"50 minute dive, back on the boat with 50 bar."

As recreational divers this is a typical instruction we're given during the skipper's briefing.

As thinking divers we need to convert this into a practical dive plan that will help to keep our team safe. Here's one approach to doing that. We need to manage four things: our dive time, our breathing gas, our decompression and our dive profile and route.

Dive Time

Work backwards. Whatever number of minutes the skipper gives you is the latest time back on the surface. You'll need 3 minutes for a safety stop and 1 minute to ascend to the surface. So deduct 4 minutes from the total time to work out the latest time to be starting your safety stop e.g. 50 minutes - 4 minutes = 46 minutes. (That's your BSAC dive time.)

Divide your BSAC dive time by two and you have your latest turnaround time e.g. 46 minutes / 2 = 23 minutes. This is the latest time at which you should turn around and head back if you're doing an out-and-back dive.

If you find yourself at, say, 45 minutes and you're still not back at the line, then you still need to ascend to 6m, complete your safety stop and surface no later than the 50-minute maximum. If possible, ascend near a visual reference such as a wall. If you have the skill to do so, deploy your DSMB.

Breathing Gas

Again, work backwards. Add the gas you need for your safety stop and ascent to your 50 bar reserve. For most divers that's about 10-20 bar. So aim to start your safety stop when the first member of your team reaches 60-70 bar.

As a rule of thumb start your ascent from 18m when the first member of your team reaches 100 bar remaining. (That's why most dive guides ask you to tell them when you reach 100 bar.)

50 bar back on deck is a common requirement for local commercial operators. BSAC, however, uses the Rule of Thirds. This means having a reserve of 70 bar at the end of the dive, not 50 bar. In this case just add an extra 25-35 bar to the numbers above. Begin your ascent from 18m with 135 bar and start your safety stop with 80-90 bar.

There is one final gas check point to be aware of. In an out-and-back dive you should turn around when you hit the 2/3 of your total gas. Assuming you start with 210 bar then the minimum gas before turning back is when the first member of your team reaches 140 bar.

The gas reserve is for your buddy in case of an out of gas emergency. If you plan and conduct your dive to have 100 bar at 18m, 85-90 bar at 12m and 60-70 bar at 6m then you will have sufficient gas to get yourself and your buddy to the surface in an emergency from each of these depths. If you don't do this - if you stay deep and run down your gas supply - you may have enough gas to get yourself to the surface but not for you and your buddy if you need to share your gas supply.

Decompression

Use BSAC tables or your dive computer to estimate the maximum no-deco dive time that you will have for your planned dive. If the skipper tells you 50 minutes for your second dive of the day on air, you will know that you need to plan a shallower dive and carefully monitor your NDT (No-Decompression Time) Remaining.

If you are using BSAC tables to plan instead of a dive computer, then you need to plan your decompression in case you exceed your planned dive time by 2 minutes, your planned depth by 2m, or both. Write the four decompression plans out on a BSAC dive-conduct slate that you carry with you and can refer to during the dive.

Plan for and complete a safety stop. BSAC advises 3 minutes at 6m. If you get within 10 minutes of going into deco, if conditions are adverse or you have worked hard on your dive consider a longer stop. Five minutes may be more prudent.

Take one minute to get from safety stop depth to the surface - 1m every 10 seconds. This is where the greatest pressure change occurs. Other ascents should be slow as well: 10m per minute is recommended - or 1m every 6 seconds.

Monitor NDT Remaining on your dive computer. If it approaches 10 minutes remaining, and definitely if it falls under 10 minutes, ascend to a shallower depth. The NDT Remaining will increase as you ascend. Normally your NDT Remaining will decrease faster on your second dive of the day, reflecting the Nitrogen still in your tissues from the first dive.

Dive profile

The ideal standard dive profile, if possible, is a 'v' or 'tick' shape - get to your maximum depth earlier in the dive and plan to gradually ascend shallower as the dive goes on. As you ascend the profile should be working in your favour as your gas consumption reduces as you ascend, making the most of your remaining gas.

Obviously the standard plan and profile is often not possible. That's fine as it's just meant to be a starting point. It can and should be adapted by the thinking diver based on actual circumstances. You may have larger or smaller cylinders, it may be a drift dive conducted at one depth, you may be diving a circuit rather in an out and back line.  But if you know the standard then you can adjust it for different situations. If you have to start your dive swimming with the current, for example, you will not wait for the 140 bar/23 minute turn point. Instead you would turn back earlier to have more time and gas in reserve for your return swim against the current.

Route

Try to develop an accurate picture of the layout of the site: features and landmarks, depths, boundaries, and so on. Distance and direction of the site from the dive platform is useful information. If the site is unfamiliar to you or there is poor visibility and you're worried about losing your way then take a conservative approach, making sure you can follow landmarks back to the platform. Diving a hub-and-spoke or cloverleaf pattern from the line can be a useful approach at some sites. Using a distance line may be a good option if you're trained in their use.

If there is a current, ideally you would plan to swim into it at the start of the dive when you have a full cylinder and lots of energy. Ride the current back when returning to the platform, thus having it work in your favour at the end of the dive.

Adapting the plan to the situation

Maintain good situational awareness. Monitor changes in conditions and relevant events that occur during the dive, and change the plan if required. If it becomes unsafe to continue as planned, for example if visibility deteriorates, current increases or a team member becomes cold or anxious, then you should develop a new plan or terminate the dive if appropriate. Don't become a victim of 'push-onitis'!

BSAC teaches the MAPP principle: Monitor conditions and events, Analyse new information, Predict what could happen in the new situation and Plan (or change the Plan) as required for safety and enjoyment of participants. This is covered in the Dive Leader course and in the Dive Planning SDC.