A LITTLE PREPARATION BEFORE ATTEMPTING ANYTHING NEW goes a long way, as we all know, but this maxim takes on special meaning when venturing below the waves. The consequences of anything going wrong on a dive can be serious.
On most advanced or technical-diving training courses, much time is spent discussing dive-planning, because decompression diving has many more variables than no-stop diving. Planning to take enough of the various breathing gases is paramount, as is deciding between the plethora of decompression software planners and tables competing for your attention.
Lets plan to take you through from seeing that tantalising depth-sounder image of a possible wreck to having a well-prepared dive slate.
Well assume that you have a twin-set of a sensible size, say twin 10s or 12s. There is no real need to use a working pressure greater than 232bar, because gases dont compress following ideal gas laws at higher pressures, and minimal gains in volume are negated by serious weighting issues.
You will also need a decompression tank, ideally 7 litres and made of aluminium for its better buoyancy characteristics.
From here on, its a matter of looking at the theoretical planning considerations of deeper dives.
Practical concerns such as buoyancy control of drysuits and wings, surface marker buoy deployment and tides and currents are equally important but we will put these aside for now.
Before planning a dive that requires formal deco-stops, find out exactly how many litres you breathe per minute.
On your next 20m-or-so dive, make a note of how much your contents gauge falls on a 10-minute swim. For the air calculation to work, you must maintain a constant depth and try to maintain your usual swim speed.
It can also be instructive to do the same test at various dive sites, to find the differences in consumption between slack water and drift dives, and on shotline descents versus free descents.
Whatever the conditions, if you used 60bar of pressure in 10 minutes at 20m you can covert this to an SCR (Surface Consumption Rate) by doing the following arithmetic:

60 (bar) Ã 10 (min) Ã 3 (ata or atmospheres @ 20m) = 2

Multiply this 2 by the cylinder capacity, ie 12l x 2 = 24l. This is how much you will breathe per minute - its your SCR or RMV (Respiratory Minute Volume). Unless you dive every day, an RMV rate of around 20lpm is average.
Knowing your breathing rate will allow you to better use the gas-planning function of software decompression planning programs.
If you plan a dive to 40m for 30min, you could use any combination of air/nitrox/trimix plus single or multiple decompression gases.
For our example we will use a decompression gas with 50% oxygen, breathing nitrox 30 on the bottom.
Hard-printed dive tables for air decompressions are available, Professor Buhlmanns being a popular one. Another source are the recently updated US Navy tables.
However, tables that allow you to plan decompressions using enriched air mixtures such as nitrox 50 or 100% oxygen are less common, and nowadays fall solely into the realm of computer-generated tables.
Using such dive-planning software allows you to plan both decompression obligations and gas requirements very quickly, but its all too easy to think that the dive will be as easy to execute as it was to plan.
Another minefield for would-be technical divers is the sheer number of software titles available, and the practice of downloading older versions free of charge - but usually not
without cost.
Many dive-software writers initially set versions adrift on the Internet for evaluation, but these copies are quickly revised as incidents of decompression sickness are reported.
So youll find that you have to pay for almost all of the useful dive programs, but only up-to-date products should be used to plan actual decompression dives.
Some are updated monthly, so check the website regularly.
Table A shows a schedule of decompression stops with, in the last column, the appropriate longer times should a specific decompression gas be unavailable.
Just as importantly, many desktop dive-planners provide a summary of gas volumes required. Volumes should include a sensible reserve that provides extra gas not only for your own needs but also for those of a buddy.
During advanced diver training, instructors reinforce the need for gas supplies to include a third extra. The thirds rule was introduced some time ago to add safety to cave dives.
The idea is that each diver uses a third of available gas to venture inwards, and another third to get back to the start, leaving a third for a buddy to share.
This system is fine for planning reserves for fairly predictable cave- and wreck-penetration diving, but is quite conservative for technical dives in the open sea. When you know your breathing rate, its fairly straightforward to calculate the volumes needed to complete the planned dive yourself.
Once bottom-mix gas volumes are known, you must of course add the gas needed to ascend to the depth of the first decompression gas change.
The final volume will be multiplied by 1.5 to find total volume as dictated by the thirds method.
Table B shows a software prediction for gas reserves needed for the same 40m for 30min dive outlined earlier. Dont worry about how these computer-generated figures were arrived at - we will plan the dive longhand and try to arrive at similar numbers.
We will calculate for an extra third of back gas and ascent gas, but will take double for any formal deco-stop gases to help with sharing, or with unforeseeable occurrences such as free-flows.
Warning: this can be long-winded!

