What is dive planning?  Is it simply picking a location, packing your gear, jumping in, looking around and surfacing again?  Well, that may be the case for some divers, but its not really the best approach to ensure a satisfying, safe and enjoyable dive.  The underwater world is amazing, but it is also not the natural environment for us human beings and as a result, we need all sorts of specialized equipment in good working order to ensure our safety.  But, the proper gear isn't the only thing we need, we also need to manage our limited resources, primarily the breathing gas we are carrying, to let us exist underwater and safely return us (and our buddy) back to the surface at all times.  In this multi-part series, we'll discuss various aspects of dive planning that will help put you in control of your future dives.

Before we get into dive planning, I'd like to start with what dive planning is not.  You may have been on a dive charter somewhere and while listening to the dive briefing (hopefully there was a briefing) you were told to be back at a certain time with no less than 500 psi.  This is pretty common, but should NOT be mistaken for a dive plan.  I can't tell you how many times I've seen divers run out of air and need to utilize an emergency air source hanging off the boat at around 15 feet to finish their safety stop.  Add to this that if you or your buddy had an issue and were not near this emergency air source, you would not be in a position to return safely to the surface.  Issues and failures can and do happen underwater, but with a proper dive plan, they mean that the dive is over with a safe return to the surface rather than being a life threatening event.

Lets start by looking at things we want to consider as part of a dive plan: 

1. Where are we going to dive?  It may seem obvious, but knowing where you are diving and some basic aspects like weather, depth, possible current, visibility, time of day, will play a part in a comprehensive dive plan.

2. What is the goal of the dive?  Are you just looking around, do you want to find a certain feature or marine creature, are you planning on photographing something or entering a cave or wreck.  The degree of task-loading during your dive needs to be accounted for in your dive plan.

3. Do you have all the appropriate equipment to safely make the dive.  Yes - you have your tank, BC, regulators, etc., but to you have the other components necessary?  Do you have a visual (typically an SMB) and audio (whistle) signally device?  Is your thermal protection adequate for the water temperatures?  Do you have at least one cutting device and at least one light (more if you are diving at night)?  Working with your buddy to ensure that you are properly equipped for a dive is extremely important

4. Finally, how long and how deep do you plan on diving AND do you have adequate breathing gas based on how quickly you and your buddy historically consume it?  These final three aspects (depth, time and consumption rate) are of critical importance and without them, you don't have a plan.

For the rest of this series, we are going to focus on the aspect of dive planning related to depth, time and air consumption so lets get started.  

We've already said that "come back with 500 psi" is not a dive plan, so what is involved.  To begin, lets start by looking at our breathing gas consumption. There are two common methods of measuring consumption, SAC rate and RMV.  You may have heard of SAC rate before (Surface Air Consumption), which is a measure of the amount of air a diver uses on the surface in one minute and is measured in PSI.  For example, a SAC rate of 30 means that a diver consumes 30 psi per minute at 1 atmosphere of pressure.  One thing to remember, however, is that a SAC rate is dependent on the volume of the scuba cylinder being used, meaning that 30 psi per minute breathing from an 80 cubic foot tank is not the same as 30 psi per minute breathing from a 100 cubic foot tank.  Instead of keeping track of our rate for each type of tank we may use, we instead translate our consumption to RMV (Respiratory Minute Volume) which is based on the volume of gas consumed instead of being of PSI, and therefore can be used across a variety of tank sizes and configurations.

In the next part of the series, we'll look at SAC and RMV in greater detail and how to quickly calculate and use these as part of your dive planning.

Last week we discussed some of the basics of dive planning and introduced SAC rates and RMV.  This week, we'll get into air consumption in greater detail, which will setup up our future discussions on dive planning in greater detail.

Remember that both SAC and RMV were measures of the amount of breathing gas consumed by the diver at the surface, but differed what the represented.  SAC was focused on the PSI consumed while RMV was the actual volume.  So lets look at how both are calculated and how they are utilized.

As we saw, SAC rates are given in units of pressure, either psi (imperial) or bar (metric) and are specific to a particular cylinder volume (i.e. Your SAC rate for an Aluminum 80 will be different than for a Steel 80 or Steel 100).  The first question you may ask is why is SAC different for an aluminum 80 v. a steel 80?  The answer is in how tanks are manufactured using these different materials.  The volume of an aluminum tanks is generally less than their listed size (typically 77.4 or 78.2 cuft,  depending on manufacturer) while steel tanks often match their size.  With that in mind, we use the following formula to calculate SAC rate: 

Surface Air Consumption = (Pstart - Pend) / Time / ATA - Where:
Pstart = Tank pressure at the start of the dive
Pend = Tank pressure at the end of the dive
Time = Duration of the dive in minutes
ATA = Atmosphere's for the average depth of the dive (from your computer)

Example:
I start my dive at 3000 psi and dive for 20 minutes at 33 feet 

in the sea.  At the end of the dive, I have 1800 psi remaining.

SAC = (3000 - 1800) / 20 min / 2 ATA = 30 psi per minute

Different from SAC, RMV rates are expressed in cubic feet per minute (imperial) or liters per minute (metric), Unlike a SAC rate, an RMV rate can be used for calculations with tanks of any volume. RMV is commonly used by most dive planning software.  RMV relies on something called a Tank Factor to convert from SAC into RMV and is a measure of the actual volume of the tank divided by the rated pressure of the tank.  Written out, this looks like: 

Tank Factor = ft3 of cylinder / rated pressure

Using tank factor, the RMV calculation then looks like: 

Respiratory Minute Volume = Tank Factor * (Pstart - Pend) / Time / ATA  which is simplified to Tank Factor * SAC

Example: 

I’m using a single aluminum 80 cylinder rated for 3000 psi and actual volume of 77.4 cuft.  I start my dive at 2800 psi and dive for 20 minutes at 33 feet in the sea.  At the end of the dive, I have 1800 psi remaining.

RMV = ((2800 - 1800) / 20 / 2) * (77.4 / 3000) = .645

A good practice is to track your consumption for each dive.  Some computers or their associated software will perform some or all of these calculations for you.  There are also some very good mobile tools that can help you quickly calculate these values such as Deep Tools available for IOS and OSX devices.  Note: There is not good or bad SAC and RMV rates.  They are simply a measure and can vary between body types, environmental conditions, propulsion and trim techniques and a variety of other aspects.  The important thing is to track them for yourself so that you can use them to your advantage as we get into greater detail on dive planning.

In the next post, we'll start to look at some different methods of dive planning.