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Gradient Factors May 4, 2009

Posted by Chris Sullivan in Technical Diving.
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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

Depth Time O2% He% Start End
100 3 21 0 2 3
180 7 21 0 4 11
150 10 21 0 12 22
120 10 21 0 23 33
70 1 21 0 35 36
60 1 21 0 36 37
50 3 50 0 38 41
40 4 50 0 41 45
30 5 50 0 45 50
20 32 50 0 51 83
0 83

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.

Depth Time O2% Start End
100 3 21 2 3
180 7 21 4 11
150 10 21 12 22
120 10 21 23 33
70 1 21 35 36
60 1 21 36 37
50 1 50 38 39
40 4 50 39 43
30 4 50 43 47
20 24 50 48 72
0 72

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.

Depth Time O2% He% Start End
100 3 21 0 2 3
180 7 21 0 4 11
150 10 21 0 12 22
120 10 21 0 23 33
50 1 50 0 36 37
30 2 50 0 37 39
20 21 50 0 40 61
0 61

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.

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Comments»

1. Shearwater Research Inc. - May 16, 2014

The Gradient Factors is a big help in terms of diver’s safety. like, preventing decompression sickness and any ailments which can occur in the divers.

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2. Chris Sullivan - August 1, 2014

Hi Patrick,

I most definitely agree. Shearwater computers are very popular at my dive club. I’m glad to say I was customer #1 for a Pursuit (now converted to a Predator) in the club and I’ve been delighted with it. I’ve known some people who will set their computers to GF 90/90 to make it equivalent to a recreational computer so it doesn’t go into deco, then add a 3 minute “safety stop”, even though at 30/85 on the same dive they’re not going to get more than 3 minutes of deco anyway.

What I really like about the computer is the GF99 display. I’ve never had recourse to use it, but knowing I can balance decompression risk against other dive safety factors (like hypothermia, low air supply, or dangerous environment) is reassuring.

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