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Enriched Air Diver Specialty June 24, 2009

Posted by Chris Sullivan in Training.
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I was supposed to teach an Enriched Air Diver class tonight. Unfortunately it was cancelled as the students did not commit to showing up (presumably they’ve paid and were studying all the while). I’d done a fair amount of prep for the class and will now have to wait until it is rescheduled. I had written the following in anticipation of my first real diving class.

To make the experience real, I designed a simple scenario. Last week I dived the Kinghorn and Daryaw wrecks in the St. Lawrence river which are 95′ and 90′ depth respectively. Using the air RDP table, the maximum bottom time on the Kinghorn is 20 minutes, and after a 45 minute surface interval there adjusted NDL is a mere 12 minutes with a residual nitrogen time of 13 minutes.

The eRDPml, PADI’s new calculator that is similar in function to “the Wheel”, takes advantage of 5 foot depth  increments to provide an NDL on the Kinghorn of 22 minutes, but curiously this reduces the ANDL on the Daryaw to 10 minutes. By limiting the Kinghorn to 20 minutes as with the previous paragraph, the eRDPml gives a single extra minute of bottom time to the Daryaw dive with 13 minutes total.

So what about Nitrox? There are a bunch of ways to calculate limits when diving Nitrox. There are standard RDP tables for EAN32 and EAN36. You can also use either the RDP or the eRDPml using equivalent air depth (EAD), which adjusts the depths for the calculations to one with the same partial pressure of Nitrogen as when diving with air. The EAD can be calculated with a formula or by using the EAD table, which will introduce additional rounding errors into the calculations because of the 10 foot increments.

Finally, you can use standard mixes, or use a formula to calculate the best mix for a given depth. With these dives, it gives a very modest advantage to do that. So there are a plethora of options as follows:

  1. Use custom RDP tables, which are limited to EAN32 and EAN36
  2. Use the EAD table with the air RDP table
  3. Use the EAD table with the eRDPml
  4. Use the EAD formula with the RDP table
  5. Use the EAD formula with the eRDPml

For options 2-5, you could either use a standard mix or a custom mix. Of course, most  people would just use a dive computer, and PADI has just announced that the Enriched Air Diver course will use the dive computer as the primary method of calculation, with the RDP methods as an option. The Open Water course is going that way as well, so the tables will eventually be a distant memory.

Just for fun I’ve calculated the dives based on 20 minutes on the Kinghorn to see what the maximum time on the Daryaw would be. With air, we’ve got 12 and 13 minutes. What will it be with Nitrox? I’m going to use the depths of the wrecks as if they were seawater. When I’m doing my own dive planning, I sometimes take advantage of the lower density for fresh water diving, but I’m not going to encourage my students to push the limits to that extent.

1. Custom RDP tables

Both wrecks can be dived with EAN36. You can’t get that information directly from the tables, however, because they are laid down in 10 foot increments. This means that for the Kinghorn you need to use 100 feet as the planning depth, finishing the 20 minutes in repetitive group K. If we used the EAN36 table instead, in the knowledge that at only 95 feet we’re still within our maximum PP02 of 1.4 atmospheres, we come out at repetitive group H. After 45 minutes we’re at E and C respectively.

For the Daryaw at 90 feet, there’s no question that we can use EAN36. This allows 25 or 28 minutes of adjusted no decompression limit, depending on whether we used EAN32 or EAN36 on dive #1. This is definitely much better than the 12 or 13 minutes we got with air.

2. EAD table with Air RDP Table

For this option, we can at least customize our mix to the depth, although the 10 foot increments on both tables will limit the precision with which we can do this. On the Kinghorn we will dive with 34% Oxygen, with which we will have an equivalent air depth of 78 feet. This too will result in us coming out of the water in repetitive group K, dropping to E with the 45 minute surface interval.

On the Daryaw, the 90′ dive will let us use EAN37 (as it’s fresh water we could actually use EAN38, but we’re using the tables here so we go with 37%). The equivalent air depth is 65 feet, which as an E diver will have an adjusted no decompression limit of 25 minutes. Not bad, considering the 10′ increments in both tables, but its at the cost of using non-standard mixes of 34% and 37% rather than getting the same dives in with the more common 32 and 36 percent mixes.

3. EAD table with the eRDPml

This will be very similar to the above, except that we get slight advantage from the fact that the EAD on the Daryaw is 65 feet, because the eRDPml uses 5 foot rather than 10 foot increments. Using this method, we get 29 minutes of adjusted no decompression limit, the longest yet.

4. EAD formula with RDP table

So lets go all the way now. In fresh water, the limits are 36% and 38% for the two wrecks, which gives EAD of 71 feet and 64 feet respectively. Diving the Kinghorn for 20 minutes gives us our old friend repetitive group K,  going to group E after 45 minutes letting us dive the Daryaw for 25 minutes. OK, so no difference here unfortunately we still get a maximum of 25.

5. EAD formula with eRDPml

Using the greater precision of the eRDPml puts us in repetitive group J after dive #1, going down to D after the surface interval. Clearly, the EAN36 table used in part one was a better bet. However, on dive 2 our adjusted NDL is 31 minutes, which is slightly better. So this method wins by a nose.

The bottom line is that Nitrox can be the difference between a really short dive a decent one at 60-100 foot depths, just like they say it is.

By the way, if you really wanted to max out the time, in example 5 you could legally cheat by assuming that the eRDPml was calibrated to salt water, and converting the EADs from fresh to salt. So 71 feet becomes 69 feet. This allows us to spend 33 minutes on the Daryaw. Ladies and gentlemen, we have a winner!

So by playing around a bit, we got 25-33 minutes time, or 12-20 minutes more than with air on the second dive. In practice, most of us are going to use our computers anyway, but the results are going to be similar with much more liberal bottom times allowed when using enriched air.



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