JOIN THE LAPTOP PLANNERS

I have always used a dive computer to work out my decompression schedule. Recently, during a day's boat-diving, I met a group of technical divers using a laptop computer to plan their dive and decompression schedule. Could you tell me more about this type of dive planning?
Alan Mackie

These divers would have been using a PC-generated decompression table. Many different software packages are now being used, including Proplanner, Voyager, Andi Plan and Abyss.
The benefits of these decompression tools are many but you need to do some work too, feeding in pre-dive information to allow it to calculate your decompression schedule. This includes maximum planned depth, time at that depth and whether you plan to use air, nitrox or mixed gases.
The schedule also depends on the gas you breathe during the ascent phase of the dive. You can enter various nitrox mixes for different levels of the ascent and the software will calculate stop depths and times. Most PC-generated tables have an algorithm based on Buhlmann tables, but some have selectable algorithms.
Advantages of using such decompression planning tools include:

• Extra safety percentage factors- read the manual carefully to find out what is taken into account. Some tools add theoretical inert gas to the mixes you plan to use, or extra depth to the planned dive. Others ask if you plan to work hard during the dive (something I would never do). With these extra safety factors built in, they will make you carry out more deco stops.
  
• They track oxygen uptake and calculate the partial pressure of oxygen (ppO2) at all depths of the dive, breathing the various mixes. With this data they can calculate oxygen CNS% (central nervous system percentage) and UPTD (unit of pulmonary toxicity dose) or OTU (oxygen tolerance unit) loading during the dive, and warn you if safe levels are exceeded.
  
• Some calculate the gas required for the dive once you have entered a breathing rate. They do this for the various gases you plan to use and also ask you for a reserve gas percentage. From this you can work out what cylinders you require.
  
• They can calculate multiple dives. Enter your surface interval time and the second dive information, and they will calculate your schedule, taking into account residual inert-gas loading from the previous dive.
  
• Some can be used to plan altitude diving by entering current atmospheric pressure.
  
• You can print out the final planning sheet, giving the full dive with depths, deco stop times, gases at various depths and a useful run-time schedule. Run-time gives you a list of times at which you must leave various depths, from the bottom to all the decompression levels. This means you don't have to think about how long you need to be at the stop depth. The software measures elapsed time, just like your computer, but make sure your ascent rates are correct (normally 10m a minute).
  
• Depending on the make and type of software, various other functions can be carried out.
  

There can be a downside. For example, when the schedule tells you to be at 6m for 20 minutes, you need the skill (correct buoyancy) and discipline to do exactly that. This is not like a dive computer that will recalculate your stop time if you move a little deeper.
I use PC-generated decompression tables for most of my dives, certainly all those on trimix. I like the flexibility it allows me in planning dives and the back-up it gives me when using dive computers.
And I can always fall back on the run-time information should my computer decide to go on strike during the dive.

BRACKETS AND BANDS
I have been looking for an effective latching system that can be used to mount decompression gas cylinders of different sizes to the main cylinders on my back. Can you help me?
Mike Everett

Various options are available. The prime consideration is that the cylinders should be firmly attached and unable to swing around or fall out during the dive.
The Ultimate Pony Bracket (UPB) from Aqua Explorers in the USA uses a camband system to attach the cylinder to the main twinset. It is very effective, holds the cylinder rigidly and works well on deco gas cylinders up to about 5 litres.
For larger cylinders, SDS in Sheffield produces custom-made bands and an attachment system that works very well. It takes most sizes of cylinder and I have used it with two 10 litres mounted either side of my main twinset.
If you plan to use a quad system (four cylinders on your back), you need plenty of buoyancy to take that extra weight. Remove your set in the water rather than struggling up a ladder with it, which is provocative in terms of DCI.

USING UP DECO GAS
At the end of a decompression dive, once I am back on the boat, I always spend time breathing the last of my deco gas. Recently I was criticised by another diver for doing this. Can I have your opinion?
Barry Pargiter

I cannot understand why this other diver criticised your technique. I'm with you- I spend as much time as possible after the dive breathing down my decompression gas. It has to be better than breathing air, with its large amount of inert gas (79 per cent nitrogen), and is surely beneficial to the off-gassing still going on in your body.
It also seems a terrible waste to empty the balance into the atmosphere just to be ready to get another fill for the next dive.
I take the decompression phase even further, after a long dive. I make the final ascent from 3m very slowly (one minute), then spend time floating on the surface, still using the deco gas, as long as surface conditions allow.
I remove my stage cylinders and main rig in the water to avoid any extra exertion climbing up a boat ladder.
Skippers in Weymouth who cater for technical divers have helped us even more by fitting cage winches to lift divers out of the water (the first to do this was the late Andy Smith, in my opinion the greatest skipper of all time).
Once back on the boat, I breathe either the deco gas or oxygen for at least 30 minutes. Please do not be put off building in extra safety factors to avoid DCI; it is worthwhile.

GAS-SWITCHING WITH FULL-FACE MASK
I have been using an Interspiro Divator Mk 2 full-face mask recently, with great success. However, I would like to use it for my decompression dives and switch gases during the ascent phase. How can I do that with this mask?
C G Dempsey

The simplest way is to use a gas-switch block. Various hoses from different cylinders are plugged into a manifold block which is fed to your mask. When you wish to change gas, you either open the correct valve, or plug in the correct hose of the gas required.
This sounds very simple, and can be. However, it is vital that you know which hoses feed which gases, or the consequences could be fatal.
Various gas-switching systems are available to full-face-mask divers. One is the RSV-1 (Redundant Supply Valve) from Sartek Industries in the USA. This type allows various mixes to be plugged into the second stage of the mask. I believe it is more useful as a redundant gas source than as a full-blown gas-switching manifold.
Purpose-built gas-switching manifolds are made by companies such as Thompson in the UK and Underwater Applications in the USA. Both allow you to plug in various decompression gas mixes and can be used with the Divator.
Remember that all your gas supply is flowing through one second stage, and you need to think about redundancy in case of an emergency.
It's worth carrying a spare half-mask and having a redundant gas source fitted with a standard regulator.
If you had a problem with the full-face mask, you could revert to the back-up mask and regulator.

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Jack Ingle is the BSAC's Technical Diving Adviser. He is a BSAC National Instructor, an IANTD Technical Instructor, a TDI Tri-mix instructor and co-author of NSAC nitrox courses.

Keep your queries coming Submit them, marked "Technical Questions": by letter, addressed to Diver; by fax on 0208 943 4312; by e-mail to editorial@divermag.co.uk; or on Divernet's Technical Q&A page. This last can also bring rapid responses from other readers (though these should of course be treated with caution). We regret that questions cannot be answered on the telephone or, generally speaking, replied to individually.

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Appeared in DIVER - July 2000