Using GF99 and SurGF

[JohnN]

This popped up in my Chrome feed

Excellent article about Shearwater computers and GF99 and SurGF

Shearwater Dive Computer Settings: Become A GF Power User!

An illustrated guide to getting the best out of your Shearwater dive computer settings. Don't be intimidated by the advanced display options!

scubatechphilippines.com

 

[Bazzathemammoth]

Great article. I recently bought a peregrine and it’s been a bit of an eye opener with regards to how decompression theory works in practice. It really showed me how great the increase in depression stress is during the last few meters of ascent.

That article will help me to figure out a safer way to dive for sure. I think I’m going to do my safety stops shallower in general, and try and slow my ascent down even more. I used to ascend way too fast before I got a dive computer (and I was originally taught 18m/min).

 

[BoltSnap]

@boulderjohn

Interesting, you should study

"...every dive is a decompression dive; even if you stay within no-stop diving limits."

 

[boulderjohn]

So are you saying that because someone else is misusing language, that should lead me to misuse it as well?

The second sentences above should have been written "However, every dive requires decompression, even if you stay within no-stop diving limits.

Note that I did not say anything about the punctuation errors in the quote.

 

[davehicks]

This popped up in my Chrome feed

Excellent article about Shearwater computers and GF99 and SurGF

Shearwater Dive Computer Settings: Become A GF Power User!

An illustrated guide to getting the best out of your Shearwater dive computer settings. Don't be intimidated by the advanced display options!

scubatechphilippines.com

Thanks for sharing this article. It's really well done and very informative. I was already well aware of the SurfGF functions, but I got a better understanding of the usefulness of GF99 while reading. The tip about optimizing your safety stop depth to get GF99 into a effective value to speed offgassing is golden.

Highly recommended!

 

[Posinator]

This popped up in my Chrome feed

Excellent article about Shearwater computers and GF99 and SurGF

Shearwater Dive Computer Settings: Become A GF Power User!

An illustrated guide to getting the best out of your Shearwater dive computer settings. Don't be intimidated by the advanced display options!

scubatechphilippines.com

That's a keeper! Thanks for sharing. As just an AOW diver (and Shearwater fanboy) right now I find myself having to go back a refresh my understanding of this from time to time. Here's a detailed video you may find interesting as well.

 

[Brett Hatch]

Nice article, thanks for sharing. I find the last example somewhat confusing:

I think I understand what he's trying to say here: by watching your SurfGF, you can decide the length of your safety stop dynamically. And by watching the GF99, you can select the best depth to do so. In some cases, it will make sense to do less than 3 minutes @ 5m, or even skip it, and in other cases it will make sense to draw it out. This example illustrates the latter, where the diver chooses to hang around for a few extra minutes.

The problem I have here is with the three SurfGF numbers: 85%, then 79% after 3 minutes, then 70% after another 3 minutes, while maintaining the same depth. So, the the second 3 minutes more effective than the first 3 minutes. That does not seem possible to me, I would expect the SurfGF change to be dramatic at first, then slow down, then eventually remain constant as GF99 asymptotically approaches zero. Am I missing something here, or is this maybe an artifact rounding, or of choosing made-up numbers?

Thanks,
- Brett

 

[scubadada]

Nice article from our buddy, Andy Davis, banned from SB several years ago. Funny the Teric was not included. There has been discussion of this topic on SB, appears to have garnered little attention.

Here is a no stop profile from the Shearwater Cloud. The GF99 does not start to increase until part way into the ascent. It reaches 54% at the safety stop and is 36% at the end of the safety stop. Over the final ascent, over a minute, it blossoms to the surfacing GF of 78%. This was my expectation by following my SurfGF.

I do not follow GF99 on my home screen but include SurfGF and use it to accurately control my surfacing GF. I'm with Andy Davis, use your Shearwater computer for safer dives. Slow your final ascent to control the rapid increase in GF99 at the end of the dive.

 

[tridacna]

So are you saying that because someone else is misusing language, that should lead me to misuse it as well?

Note that I did not say anything about the punction errors in the quote.

I assume you meant “punctuation” and not “punction”. Or maybe you were led to misuse?

 

[iointerrupt]

The thing about SurfGF, is it's tempting to think of it as a linear number, but there is a ton of complexity hidden underneath it.

Specifically, there are 16 compartments, and SurfGF is calculated from the 'leading' compartment. The leading compartment changes over the course of the dive, and thus, how SurfGF behaves changes over the course of the dive.

Purely in theory, if a middle compartment is leading, but off-gasses enough to make a fast compartment leading, the rate of change could be as the example shows, slower then faster.

Is that a likely situation to happen during a typical recreational dive? I don't know, but I wouldn't worry too much about it either. Dives can have vastly different profiles, resulting in vastly different tissue compartment states. Examples are rarely going to line up with what you see on your own particular dives.