SHOULD I USE COMPUTER OR TABLES FOR NITROX?

I have been using a dive computer for a number of years. Now I have qualified as a nitrox diver I want to use this as a decompression gas. What is the best way to plan my dives now - with a dive computer or deco tables?
Darren Speedie

You can use either system, as both have various advantages. Assuming that the computer you already own is for air, you can go on using this and carry out the decompression stops it demands while using your nitrox as the decompression gas. This gives you a great safety margin, as the computer assumes you are breathing 79 per cent nitrogen (as air) during the decompression but you are in fact breathing a greater amount of oxygen.
You could go to the expense of buying a computer that can be programmed for nitrox mixes, and get the advantage of shorter deco-stop times. You can also use these computers with air. There are various types on the market, but I suggest you go for one that allows at least two gases to be programmed, one as a bottom gas and the other for decompression.
Various hard-copy tables are available. BSAC nitrox tables cover various mixes - 21 (air), 27, 32 and 36 per cent - and these allow no-stop and deco-stop diving to be planned. You get the advantage of longer no-decompression-stop times in comparison to air, or reduced decompression stops. Other hard-copy tables work on the Equivalent Air Depth (EAD) principle to calculate decompression requirements for various nitrox mixes. Only the BSAC nitrox tables have been calculated from base algorithm.
Another tool you could use is PC-generated decompression software. The various makes on the market all plan air and nitrox dives and you can programme the tables with various gas mixes at different depths, which will give you the decompression requirement for that precise dive.
As with hard-copy tables you must be precise with your dive times, deco times and stop depths. And unlike the dive computer, the table must assume that you will be where you told it you were and for the length of time specified.
Please ensure that you read the manual carefully before using any new decompression tools.


Grill your instructor
I have been diving for about three years and I am very interested in the articles I read about technical diving. I would like to learn more about the techniques used by technical divers and the various gases they use to carry out deeper dives and decompression. What is the best way to get started?
Dawn Pinker

Education is the keyword. Various training agencies have good training programmes, from nitrox through to mixed gas and rebreathers, but any course is only as good as the technical instructor teaching it.
You want somebody who has a lot of experience in the field you are looking into, so when you speak to an instructor, ask him or her some pointed questions. What are their qualifications? Fine - ask to see them. How long have they been diving nitrox, or rebreathers, or whatever they claim to be expert at.
Most importantly, how long have they been teaching technical diving? Just talking to the instructor should give you a good idea of their experience, or seek recommendations from divers you know. Avoid at all costs an instructor who doesn't know his stuff.
I strongly suggest that you start with an advanced nitrox course, as this is the foundation of many technical courses. It will give you a solid understanding of other gas mixes, their advantages and disadvantages; the use of decompression tables apart from air, and the principles of good decompression practice.
Don't rush from one course to another; ensure that you spend time after the course practising the new skills you have learnt.

Shot saves back but takes longer
Is a shot weight harness better than standard weight set-ups, and does the addition of more straps add too much clutter?
Scott

Many technical divers feel no need to wear weightbelts, as they become unnecessary with the amount of weight in the other equipment being carried. Harness weight systems do work well, taking the weight from the middle of a diver's back and spreading the load more evenly. However, it can be time- consuming with shot systems to adjust the weight for various diving conditions.

Build from the bottom
I'd like to do a two- or three-day introductory course on the Buddy Inspiration rebreather without having to get formal nitrox qualifications. I regularly dive and decompress with nitrox and have some "extended range" experience but it's all either self-taught or gleaned from more experienced divers.
Rob Jones

It's vital that you get the correct education for all types of diving and especially use of rebreathers. I'm sure you're a very competent diver but can you be sure the information you have been given by those "more experienced divers" about nitrox and extended-range diving is correct? Referring you back to "Grill Your Instructor", I would urge you to participate in an advanced nitrox course before doing an Inspiration course - and pick the right instructor!


Central heating
I am from Italy but I now live in the UK. I have been diving in England all through last summer and I would like to continue through the winter months. I am diving in a drysuit but still get quite cold throughout the dive, so do you have any suggestions on how I can stay warmer?
Paul

There are a number of ways in which you can maintain heat, or at least reduce heat loss during a dive. I'm pleased to hear that you are using a drysuit, which is essential in northern European waters in winter and highly desirable for long dives even in our so-called summer. However, is your drysuit really dry? Many divers get out of the water with damp patches because their suit has a slight leak.
Make sure that yours fits, is well-maintained and does not let in water.
Undersuit clothing is the real key to staying warm. Some divers use layers of standard clothing under their drysuits: tracksuit trousers and T-shirts, with sweatshirts over the top. The idea of several layers is good, but the materials used are all-important.
Have a wicking-type fabric next to the skin, a Thinsulate layer over that (they come in various grades) and a lightweight polypropylene top cover. Various undersuit makers offer this type of suit, so try out some variations to find which suits you best.
There are also various ways of heating the diver during the dive. Commercial divers use hot-water systems but these require a link with the surface.
You can however find heating systems for sports divers in the USA, electrically heated garments. These fit over the diver's chest and back (like a bib) under the drysuit. They require a battery, which is attached to your cylinder with a connection plug through the drysuit.
Finally, you can be warmed by the gas used to fill your drysuit for buoyancy control. Divers using helium mixes use either a weak nitrox mix (their travel gas) or argon, which has advantages over nitrox or air, being denser. You need a separate small cylinder for it, clearly marked to the effect that it is not a breathing gas (it has no oxygen content).
If you plan to use argon, remember to flush your suit with the gas before you enter the water, to get rid of the air.

---

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.

---

Appeared in DIVER - April 2000