Chronicle of an older diver

On my second Oriskany dive, we had a discussion prior to the dive about the profile. We thought the deco requirement was pretty long, and seemed unnecessary.

The first profile was something like this, for a dive with 40 minutes of bottom time, including descent. Bottom gas is air, and deco gas is EAN50

I decided to try the same profile, changing the gradient factors from 30/70, to 30/85. It had a fairly dramatic effect on the dive. I looked at the boat’s divemaster and said “that was easy” and he seemed to agree that it was a reasonable thing to do.

Now it was 72 minutes, a full 11 minutes shorter. Of course, it’s the same dive, so we’re increasing risk by shortening the time, so why would there be two such far different profiles, and are they both right? The way the original Bühlmann algorithm worked was in effect with the gradient factor at 100/100, which results in an even shorter profile. Now we’ve shortened it by another 11 minutes, to 61, with only 24 minutes of decompression stops. If you’re wondering, using a 10 foot stop instead of the 20 foot stop shortens the dive by a mere two more minutes.

So gradient factors have a big effect on the dive time. For an explanation of how gradient factors work, check out Clearing up the Confusion about Deep Stops, by Erik Baker, who came up the concept.

Some things I’ve come to consider in my diving:

1. I feel better about adding conservatism to the basic Bühlmann algorithm.
2. I’ve seen comments regarding additional conservatism for repetitive dives, so I’m thinking that on dive #1 I’d go with up to 90 on the second (shallowest stop) gradient factor, and I’d subtract 5 for each subsequent dive.
3. The first factor controls deep stops. Lowering it to 10 only adds a couple of minutes to a profile like this one and deep stops are a good idea, I think, so I’m going to consider lowering this factor to 10 or 15 to encourage deeper stops, although one effect of this is to make some of the shallower stops more aggressive.
4. I’m going to continue to use my computer to compare my dive against the plan like I did with Oriskany dives #1 and #2. This is good discipline, I think.

None of these ideas are necessarily right, and I imagine I’ll further change or refine them over time.