A dive computer is one of the most useful toolsfor divers that is available these days. While we still train to use the dive tables, they are absolutely conservative and cumbersome. Dive computers calculate your No Decompression Limits (NDL) on the fly and are constantly adjusting for changes in your depth. This distinction is why computers are almost a mandatory piece of equipment and is where the dive tables suffer the most.
Styles of Dive Computers
When selecting a dive computer, the first decision you must make is whether you want a wrist mounted computer that you wear like a watch or a console mounted computer that is attached along with your pressure guage and possibly a compass on your high pressure hose. There are pros and cons to each style of computer and we will discuss some of them here.
The Wrist Mounted Computer
While wrist mounted computers come in a wide variety of individual styles, they all attached to your wrist like a watch. What I consider to be the greatest benefit to a wrist mounted computer is the simple fact that you are wearing it on your wrist - whether you are in the water or killing time during a surface interval. When you want to check on your surface interval time you don't have to wander back to your equipment and dig out your console. On the down side is that if you are interested in features like air integration(seeing your tank pressure on your computer), you will need a wireless transmitter connected to a high pressure port on your first stage. Computers capable of interfacing with a wireless transmitter are slightly more expensive and the wireless transmitters themselves are an additional, not unsubstantial cost as well. Wireless air integration is going to cost you an extra $400 - $600 on the price of your dive computer. Wireless transmitters do not transmit a great distance and can lose their connection to your wrist is something large gets between your first stage and your wrist.
The Console Mounted Computer
A console mounted computer is relatively permantly connected to your first stage. On the down side, when you wish to check on your surface interval time you will need to head over to your gear and dig out the console. If, on the other hand, you are interested in air integration with your computer, console mount computers can do so my connecting directly to your high pressure hose. This direct connection is slightly more reliable that using a wireless transmitter with a wrist mount computer. Another potential issue with air integrated consoles is that should the battery die or the computer fail in any majory way, you have no way of reading your tank pressure unless you wisely keep a good old analog pressure guage on your second high pressure port as a back up.
All dive computers will provide you will the basic information required to successfully and safely complete a dive. These standard features include:
- Bottom Time
- No Decompression Time Remaining (NDL)
- Maximum Depth
On top of the basic features, there are other features you may wish to consider.
The vast majority of dive computers available are programmable for Enriched Air Nitrox as well as just air. The difference in price for a Nitrox ready computer versus an air only computer is minimal, so buying a computer that is Nitrox ready is a good idea. most recreational computers are capable of setting a Nitrox mix up to at least 40% - which is adequate for almost all recreational diving. Some computers can be programmed for Nitrox mixes of up to 99% and will be viable for use if you should begin technical diving.
Air integration is a fun feature to include in your dive computer. It shows you the tank pressure and can also help you look at your gas consumption at various points in the dive when you review the logs after the dives have been completed. It is a costly feature, whether you are diving a console or a wrist mount computer, and should you get to the point where you want to dive muliple tanks of gas your air integration quickly becomes less useful. You would need multiple transmitters -one for each tank of gas you carry - or you would not have the integration available for anything outside of your primary gas. For this reason, I would recommend considering spending your extra money on better computer rather than an air integrated computer.
Different manufacturers use different algorithyms to calculate your NDL's. Some manufacturers like Suunto are known to have a more conservative calculation over others like Mares or Oceanic. Within each computer there are also settings to allow for more or less conservative diving. There is nothing inherently wrong with a more conservative dive computer however you may find that your NDL is a bit shorter than your buddy's. Again, this is not necessarily a bad thing. Diving more conservatively is safer than diving less conservatively.
Along with varying NDL's, some computers have what are referred to as technical features, namely decompression planning. Where a typical recreational computer is not designed for decompression and uses a very conservative decompression model to get you out of an accidental crossing over from the NDL's into a decompression dive, others are able to plan and execute decompression profiles using either the VPM or Buhlmann algorithyms. If you are considering venturing into the land of technical diving someday, keeping these features on your wish list may be a good idea.
Many recreational dive computers have programmable alarms to indicate if a preset depth has been surpassed or if a preset dive time has been surpassed. The computer will audibly indicate that you have exceeded your planned depth or time. Many will also audibly alert you if you are ascending too rapidly.
Some higher cost dive computers also have a built in electronic compass which have become very reliable and useful in recent years. Combining the feature of a compass into your dive computer means one less item to carry and means you can be a just a little more stream lined.
Multiple Gas and Gas Switching
Newer Nitrox computers offer the ablility to define multiple gases rather than a single gas. This allows the diver to have their common Nitrox blends ready to toggle rather than reprogramming the computer before each dive. Many will also allow you to switch gases during the dive so that if you switch between your back gas and a pony bottle during the dive and they happen to be different Nitrox or air mixes, you can switch the mode on your computer to account for it in your dive profile. This feature is very useful if you consider technical diving in the future.
Most computers will connect to your computer to allow you to download your dive information to your personal computer. This allows you to easily keep a digital dive log with all the information from each dive. This can be down using a plug-in cable interface, infrared connection and even blue tooth. Consider whether the computer you are buying comes with the interface cable or if it much be purchased seperately. Some interface cables can cost as much as an additional $100 on top of the price of your dive computer so finding a computer that includes this interface in the box may save you money.
