Dive Planning
ODIGTML
This system was designed by Terrence N. Tysall, Co-owner of Benthic Technologies, this is an important process for all dives, not just the technical dives. A dive plan should be made for each and every dive we do. From a reef dive to that deep, multi-gas, exploration dive. Setting up a dive plan can take as little as a few minutes for a simple dive or longer for the more complex ones.
Oxygen – Here you will determine your CNS, whole body exposure, and 02 clock. NOAA has exposure tables for PPO2 that can be found on the web or in the NOAA diving manual. If you figure out that your CNS clock is less than 100%, you are good to dive. If you are diving within the recreation limits, then just stay with in the no decompression time. When you progress to the deeper and more complex dives, you have to take into account the different Oxygen percentages and what effect that would have on the body for a day, week, what ever amount of time you will be diving.
To determine CNS exposure, you must find the particle pressure of oxygen for the desired depth. This means you must pick a gas that has max pp02 of 1.6. You must also figure in your descent and ascent times. If you are planning multiple dives in a single day, remember that your CNS exposure decreases by 50% every 90 minutes (remember Residual Nitrogen Time +Actual bottom Time = Total Bottom Time)
If you are planning deeper and more complex dives, you have to start tracking your OTUs (Oxygen Toxicity Units). Dr. Bill Hamilton came up with the REPEX method to help track a person’s OTUs. Unlike CNS, OTUs do not decrease during a standard surface interval, but will start to decrease after 12 hours of not diving. An example is when a diver takes an “air break” if he/she is decompressing from pure 02.
Decompression – This is the part of the dive plan where you will plan what gases to use, what decompression gases to use. What Maximum Operating Depth (MOD) of the gases are, what you want to keep the PPO2 at. This will help you figure out if you will need stages or multiple gas mixes. To do this you can any of the number of table
Inert Gas Narcosis- Inert gas simply means what narcosis level you will have at depth. If you are diving a deep wreck, you are going to want to use a Tri-Mix that will have EAD (Equivalent Air Depth) that you are comfortable with. You need to know what you are comfortable with before the dive.
Gas Management – Gas management deals with several issues. First off what is your RMV while you are diving, when are you going to turn the dive (i.e. 1/3 or ½ plus 200), how much gas you and your team have in the tanks you are diving with.
You can calculate your RMV by diving at different depths with varying degrees of work for a specific time period. Find out how much gas you have consumed and use the following formula:
RMV: (gas consumed/Pt)/Time
Pt – The total pressure in ata.
Now we know what are RMV is. Next, we need to figure out when we will turn the dive. If you are diving in an overhead environment, it standard practice to turn at 1/3 of your total gas volume. This will leave you with 1/3 to exit and 1/3 for emergencies (out of air situation).
Next you will have to figure out how much gas you are going to need for your planned dive. To do this, you will use the following calculation:
RMV at the planned Depth (in ata.) X Time at Depth
Once you determine the amount of gas needed, you will need to find out how much gas you actually have in the tanks you are using. To do this, you will need to find the baseline for your tank. The baseline is the amount of gas you have in psi. An example would be at set of LP 104s. They are rated to 2640; divide 104/2640 and you have your baseline. (If you are using doubles, you will take your answer and multiply it by 2) This never changes and different tanks have different baselines. Now take your baseline and multiply it by the amount of gas you actually have in your tanks.
Now you know how much gas you have in your tanks, if all members are diving the same size tank, you have completed your Gas Management phase. If not, you will have to calculate dissimilar turn pressures. The person with the smallest tank will determine the turn pressure.
Once you have completed that you are all set. That is why it is a good idea for you and your buddy to dive the same type of tanks.
Thermal – Thermal deals with what type of exposure suit you are going to be using. If it’s a drysuit, you also have to decide what type of undergarments you will be using. Will you have to include a dry hood, dry gloves, thick underwear, electric underwear? If it’s a wetsuit, will you include a hood, gloves, hooded vest? Or are you just going to use a Dive Skin? These questions that you are going to have to ask yourself: What is the water temperature? What depth will we be at? Time at depth? How cold is the water on the surface? The bottom?
Mission – The mission can be as simple as watching the colorful reef fish on a shallow dive. It does not have to be a complex series of gas shifts, and taking water samples, but it can be. Every time we jump into the water we have an idea of what we are going to do (i.e. look at a shipwreck), but it’s a good idea to let others know what that mission is and if Mr. Murphy is going to strike, having a plan helps quite the “monkeys” in your head
Logistics – Logistics covers what you will need to set up and complete your dive. This pertains mostly to expeditions to areas where diving gases, compressors, even a chamber will need to be transported along with the tons of tanks, BCs, and drysuits.
Once you have gone over all of these items, you have created a dive plan that will work for every type of dive you do.