Woody’s Simple Gas Planning Worksheet For Rebreather Regular Air and/or Normoxic Trimix (Maximum Depth Limit 200 Feet, O2 Minimum % For “Deep Gas” = 16%) Imperial Based
1. Set Point (planned maximum PO2 at depth) –> there are two different concepts here. There is a maximum single dive exposure and there is a maximum 24 hour exposure. Here are the numbers:
For a SINGLE DIVE and for 24 HOUR EXPOSURE, here are the maximum times by set point:
1.6 -- 45 min (single exposure) and 150 min (24 hour exposure)
1.5 -- 120 min & 180 min
1.4 -- 150 min & 180 min
1.3 -- 180 min & 210 min
1.2 -- 210 min & 240 min
1.1 -- 240 min & 270 min
1.0 -- 300 min & 300 min
2. Shallow Gas can be EAN 50 (50% Oxygen, 50% Nitrogen). Remember, this is used for bailout purposes (usually) therefore, don’t breathe it if deeper then 70 feet (at 70 feet, your PO2 on 50 EAN is 1.56).
3. Equivalent Nitrogen Depth”END” (how narced do you want to feel) is no more then 100 feet, since most people start feeling narced at 100 feet.
4. Maximum Depth for Normoxic Trimix Divers is 200 feet.
5. Minimum Oxygen % in deep gas for Normoxic Trimix Diver is 16% (the vast majority of most divers can breath 16% of Oxygen at the surface, and not get hypoxic and pass out)
6. Surface Air Consumption Rate”SAC Rate”Use the highest team members SAC rate to determine total bailout gas needed assuming a total Rebreather failure. Use a 1.5 multiple (this means we will have enough gas amount among team to get 1.5 divers to the surface from the deepest point in the dive).
7. If your dive is 130 feet maximum depth, your deep gas can be normal air and you don’t need a shallow gas. Helium is expensive. Therefore, you my only want to use Helium on dives that will be deeper then 130 feet as Helium is used to lessen narcosis.
8. Minimum equipment required for a Normoxic Trimix Dive--- surface signaling device, a reel with at least enough line that is 1.5 times your deepest depth (deep diving could require the need for Big reels), two Shearwater dive computers, knife, O2 clean regulator for the shallow gas since it’s above 40% Oxygen gas.
9. Suggested Gradient Factor “GF” of 30/70. The way to think of this is to think how close to the M line do you want to get. So, the 30 in a 30/70 means that you will make your first stop when you are 30% of the way to the M line. The 70 in 30/70 means that your last stop will be at 70% of the way to the M line. 30/70 is considered a conservative and widely adopted deco profile setting. You will need to enter this in your two Shearwater computers.
1. FN2 Formula (FN2 = fraction of nitrogen in the breathing gas). End = equivalent nitrogen depth (how narced do you want to feel)? I suggest limiting your END to 100 feet as most people start feeling “narced” at 100 feet.
FN2/.79 (which is the % of nitrogen in regular air) = (END+33)/(Depth+33)
2. Atmospheres = (Actual depth/33)+1
3. Helium percent = 100%-Oxygen%-Nitrogen%
4. SAC Rate (surface air consumption rate):
A. Swim at 33 feet for 10 minutes and determine how much PSI you use on open circuit. For example purposes, assume an 80 cubic foot aluminum tank was used and you used 400 psi.
B. Determine how much volume of air is represented by each psi in your tank. An 80 aluminum that started at 3000 psi is 80/3000=.026 cubic feet of air per psi in your tank (remember psi is pressure, and cubic feet is volume).
C. Convert the psi you used into cubic feet. 400 X .026 = 10.4 cubic feet of air.
D. Convert depth you were swimming at to atmospheres. (33/33) + 1 = 2
E. Calculate the SAC rate. 10.4 cubic feet used in the 10 minute swim / 10 minutes = 1.04 cubic feet per minute. Then 1.04 / 2 atmospheres = .52 cubic feet per minute. So your SAC rate is .52 Most people round up to be conservative,, so in this example the SAC rate would be .60 (rounded up). Enter this (.60) into your V planner and Shearwater.