Back-gas volumes are calculated as follows: 5ata (40m) x 30min (bottom time) x 20 (lpm breathing rate) = 3000l
Ascent gas is calculated like this:
40m - 21m (1st stop) = 19m. To travel this distance takes approx 2min.

Average depth for the ascent gas volume is calculated as follows:
40m - 21m (first stop) = 19m, divided by 2 = 10. Add this 10 to 21m (1st stop)
= 31m. This translates to 3.1ata.
3.1ata x 2min x 20 (breathing rate)
= 124l ascent gas.

3000l + 124l = 3124l x 1.5 (thirds) or 4686l.

4686l à 25l (2 x 12.5l back tanks) = 188bar needed - close enough to the software prediction in Table B.

Decompression gas requirements can be calculated like this:
3.1ata (21m) x 1 (min) x 20 (lpm) = 62l
2.8ata (18m) x 1 (min) x 20 (lpm) = 56l
2.5ata (15m) x 1 (min) x 20 (lpm) = 50l
2.2ata (12m) x 2 (min) x 20 (lpm) = 88l
1.9ata (9m) x 3 (min) x 20 (lpm) = 114l
1.6ata (6m) x 5 (min) x 20 (lpm) = 160l
1.3ata (3m) x 12 (min) x 20 (lpm) = 312l
Total = 842l needed.

A common approach to decompression gas volumes is to double them, which will allow for the possibility of sharing: 842 x 2 = 1684l. Divide by 7(l) = 240bar of nitrox 50 needed.

As you can see, its much easier to compute gas volumes using deco software, but you must use adequate reserves and realistic breathing rates or the numbers are - just numbers.
In the last calculation it becomes obvious that the gas required should one or both decompression tanks be lost would swell to enormous quantities - 265bar in our Table B example.
The solution to this must be managed either by a support diver or by staging decompression tanks. Some divers even take their own spare gas on appropriate-length ropes that a skipper can lower over the side and clip to the divers SMB line.
Any prudent dive-planner should consider gas reserves/options in case of deco-gas losses, as certain back-gas options simply cannot be decompressed from within the divers lifetime!
Desktop deco software may be becoming more widespread for dive- and gas-planning, but pen, paper and calculator skills are every bit as important to help avoid real dramas.
Its easy to see why successful technical diving even at a moderate level involves a team effort, because to do it alone simply leaves too much to chance.
Throughout the descent and bottom phase of the dive, check the decompression tanks to ensure that the regulator works, and that the pressure gauge is pressurised to stop water getting in.
With two identical decompression regulators, it is easily possible to switch them from tank to tank under water if necessary - as long as they use DIN or INT attachments, of course!
This skill should be practised regularly, and be sure to carry a supply of spare DIN O-rings.
Should the deeper decompression tank be inaccessible, you could switch regulators from the other tank or (better) ascend a little shallower, remembering to extend the deco-stop times to take into account that you are not breathing a rich decompression gas.
In deeper water, it is often OK simply to double the stops taken on air instead of, say, nitrox 50 - but run such changes through the software planner to get the facts.
Few divers carry an extra second stage on decompression gases for some reason (mostly tradition).
This makes sharing deco gases very difficult unless both divers are skilled at sharing a single second stage.
Buddy breathing is a useful skill for all technical divers and should be practised regularly so that you feel comfortable and safe doing it for possibly lengthy periods.
An alternative is to add octopus regulators to decompression regulators, or even a low-
pressure inflator hose to each decompression regulator first stage, so that an inflator/regulator combination can be employed for any tank when necessary.
However, the best way to fix a decompression regulator problem is to plan to take additional bottom mix and have a stop plan to cover its use. Avoid complicated high-performance regulators on decompression tanks, and prefer simpler and ultimately more reliable basic units.
Practice makes perfect. Learn to stay calm to effect a drama-free outcome in an emergency.
Taking two dive slates (one spare) and two depth/time gauges is the norm for all decompression dives, but multi-gas dive computers are getting reliable, and if youre feeling flush, get two!
Technical diving is a bit like learning to be a boxer. If you box above your weight or experience, youre quite likely to hit the canvas in the form of a multiple recompression-chamber ride - if youre lucky. Safe diving.

Examples of deco-planning tables available on line
dive slate and computer
the sort of side-scan sonar image of a plane wreck that would inspire any diver to get planning
Hanging out after a well-planned dive.
Decompression regulator. Avoid complicated high-performance models on decompression tanks
Table A: Sample Deco-stop Schedule
DepthGas StopRun timeNO 50%
Surface breathe 50% for 10min - relax
Table b: Software-generated gas predictions
2 x 12.5l30%75bar4875195bar265bar!
(includes reserve)
1 x 7l50%120bar840240bar-