Where Is Your GF?

[PfcAJ]

For people doing dives with longer TTS, it would also be interesting to know what gas is being used vs what gas is planned with.

I just put in the gases that I'm using. GFs are 60 or 70/85, TTS 400+mins.

I don't change any of these parameters for shorter dives.

 

[stuartv]

Typical stuff for me is 50-100 TTS. I run 50/85, though will typically extend my last stop so that my surfacing GF is below 80. I do put a lower helium % in my computer though.

You're saying you put a lower helium number in your computer than what you are actually breathing?

Is that to reduce "the helium penalty" that the dive computer would apply to your deco stops on your ascent?

 

[njdiverjoe]

You're saying you put a lower helium number in your computer than what you are actually breathing?

Is that to reduce "the helium penalty" that the dive computer would apply to your deco stops on your ascent?

Yes that's what I do -- and I obviously don't recommend anyone go give it a try because they read about it online.

 

[Jay]

Using @EFX' latest sheet, Adding in a 5min 5m SS to a 30m dive on air for 18min (GFs 100), produces a 10% 7% SurfGF/%AoM pickup (85%-78%). (link to the s.sheets: Dive Calculation Spreadsheet)

I wonder if the SS was skipped, how much of a longer SI would it take to get the GF back down to the same level?

Here's the 2 s.sheets:

 

[EFX]

Using @EFX' latest sheet, Adding in a 5min 5m SS to a 30m dive on air for 18min (GFs 100), produces a 10%SurfGF/%AoM pickup. (link to the s.sheets: Dive Calculation Spreadsheet)

I wonder if the SS was skipped, how much of a longer SI would it take to get the GF back down to the same level?

Here's the 2 s.sheets:

Thanks jay for mentioning my spreadsheet. I think it is a useful tool for exploring the types of scenarios you are studying: SurfGF (%AoM), the impact of GF's on dive profiles, etc. I entered your dive profile with the safety stop set with a SI of 10080 minutes (7 days). It matched your %AoM of 85% after the ascent to surface. How long of a SI is required to bring the %AoM from 85 to 78 (the same profile without the safety stop)? 5 minutes is what I got using your safety stop profile and decreasing the SI until the %AoM equalled 78%.

Such a short SI was a surprise for me. I started with 4 hours (240 minutes) and worked down from there. This demonstrates how quickly the change in pressure occurs in the last 5 meters and the associated increase in offgassing rate.

 

[rddvet]

You're saying you put a lower helium number in your computer than what you are actually breathing?

Is that to reduce "the helium penalty" that the dive computer would apply to your deco stops on your ascent?

I was taught by my instructor that for 30/30( a nice mix for some of our caves) just consider it 32%. Of course that was after an exhaustive course and running over the difference between mixes with and without helium and running numbers on a computer. He didn’t say that’s what we should be doing, but he did say that’s what many are safely doing. I also know a few people that drop the helium from their computer to avoid it affecting deco because many don’t believe in the helium penalty.

 

[Jay]

Thanks jay for mentioning my spreadsheet. I think it is a useful tool for exploring the types of scenarios you are studying: SurfGF (%AoM), the impact of GF's on dive profiles, etc. I entered your dive profile with the safety stop set with a SI of 10080 minutes (7 days). It matched your %AoM of 85% after the ascent to surface. How long of a SI is required to bring the %AoM from 85 to 78 (the same profile without the safety stop)? 5 minutes is what I got using your safety stop profile and decreasing the SI until the %AoM equalled 78%.

Such a short SI was a surprise for me. I started with 4 hours (240 minutes) and worked down from there. This demonstrates how quickly the change in pressure occurs in the last 5 meters and the associated increase in offgassing rate.

Thanks - very interesting indeed.

 

[DeepSeaExplorer]

I noticed before I switched to a Helium computer that just putting in the O2 content in a Nitrox only Nitek computer came out very close to the tables.

Once I got the Nitek HE, I was shocked at the Helium penalty, an additional 45 minutes on one dive. I actually sent it back thinking it was defective. I got a nice 2 page letter from Lemar explaining the error of my ways. However, I couldn’t use the computer in Helium mode after that because it gave much longer runtimes than the group I was diving with.

 

[Jay]

Using @EFX' latest sheet, Adding in a 5min 5m SS to a 30m dive on air for 18min (GFs 100), produces a 10% 7% SurfGF/%AoM pickup (85%-78%)

How long of a SI is required to bring the %AoM from 85 to 78 (the same profile without the safety stop)? 5 minutes is what I got using your safety stop profile and decreasing the SI until the %AoM equalled 78%.

Adding a 5min SS is the same as an extra 5min SI. This seems very odd.

 

[EFX]

The reason it only takes 5 minutes of SI to bring the %AoM down from 85% to 78% is the CTC is 6 which has a HT of 38.3 minutes. This is a fairly fast compartment so it doesn't take long to offgas. Increasing the SI to 15 min moved the CTC to 7, 30 min to 8, and 40 min to 16 the slowest compartment.