5. PO2= (partial pressure of oxygen)atmospheres times % of Oxygen in the mix, PN2 (partial pressure of nitrogen)atmospheres times % of Nitrogen in the mix.
Example: Assume a dive to 200 feet, and planned time at 200 Feet (deepest point in the dive) is 30 minutes.
Gas Planning Calculation:
Step 1 – Figure out the FN2----->FN2/.79 = 100 (which is your END)+33/200 (planned depth) + 33
Cross multiply to solve for FN2 as follows:
233FN2 = 105.07
FN2 = 105.07/233
FN2 = 44%-----this means your % of nitrogen in the deep gas is 44%
Step 2 – Calculate the % of helium in the mix as follows:
1.0– 16% oxygen – 44% Nitrogen = Helium % in the deep gas
Helium % = 40%
Step 3 – Use V Planner or IDeco Pro To calculate gas needed for the team. You will need to enter: A. Set point of 1.2 (this gives you 4 hours of diving for the day),
B. Gas mixes which is 16/40 for deep gas and 50/20 for shallow gas,
C. Planned time at deepest depth – 30 minutes
D. SAC Rate – enter whatever the highest members SAC rate is in the team.
Step 4: Make sure the team is carrying enough deep and shallow gas to accomplish a bailout at the deepest point in your dive (assuming a complete rebreather failure). Ideally, you should carry enough gas on your own to accomplish this bailout as if you were diving alone. Adjust your dive time accordingly.
Step 5: Make sure you enter in Both of your Shearwater computers your deep gas mix and shallow gas mix and turn them on in the computer (and if team planning, enter the highest SAC rate of the team). Also, make sure your dive profile GF (gradient factor) is set at 30/70.
Step 6: Determine the actual gas mix you are really breathing in the loop. What are you actually breathing at 198 fsw (7 ATA) if diluent is, say, 16/40 and your set point is 1.3. Note if deep gas is 16/40 (meaning 16% oxygen, 40% helium), then nitrogen in that gas is 44% (100%-16%-40%).
A. Determine the % oxygen in the loop (this is known as FIO2, but use your PO2 formula to calculate): PO2 (partial pressure of oxygen) = Atmospheres X O2% in the mix. ATA is abbreviation for atmospheres.
Atmospheres = (198 feet/33 feet) + 1 = 7 atmospheres
Now solve for the % of oxygen using the PO2 formula as follows:
1.3 partial pressure (your desired set point) = 7 (atmospheres) X O2%
O2% = 1.3 ATA / 7 ATA = 18.57% (this is also known as your FIO2)
B. Determine % of inert gas in loop (mostly made up of nitrogen)
100% - 18.57% (oxygen) = 81.5% inert gas in loop
C. Determine the % helium in the loop by using this formula:
81.5%-->which is the inert gas in the loop x [He/(He+N2)]
Note, N2 is the 44% nitrogen (inert gas) in the deep mix which we calculated by taking (100% - 16% oxygen – 40% helium) = 44% Nitrogen (inert gas)
81.5%-->which is the inert gas in the loop x [He/(He+N2)]
He is 40, N2 (nitrogen) is 44%, so plug these into the formula above and solve for helium in the loop as follows:
81.5% X [(40/(40 + 44)] = 81.5 X (40/84) = 38.81% helium in the loop.
So, basically, at 200 feet (fsw) with a set point of 1.3 and a diluent of 16/40, you are actually breathing 18.5/38.8 (basically 19/40) this means 19% oxygen, 40% helium.
Another Example using these formulas, just for reference purposes:
At 99 fsw (4 ATA), this would be the math for the same 1.3 set point and the same 16/40 diluent. Remember, FI02 means % of oxygen.
So, oxygen in the loop at this depth really is:
FIO2% of oxygen in the loop at depth = 1.3 (set point) /4 (ATA) = 32.5
% of inert gas in the loop (nitrogen) = 100% - 32.5% = 67.5%
Now we use our formula to solve for helium in the loop as follow:
Inert gas in the loop x [He/(He+N2)] % helium = 67.5% X [40/(40+44)] = 32%. This means at 99 feet, you have 32% helium in the loop.
So, at 99 fsw (4 ATA) you are breathing 32.5/32, not 16/40.