These are just a few of the many things to consider when selecting a dive computer. It is often better to spend a little extra money on your first purchase than to need to spend a lot of money upgrading to a new computer down the road.
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The debate over which tanks are better, steel or aluminum has raged for many years. Some divers prefer aluminum while some divers prefer steel. As a new diver buying equipment, you need to know which tanks are best for you.
Aluminum scuba cylinders most commonly come in an 80 cuft (actually 77 cuft) variety. They tend to be cheaper, lighter and also come in a wide variety of colours. Aluminum 80's or AL80 are the most common rental tank for dive shops around the world. They also come in a less common 100 cuft variety. The AL100 is larger - but also much heavier.
Steel scuba cylinders are a bit more complicated. While the options for aluminum tanks are fairly limited, steel tanks come in a greater number of sizes and two different working pressures. Low Pressure or LP steel tanks have a working pressure of 2640 PSI / 184 BAR while the High Pressure or HP steel tanks have a working pressure of 3442 PSI / 237 BAR. They are less commonly used as rental tanks but have gained popularity because of their higher gas volumes.
A quick breakdown of some common scuba cylinder characteristics ....
|Buoyancy(lbs) @500 PSI:||+2.8||+1.8||+1.16||-0.35||+1.46||-2.38||-2.66||-1.73||-1.19||-0.08|
|CuFt @500 PSI:||12.83||15.15||16.1||17.99||20.45||22.73||11.62||14.53||17||19.32|
*Always check the manufacturer specifications for your own model of tanks for the most accurate information.
The working pressure is the fill pressure to which the tank has been rated and is directly related to it's fill volume. The working pressure is always marked permanently on the shoulder of a scuba cyliner as well as its full volume. Many different sizes of tanks have the same working pressure ratings, so it is the volume that is most important. If someone tells you they are starting their dive with full working pressure of 3000 PSI, you really do not know how much air they have unless you also know the volume of the cylinder. A 3000 PSI fill in a 19 cuft tank is vastly less air than a 3000 PSI fill in an 80 cuft tank. Though the working pressure is the same, the 19 cuft tank is much smaller and has 1/4 of the internal volume. There are three common working pressures found on tanks. The most common is the aluminum cylinders (AL) with a working pressure of 3000 PSI, nearly as common is the low pressure (LP) steel cylinders with working pressures of 2640 PSI and finally the high pressure (HP) steel cylinders with a working pressure of 3442 PSI. There are other, less common working pressures as well.
Hydrostatic Testing and the mysterious PLUS rating.
Both aluminum and steel scuba cylinders must be hydrostatically tested every 5 years in Canada and in the US.
In the US, a typical 2400 PSI low pressure steel tank can be tested for a PLUS (+) rating which allows the filling station to increase the fill pressure to 2640. They come from the factory with this plus rating, however it must be specially tested to be renewed every 5 years otherwise the working pressure rating on the tank drops back down to 2400 PSI.
Transport Canada does not recognize the (+) system and tanks are marked according in the metric pressure rating (BAR) and you will notice on your LP steel tanks (Marked 3AA) that while the rating in PSI is 2400, the rating in BAR will be 184. (184 BAR = 2669 PSI!!!)This is the normal working pressure as determined by the permit issued by Transport Canada. There is no (+) required, ever.
When air is compressed to fill a scuba cylinder, it heats up dramatically which causes the air to expand. Though the initial fill pressure may read 3000 PSI, that pressure will drop as it cools back down leaving your cylinder less than full. For this reason, it is always best to take your time when having your tanks filled. If possible, leave them at the dive shop over night so that they can be filled slowly.
Different scuba cylinders will become more or less buoyant as they are emptied. It is important that you plan carefully for this additional buoyancy and ensure you have enough weight on your weight belt to do a comfortable safety stop at the end of your dive. If you do not have enough weight you will find yourself fighting to keep from popping up at the end of your dive.
In Recreational Scuba diving, it is important to remember that you are carrying the breathing gas for not only yourself, but for your buddy in the event there is an emergency. Ensure that you are aware of what is a safe 'turn pressure' (at what point you must end your dive) so that you always have ample breathing gas to share with your buddy to make a safe ascent should something go wrong. Remember that the turn pressure will vary depending on the size of your scuba cylinder. As shown in the chart above, 500 PSI in an AL80 is quite a bit less than 500 PSI in an LP120. It is always best to plan to end your dive with between 13 and 15 cuft of gas remaining in your tank, which for most cases should be ample reserve. For this reason it is a good idea to understand how many PSI or BAR 15 cuft of gas is in your individual tank(s).
So ... Steel or Aluminun?
For the kind of diving we do in Canada, which tends to be deeper, colder and a bit more challenging, I prefer steel tanks. They have better buoyancy characteristics and they are available in a larger variety of sizes that can give you more bottom time without sacrificing your safety margin. With proper care and maintenance, steel tanks will last the same or longer than aluminum tanks and are quickly becoming the status quo for many divers up north.
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