As someone who teaches scuba diving, one of my pet peeves has always been the tendency of some instructors to tell students what they should do while at the same time neglecting to tell them how to do it.
"Plan your dive and dive your plan" ... we've all heard it, but what does it really mean?
At a conceptual level it implies that before getting in the water, you should have an idea of where you're going, who you're going with, how long you're going to be there, and how you're going to get back.
It's no different, really, than taking a trip in your car. You usually don't just jump in the car, drive around, and head back when you feel like it. You have a purpose, and you ... at some level ... plan before you go how you're going to use the car to achieve that purpose.
With diving, "planning your dive" at a minimum should involve determining how deep you want to go, how long you intend to stay, and how much air you will need in your tank when you begin your ascent. "Diving your plan" means, simply, that once you've determined the above ... impose enough self-discipline on yourself to stick to what you had decided to do.
"End the dive with 500 psi" ... that's another one of those things they always tell you without giving you a clue how to do it. Well, it's also part of dive planning, and you should be thinking about your air supply before you ever get in the water ... not waiting till you're swimming around and thinking about how much you want to have when the dive's over.
Gas management is one of the most important skills not taught by the major agencies. You would never consider taking your car across a wilderness ... knowing that there are no gas stations along the way ... without having a pretty fair idea of whether or not you had enough gas to make it. And yet that's exactly what scuba divers often do when they jump in for a dive without knowing how much air they breathe, on average, and whether or not the air in their tank is sufficient for the depth they plan to go to ... or the amount of time they intend to stay. Think about it ... the air in your tank is all that you will have available until you get back to the surface ... wouldn't it be great to be assured that you're not going to run out before you've completed your dive?
There are straightforward ways to determine that ... although at first it involves some measuring and some arithmetic. Once you know your consumption rate, though, you can actually plan a dive based on the amount of air you HAVE ... rather than having to abort the dive when you notice your pressure gauge getting low.
Many divers get really fascinated with depth ... some chasing "personal best" depth records without really understanding either the risks involved or the preparation needed to go to that depth safely. Understanding how much gas you consume, and how that consumption relates to depth, will help you determine whether or not your depth goals are realistic ... or even safe. It's all part of the "plan your dive" thing.
I use a rule of thumb for my new diver students ... don't go deeper than the cubic feet of gas your tank will hold. In other words ... if you're diving an AL80, limit your dive to 80 feet. Sure, you CAN go deeper ... but it'll be either a short (bounce) dive or you'll be risking running out of air before you can complete your ascent safely.
Planning your dive also involves considering your own skill and training level, and that of your dive buddy. Ask yourself if it's adequate for the dive you want to do. What's your comfort level in the water? Can you hold a safety stop without having to hang onto something? Do you need to kick your fins in order to hold your position in the water? Are you comfortable in current ... or surge? Make sure you get some information on the type of conditions you're likely to encounter, and ask yourself if you're ready for those conditions ... and be honest with yourself, because we all have a tendency to think we're more skilled than we actually are.
Another part of planning ... how am I getting out of the water? Am I diving from a boat or from a shore site? If a boat, do I need to get back to the anchor line or can I just surface anywhere and the boat will pick me up? If from shore, do I need to exit at a particular place? What kind of hazards (e.g. current, waves, boat traffic) might I anticipate on my exit? All of this must be considered in the "plan your dive" phase ... never get into the water without first having determined how you're going to get out.
Planning your dive, really, means thinking about all of those things. Determine ... before you ever get in the water ... what type of dive you want to do, and ascertain that it's within your capability. Consider your skills and equipment, and those of your dive buddy, and determine that they are adequate to do the dive with a relative amount of safety and comfort ... diving's all about having fun, after all ... and terrorizing yourself with the unknown isn't much fun for most of us.
Diving your plan is the execution part ... don't go deeper, don't stay longer, don't go off on a whimsical junket ... and begin your ascent when your gas plan had determined that you should. There's a great peace of mind that comes from knowing ... because you thought about it in advance ... that you can safely handle the dive you're doing. It helps you relax ... and that helps you enjoy the dive more. And enjoyment, after all, is why we dive.
... Bob (Grateful Diver)