unexpected behaviour toggling aGF and back

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hannes
Posts: 4
Joined: Thursday 9. June 2022, 08:19

unexpected behaviour toggling aGF and back

Post by hannes »

For the alternative set of GFs I expect that the deco does not change after switching to aGF and then back to the original set.

I played with the simulator mode for a 60m TMX (18/45, NX50 and O2) dive. Being on oxygen at the 6m deco-stop, OSTC saying 6m 10' (TTS 10'). Then I switch to a less conservative set of aGF. As expected, the OSTC now indicates a shorter deco obligation (6m 6', TTS 7').
Then, I immediately switch back to the original set of GFs and now a deco-obligation of 9m, 1' is shown, which is weird as the 9m stop was cleared before.

I hope the description is sufficient to reproduce and find the problem, otherwise I can provide more details.
Ralph
Posts: 688
Joined: Saturday 24. June 2017, 11:31

Re: unexpected behaviour toggling aGF and back

Post by Ralph »

Hi,
the behaviour is as expected / programmed - although it is close to the bug or feature question. Changing the GF settings while in the dive is not covered by Erik Baker's GF concept and doing so poses a question on what to do regarding the GF low reference depth. This depth is not set by hand but calculated automatically during the dive from the GF low value and the tissue saturations.

Every time the GFs are toggled the OSTC recalculates the GF low reference depth. When toggling to higher (less conservative) values, the TTS will become smaller and one is likely be cleared for a shallower stop. When toggling back, the old GF low reference depth is calculated again, but as the tissue pressures have changed in the meantime the result will be different, i.e. shallower than before.

Moreover, as in your example, the 6 m stop is placed not with the GF low value but pretty close to the GF high value as the previous GF low ref depth was at say 21m or whatever. But now the GF low ref depth is shallower, so the current depth is closer to the GF low ref depth and with GF low < GF low the current stop will be controlled by a lower (more conservative) GF value - that is why in your example you get placed deeper to 9 m instead of staying at the 6 m stop depth.

Thus, the behaviour is as programmed. The valid question remains, if this behaviour is as expected, or if the original GF low reference depth should be memorised and recalled when toggling back.

Side remark: similar but less obvious effects happen when a dive switches from a deco dive to NDL and back into deco.

BR
Ralph
hannes
Posts: 4
Joined: Thursday 9. June 2022, 08:19

Re: unexpected behaviour toggling aGF and back

Post by hannes »

Dear Ralph,

thanks for the detailed explanation.
I (wrongly) assumed that the deco is internally simultaneously computed for the two different sets of GFs and therefore the results should be the same as before when switching to aGF and then back to the original GFs.
In my opinion it would be more intuitive if the GF low reference depth isn't changed (after once established). However, thats just my opinion.
After all, the desired behaviour (re-computing the GF low reference depth, or not) probably depends on how people use the aGF feature...

cheers
Hannes
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