get models with a built in electronic compass.  Eventually, we’ll probably even have GPS! In the meantime, there are some “nice to have” functions that every diver will appreciate as well as some extremely advanced features for very specialized types of diving.

Some nice-to-have features include:

Backlight for viewing the computer at night.
Alarms. Since you’re allowing the computer to do some of your thinking for you, how about letting it track some of your dive parameters? You can get computers that sound an alarm when you reach your pre-set dive time or depth, or if you exceed your maximum rate of ascent.
Nitrox capable. Even if you currently dive only with air, the utility and availability of Enriched Air (NITROX) means that most divers will eventually use it. It would be nice to have a computer that can be used for both air and NITROX up to recreational limits (40% oxygen). For a usually very modest increase in price, you can get a computer that can use both air and NITROX- in the long run, it's worth it.
More memory. More advanced computers will also have more logbook memory, keeping more dives on file before being overwritten.
Deep stop. Like the safety stop on the basic computer, more advanced computers may also include a “deep stop” calculator (usually one minute at about half of your maximum depth).
"Auto on". This automatically turns the computer on when it gets wet (in case you forget to manually turn it on.)
More control buttons make it easier to scroll through the menus and make selections.
Guidance for missed no-deco limits. More advanced computers may not offer decompression capabilities as such, but they will usually provide guidance such as “total ascent time” and depths in the event that a no-decompression limit is inadvertently exceeded.

Really Advanced Operation and Functions

Some of these functions might benefit the majority of divers while some apply only to specialized and technical divers:

Air-integrated Want to get rid of a gauge or two?. These computers incorporate your submersible pressure gauge and present a digital display of remaining tank pressure and then calculates an estimated “air time remaining”.

 “Hoseless” Why not get rid of that hose altogether? Some computers use a transmitter on your first stage to transmit tank pressure to a wrist mount computer.

PC download You can download your logbook to your home computer. By adding some simple software and a cable, many computers allow you to download the data and view it graphically along with a representation of what your computer displayed at any point during the dive.

Mixed gases And for the more technically inclined, really advanced computers have the ability to change breathing gas during the dive and have the computer seamlessly continue the calculations.

Deco diving Most really advanced computers will also include algorithms for planned decompression diving, which is beyond the scope of recreational diving.

Dive Computer Safety

Ok. Dive computers do make our lives easier but using one doesn’t mean you can stop thinking or toss aside common sense.  And,  because dive computers  track the individual diver so closely, there are some special rules that must be followed when using a dive computer.

Each diver in the group must have their own computer. Computers cannot be shared because even the slight variations of depth in a buddy pair could result in nitrogen loading that is not accounted for in the diver without the computer.

If there is a discrepancy between the no-decompression time remaining between buddy A’s computer and buddy B’s computer, use the most conservative computer to control the dive.

If your computer quits, there is no way to switch over to the tables and continue diving. Remain out of the water in accordance with the manufacturer's recommendations to allow all nitrogen to be off gassed. (This is a good reason to consider wearing a back-up computer. Also, consider keeping spare batteries in your save-a-dive kit.)

Learn your computer! Know how it functions, how to set it, how to read it, and how to interpret and react to what it’s telling you. Don’t completely trust something that thinks for itself.

Dive conservatively. Just because a dive computer will allow you to dive to the limits of nitrogen loading, doesn’t mean that you should. A computer can’t factor in all of the unique physical  characteristics that affect your nitrogen loading on any given day .  If you always go to the line, one day you may find yourself on the wrong side.

Is a Computer for You?

Every diver can benefit from the added safety, bottom time, and convenience of a dive computer.  Which computer to choose? Just figure out what type of diving you’ll be doing and find the  computer with the features that best support your needs.  Stop by the shop - we'll be happy to help you choose the right computer for you.

Now...go enjoy that extra bottom time!