"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.

We need to convert this into a practical dive plan that will help to keep our team safe. We need to manage four things: our dive time, our breathing gas, our decompression and our dive profile and route.

Plan Dive Time

Work backwards. The skipper gives you Total runtime - the latest you can back on the surface.

BSAC dive time

You'll usually need 3 minutes for a safety stop and always take 1 minute to ascend to the surface. So deduct 4 minutes from the total time to work out the latest runtime to be starting your safety stop e.g. 50 minutes - 4 minutes = 46 minutes. (The time to your first stop is the BSAC dive time.)

Latest turnaround time Divide your BSAC dive time by two and you have your latest Turnaround runtime e.g. 46 minutes / 2 = 23 minutes. This is when you should turn around and head back if you're doing an out-and-back dive. Start ascent time In this example you should start your ascent from the bottom at around 44 or 45 minutes. This gives you a couple of minutes to ascend slowly to 6m. What if you're not back at the line? Even if you're not back at the line you must still ascend, complete your safety stop and surface. If you stay down and keep looking for the line you may trigger a lost-diver incident!

Plan Breathing Gas

Minimum gas remaining

The skipper has told you to be back on the boat with no less than 50 bar. This is the normal Minimum gas remaining for commercial dive operators.

BSAC, however, uses the Rule of Thirds. Assuming that you begin your dive with 210 bar in your cylinder, this means having a larger minimum gas reserve of 70 bar on the surface.

Remember: the gas reserve is for your buddy in case of an out of gas emergency. If you run down your gas supply you will not have enough gas for you and your highly-stressed, out-of-gas buddy if you need to share your gas supply!

Minimum stop gas

Most divers use 10-20 bar during their safety stop and ascent. The minimum gas you should have at the start of your safety stop, therefore, is 80-90 bar. Note that you're diving in a team, so this applies to the team member who reaches 80-90 bar first, even if the other diver has more gas remaining.

Start-ascent gas

If safe and appropriate to do so, start 'shallowing up' your dive and swimming even closer to the exit point 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.

Turnaround gas

In an out-and-back dive, using the Rule of Thirds, you should turn around when the first member of your team hits the 2/3 of their 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. Even if you are not conducting an out-and-back dive and don't actually turn around, you should generally not be diving deeper or further away from the exit point. You need enough gas (50-60 bar) to get back to the line and ascend to 6m to conduct your safety stop.

Plan Decompression

Use BSAC tables or your dive computer to estimate the maximum no-deco dive time that you will have for your planned dive.

BSAC tables

If you're using BSAC tables, 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.

Safety stop and ascent

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 safety stop. Five minutes may be more prudent. You will need more gas to complete a longer stop.

Take 1 minute to get from safety stop depth to the surface - this is 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. NDT remaining Monitor No-Deco Time (NDT) Remaining on your dive computer. This tells you how much time you have before you will 'go into deco' at your current depth. If it approaches 10 minutes NDT-Remaining, and definitely if it gets to 5 minutes, ascend to a shallower depth. The NDT-Remaining will increase as you ascend. Your NDT-Remaining will decrease faster on your second dive. This reflects the Nitrogen still in your tissues from the first dive.

Plan 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 move shallower, the profile works in your favour as your gas consumption reduces, making the most of your remaining gas.

Obviously the standard plan and profile may not be possible. That's fine - it can and should be adapted to 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 would 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.

Plan Route and Pilotage

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 and compass bearings back to the platform. Diving a hub-and-spoke or cloverleaf pattern from the line can be a useful approach as you return to the line several times during the dive and don't get too far away.

Current

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. As mentioned previously, if you must start your dive swimming with the current, turn back earlier to ensure you have more time and gas for your return swim against the current.

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. 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.