The Importance of Dive Tables
By Thomas Gronfeldt
There was a time when dive computers were for the few, the most dedicated or simply the most flush with cash. The sheer cost of a dive computer meant that most divers went without one, relying instead on a waterproof watch and an analog depth gauge. These were the days of dive planning and dive tables. Before a dive, divers would plan the maximum depth of the dive, which was not to be exceeded, and a maximum time for the dive based on the data their dive tables gave them. Once the dive was completed, the nitrogen load of the dive would be calculated using the table. And based on this, the timing, depth, and duration of the next dive could be planned.
But as Morse’s Law came into effect (any new technology will double its processing power and halve its price roughly every 18 months), dive computers have become accessible for every diver who wants one. Small, inexpensive and all very capable of giving you dive time, depth, remaining dive time and surface time between dives, computers have become for many divers a key piece of gear. I use one myself. And several organizations either have or are considering making the switch completely to the point where dive computers are taught during dive classes and dive tables are left completely out of the equation.
The reasons given in support of this policy are quite good: dive computers are everywhere and inexpensive enough that any diver can afford one, they allow for longer dives due to their on-going calculations of nitrogen loads (rather than calculating the diver as having spent the entire dive at the maximum logged depth, as the tables do), they’re reliable, safe and much easier to learn how to use for a new diver than the dive table and its principles are. So from one perspective, safety would be enhanced by using computers instead of tables.
But we should be cautious about throwing the baby out with the bathwater. Having access to dive computers is great, and it’s a great safety asset for divers, but that shouldn’t mean we completely discard dive tables. Why? There are a number of reasons:
1. Dive tables are universal. Dive computers are all slightly different from one another, both in design, user interface and in the algorithm that calculates dive times (hence the warning never to do to consecutive dives on different computers). A dive table is not this way. If you find yourself without a dive computer, the table can always take over. And by making sure all divers are able to plan a dive according to a dive table, there will be no confusion over which computer’s data is used.
2. Dive tables don’t crap out. Yes, dive computers are safe and reliable. But they’re not 100 percent reliable. I have seen dive computers crap out during dives and on diving holidays, and I’ve had it happen to me. The normal advice when this happens is simple: stop diving for 24 hours to allow complete nitrogen degassing. However, if you’ve logged all your dives using a dive table you’re able to continue diving (after ending the dive at the moment the computer dies, of course) using any means of depth and time measuring available. That could be a backup computer in gauge mode, watch and depth gauge or a watch with a built-in depth gauge. Some of the more experienced divers out there will even make a note - mental or on a writing slate - of the time allowed at the maximum depth of their planned dive, just in case. If you have not kept a continuing log for all your repetitive dives and all you have is the dive computer’s log, there’s no other option than to abstain from diving for a full 24 hours should your computer fail.
3. Dive tables prepare divers for more advanced planning. As divers progress in skills and experience, they may start learning how to dive with more advanced gasses in their tanks. And part of knowing how to dive with Nitrox, Helitrox or Trimix is the ability to do depth and dive time calculations based on dive tables - tables that are somewhat more complex than the ones used for diving on normal air, to boot. Knowing how to use the basic tables prepares you for learning how to use the advanced ones. And it doesn’t look like the requirement to be able to plan and calculate your dive and gas needs will go away from Nitrox courses or tec diving courses anytime soon.
4. Dive tables are good practice. For many divers I’ve observed, it isn’t until they log their dives that the reality of nitrogen load really dawns on them. Working their way up the letters indicating nitrogen load is a very tangible way of experiencing that for each dive they came closer to their body’s maximum capacity for dispelling nitrogen. A dive computer doesn’t have the same effect on most people - there’s something too abstract about the digital numbers. So, in particular for new divers, it’s a good learning experience to work with tables.
The bottom line is that I believe dive computers have, in fact, made diving safer. But just like ABS braking systems and anti-skid systems in cars shouldn’t make us lazy drivers and unable to stay safe without them, dive computers should never take the place of the diver’s brain. Use them as an additional layer of planning and safety, but don’t let them replace sound judgment, good planning and reasonable conservatism. And dive tables – don’t forget the dive tables.