Re-Evaluating My GF

[J-Vo]

Back in September I was bent in my shoulder ascending from a 200’ for 30 minutes dive. I was diving two Shearwaters set to GF 50/80 and executed all deco stops as specified. 30 minutes IWR at 20’ on O2 and an hour on O2 at the surface completely relieved my symptoms.

A buddy of mine was recently bent on a benign dive above 100’ for less than 60 minutes total run time.

A recent study showed GF Low needs to be set between 55 & 70 while GF HI needs to be set below 70 for the algorithm to remain inside of Navy safe diving limits.

These things have sparked recent conversations about GF settings inside my small group of dive buddies. I got curious and started running some numbers. I’m not sure why I haven't done this before. Probably because most of my reading on the NEDU study came before I had actually done any serious decompression diving.

The NEDU study shallow stop profile:

• 170 FSW for 30 minutes
• 40' first stop
• 174 minutes run time, air decompression

To reproduce this in Subsurface I have to use GF 100/52. Most planners will not even allow inverse GF numbers as they are considered ridiculous in general.

The NEDU study deep stop profile:

• 170 FSW for 30 minutes
• 70' first stop
• 174 minutes run time, air decompression

Interestingly, to achieve this profile I get GF 53/63. This setting seems pretty reasonable if not very conservative relative to my usual GF 50/80.

GF 50/80 Produces:

• 170 FSW for 30 minutes
• 70' first stop
• 134 minute run time, air decompression

This is way more aggressive then the NEDU deep stop profile.

Given that the NEDU profile was bending the Navy divers I feel much less bewildered by my bend in September. I was also working on the bottom (with my shoulders) and cold during deco, so no surprise.

So, where do I go from here? On top of moving my GF High lower, I'm considering significantly raising my GF Low. I may even go to something like 70/70.

How would this impact the same dive while adding accelerated deco?

GF 50/80 Produces:

• 170 FSW for 30 min on Air
• 70' first stop
• 71 min run time, 50% and 100% for deco gas

GF 70/70 Produces:

• 170 FSW for 30 min on Air
• 60' first stop (gas switch at 70')
• 73 min run time, 50% and 100% for deco gas

Almost identical run times, but 70/70 has an extra 5 minutes on O2. Gas volume requirements are identical, we simply moves 2 cuft from 50% to O2. I get similar results for 270 for 20 minutes (common for my group) with an extra 3 minutes total time, 10' shallower first stop, and 3 cuft extra deco gas requirement total.

In addition, for a while I have thought it may be best to keep GF High and GF Low equal to each other. This preserves the empirically tested deco curve of Bhuelman's ZHL-16c while simultaneously introducing a flat percentage of conservatism to the original M-values.

Thanks for reading and fire away,

 

[Johnoly]

Back in September I was bent in my shoulder ascending from a 200’ for 30 minutes dive.

Just curious & assuming you were not CCR,,can you tell us what your back gas mix or air was and what was your 1st deco bottle mix was. I'm not tech, just inquiring.

 

[caruso]

Who else checked out this thread thinking GF= Girlfriend?

 

[Fibonacci]

Who else checked out this thread thinking GF= Girlfriend?

Yeah I was all ready for some salacious gossip

 

[cerich]

all I read was the title, don't re-eval your Girl Friend on Valentines day... really

 

[rjack321]

is there altitude involved too?

air @ 170 aint helping your deco or your shoulders

 

[dmaziuk]

 

[PfcAJ]

Dude, sometimes getting bent just happens.

There’s ALWAYS a chance of getting bent. Always. You have to accept that, or quit diving. Fiddling with GFs may or may not help, and really, the changes you’re talking about are very minor on a dive like this.

My suggestions:

Get rid of the air. Replace with trimix.
Consider your thermal protection.
Consider your hydration.
Consider your post dive exertion.
Consider your exertion level on the dive.

 

[Pavao]

I’ve got bent once, shoulder also, left one. 174’ max depth with multilevel profile to ~130’, 30 minutes bottom time, didn’t realize( or maybe accepted) I was bent until about 3 hours later, minor pain just kept getting more and more annoying, took some iboprufen and my O2 bottle home, laid on the couch watch TV and was fine after ~1 hour.
In my case it was pretty obvious what caused it. I bought my spergun down, held it on my right hand and was just yanking real hard on the decent line, like a big pull and glide down as much as I could with the left hand. Same profile, gases, deco, run time as many other times before

 

[Hoag]

Not trying to side track the thread, but sometimes the same acronym is used for different (and totally unrelated) meanings. At first, I read the title of this thread as "Re-Evaluating My Girl Friend" and thought WTF.

Sorry. Back to our regular scheduled programming.