Legend

Y=Yes, N=No, A=Automatic, M=Manual

 

Numbers following letters Y, N, A, or M are modifiers as described below:

 

Max Dep = Max Depth (in feet unless noted)

Air Prs = Air Pressure Integrated: 1=wireless

Tmp Gge = Temperature Gauge. 1=Used in calculations, not displayed.

Dec = Decompression: 1=limited deco info

Alt Cmp = Altitude Compensation: 1=to 10k ft, 2=to 14k ft

Act = Activation: 1=on submersion, 2=by applied air pressure

Usr Bat = User Replaceable Battery: 1=Yes on newer models

MSPR = Manufacturers Suggested Retail Price, US$

Maximum Depth

This is the maximum depth to which the computer can be taken and have it still be water proof and some of the functions still work. This is not the recommended maximum depth to which the computer should be taken! Most computers will log maximum depth. Some also tell your average depth (which is nice for computing Air Consumption rates). Newer, more elaborate computers, keep a log of depths at intervals of a few second (or minutes). Many of the air integrated computers will automatically calculate and tell you your air consumption rate.

Air Pressure

Units that have integrated air pressure will show you a digital readout of the amount if air remaining in your tank. (The readout is usually in increments of 1 to 10 PSI, check the manual for details). These computers are often a little more expensive (and have more features). Most will also tell you your air consumption rate, and use that to calculate how long you can remain at the current depth given your current air consumption rate and the amount of air presently in your tank. Most will then display this time, or the remaining no decompression time, which-ever is less.

Temperature Gauge

Newer computers are starting to incorporate a digital thermometer into the list of neat gadgets. This feature can range from a simple temperature readout on the display during the dive, to a system that logs temperature (surface and bottom) and also logs average temperature. Having this information may not be terribly important during a dive, but it is a nice addition to a dive log --- then, when you return to a spot, you can easily get an idea of local conditions before setting out on a trip.

Dive Logging

Almost all of the current dive computers store information about a few dives that you can later recall. Depending on the computer, some of that information may only be accessible using a computer interface (usually, a very expensive cable/software packages that allows the dive log information to be downloaded from your dive computer to a personal computer.) The key factor to keep in mind is: if the computer you are looking at has some wiz-bang log feature you just can't live without, find out if that information is available in standard log mode (without needing expensive interface equipment/software, or having to use the software in the dive shop where you bought the machine) before buying the computer.

Decompression

Does this computer do decompression calculations? This does not mean that the computer is necessarily a decompression computer. Many of these computers will do emergency decompression: you can use them to help you get through emergency decompression if something happens during the dive to cause you to enter decompression mode, but they will not allow you to plan a decompression dive. Depending upon the computer, the level of decompression support may vary. If this is an important feature to you, you should look into this carefully before purchasing a computer. NOTE: decompression diving is beyond the realm of sport diving. You need special training and equipment to be able to safely do decompression diving.

Altitude Compensation

Altitude diving adds a few variables to the diving game; as you get higher into mountains, air pressure decreases. However, the effect of water pressure remains constant. Therefore, to be safer, you need to add safety margins to the dive profile (call a dive to 50 foot a dive to 60 foot when calculating TNT for example). Computers deal with this in various ways: 1) ignore it (i.e. assume the person is always diving in sea water at sea level), 2) allow the person to manually adjust the altitude (which might be an exact value, or an indication of a range depending upon the computer), 3) automatically adjust for altitude (this can also be in a range, or an absolute measurement). Some of the computers that have altitude compensation will also change calculations to those for fresh water dives after a certain altitude (such as 4000 feet) or after sensing the water. Some computers will actually monitor the altitude and adjust automatically even when they are off, others self adjust when turned on.  We recommend that you select a manual adjust.  This allows you to select the highest altitude you will be at even if its higher than your dive site.

User Replaceable Battery

Your dive computer needs energy to operate. This energy comes from one or more batteries located inside the unit. Some computers have user replaceable batteries, some do not. Often, batteries requiring factory replacement must be soldered in, and are often more powerful --- i.e. they last longer. User replaceable batteries offer the convenience of not having to wait for the unit to have the batteries replaced at a service center, or mailed to the manufacturer, as well as the added comfort of being able to bring a spare set of batteries on a long trip. However, there are some items to be aware of. See the section "User replaceable batteries != service free" for more information.

Activation

Before using your computer, it must be turned on. Activation of computers falls into two categories:

Manual

You push a button to turn the computer on. If you forget, you suffer the embarrassment (and possible danger of missed on-gassing information) of having to surface and turn the computer on (I don't think that any of the computers that must be manually activated can be turned on if they are below a certain depth of water.) Many of the computers will stay on for some preset time (about 1 hour usually) before shutting off if no dive is made.

Automatic

The computer turns itself on when some condition is met: 1) it is exposed to water pressure of a certain depth (a few feet for example), 2) air pressure is applied (for air integrated systems). These computers have the added benefit of removing the responsibility from the user of having to check the computer before entering the water. It is a good and bad thing. It is nice not to have to worry about turning the computer on, but IMHO, I like to look at the computer before entering the water to verify that it's working (and see if the test mode passed when it was turned on). Some (all, I would imagine) automatically activated computers can also be manually activated.

Which system you prefer is largely a mater of choice. If a computer is automatically activated, there is the possibility that that activation will not take place (very unlikely if you have your system serviced regularly.) On the other hand, a manually activated system might be accidentally turned off (all of the computers that I know of have safeguards against this: they can not be turned off while under water, or while the theoretical nitrogen cells are still loaded). Some systems can not be turned off at all; once turned on, they stay on for 1 hour (or similar time span) unless you start a dive. Once you have been diving once, they remain on until completely off-gassed (or a fixed time has elapsed, say 24 hours), and then shut themselves off. This is an important safety guard for the person that owns the computer. However, it can get in the way of places that have computers for loan/rent (large live aboard boats for example). If diver A takes a computer for a few dives in the morning, then quits and diver B would like to use the computer that evening (to start his/her diving) a problem arises: the computer still has on-gas information for diver A. If the machine allows a user to reset it, it might be done accidentally and pose a hazard for the user.

Alarms

In the process of diving, if you do something incompatible with the programming of your computer, i.e. you violate a built in constraint (ascending too fast, omitting a required decompression stop, exceeding the maximum depth allowed by the computer, etc.) the computer needs to be able to alert you to this fact. These "alarms" can very from audible beeps (or actually speech in one case) or visual symbols on the display, or both.

PC Interface

Some computers have interfaces to personal computers (usually IBM PCs) and come with hardware and software to upload data from the dive computer to your personal computer. The information available, and number of dives stored will vary from computer to computer. The software usually includes the ability to store notes and other details so that you will have a "electronic" log book. However, some systems with a PC interface provide some information solely through the PC interface (i.e. you can not get at the information via the normal log mode of the computer). This information is usually not needed in planning dives, but if you don't want to spend the extra money for the interface (assuming it's not included with the computer) then the information in question will not be available to you.

Some computers can also be configured via the PC interface. For example, the PC interface can be used to set the percentage of oxygen in the Nitrox mix being used for the Nemesis-Nitrox. Several of the newer dive computers can be customized (by the user) using a PC interface to select items such as: aggressiveness of the decompression algorithm, ascent rates, depth alarms, Oxygen percent, altitude, sample rate for dive profile information, etc.