Can I say bollocks? Never mind…
Short version - do not use NDL times to compare computer conservatism.
Long version…
I recently acquired an AI transmitter for my Perdix. It came with a free Oceanic VT3. The seller thought it was the other way round so I got “a deal”.
Yesterday, I had no buddy and so had to use a twinset rather than a rebreather - GF’s risk assessment on new cells, new drysuit and no buddy said no.
My plan was 40 minutes at 30m, about 15 minutes of backgas deco on 50/80 using 3400l of gas.
I decided to take the VT3 in as a backup computer as it was shinier than an old Zoop. I did consider taking the Zoop too as a comparison but being early season, new suit and solo I decided against it.
I had a twinset of 32% and a choice of my regular bailouts. For this dive 80% would only save maybe 7 minutes of deco and I would rather keep it full. I decided to take a 7 of 32% as my shutdown skills are rusty, new suit etc and being solo.
I downloaded the VT3 manual and went through setting the mix. That was quite nasty with a lot of long press on two buttons sort of thing. At one point I had three random mixes set. There is something special about 50% too that escapes me. The worst thing is that the picture of which button is which (A, S and M) is at the front of the manual and so there is a lot of scrolling about. This took me 40 minutes. I see that even a Perdix two button UI is better than that.
Planning mode of the VT3 (default conservatism) showed a 31 minute NDL for 32% 30m, the Perdix (40/85) 22 minutes 31% 30m. Having measured the gas on the boat I couldn’t face trying update the gas on the VT3. BSAC tables give 28 minutes surface to safety stop.
As it turned out there was a spare diver on the boat so I had a buddy. He had a twinset of 32 and a 7 of 50% so all quite well matched for an instabuddy.
So we did the dive, nothing untoward, perfectly lovely and relaxing, max 28m, mostly at about 25m. I used the VT3 as a pressure gauge (not having set the transmitter on the default screen of the Perdix yet).
Come the ascent my buddy had a couple of minutes at 6 (his Peregrine having last stop set to 6, and using a richer gas) and I had one at 6 and 4 at 3. The VT3 was showing 14 minutes at 3. We did the time at 6 to 7 for the buddy’s Peregrine and then moved up to 5 for my deco and the the 5 minute safety stop the adaptive mode of the Perdix called for. By the end of that my surface GF was about 73% so a perfectly nice number. The VT3 still wanted 8 minutes. Our max run time at this point was only 5 minutes more so there was no chance I was keeping everyone waiting, even without that I’d not want more time in 9C water pointlessly so we surfaced and bent it.
Conclusion, despite the VT3 being apparently less conservative, using NDL as a measure, than medium setting on a Shearwater GF, it is in fact more conservative in deco.
Looking back at my logs for similar dives done with either a Suunto backup to a Petrel on CCR, or dives with buddies on Suuntos I don’t generally see long hangs once my deco was clear.
Short version - do not use NDL times to compare computer conservatism.
Long version…
I recently acquired an AI transmitter for my Perdix. It came with a free Oceanic VT3. The seller thought it was the other way round so I got “a deal”.
Yesterday, I had no buddy and so had to use a twinset rather than a rebreather - GF’s risk assessment on new cells, new drysuit and no buddy said no.
My plan was 40 minutes at 30m, about 15 minutes of backgas deco on 50/80 using 3400l of gas.
I decided to take the VT3 in as a backup computer as it was shinier than an old Zoop. I did consider taking the Zoop too as a comparison but being early season, new suit and solo I decided against it.
I had a twinset of 32% and a choice of my regular bailouts. For this dive 80% would only save maybe 7 minutes of deco and I would rather keep it full. I decided to take a 7 of 32% as my shutdown skills are rusty, new suit etc and being solo.
I downloaded the VT3 manual and went through setting the mix. That was quite nasty with a lot of long press on two buttons sort of thing. At one point I had three random mixes set. There is something special about 50% too that escapes me. The worst thing is that the picture of which button is which (A, S and M) is at the front of the manual and so there is a lot of scrolling about. This took me 40 minutes. I see that even a Perdix two button UI is better than that.
Planning mode of the VT3 (default conservatism) showed a 31 minute NDL for 32% 30m, the Perdix (40/85) 22 minutes 31% 30m. Having measured the gas on the boat I couldn’t face trying update the gas on the VT3. BSAC tables give 28 minutes surface to safety stop.
As it turned out there was a spare diver on the boat so I had a buddy. He had a twinset of 32 and a 7 of 50% so all quite well matched for an instabuddy.
So we did the dive, nothing untoward, perfectly lovely and relaxing, max 28m, mostly at about 25m. I used the VT3 as a pressure gauge (not having set the transmitter on the default screen of the Perdix yet).
Come the ascent my buddy had a couple of minutes at 6 (his Peregrine having last stop set to 6, and using a richer gas) and I had one at 6 and 4 at 3. The VT3 was showing 14 minutes at 3. We did the time at 6 to 7 for the buddy’s Peregrine and then moved up to 5 for my deco and the the 5 minute safety stop the adaptive mode of the Perdix called for. By the end of that my surface GF was about 73% so a perfectly nice number. The VT3 still wanted 8 minutes. Our max run time at this point was only 5 minutes more so there was no chance I was keeping everyone waiting, even without that I’d not want more time in 9C water pointlessly so we surfaced and bent it.
Conclusion, despite the VT3 being apparently less conservative, using NDL as a measure, than medium setting on a Shearwater GF, it is in fact more conservative in deco.
Looking back at my logs for similar dives done with either a Suunto backup to a Petrel on CCR, or dives with buddies on Suuntos I don’t generally see long hangs once my deco was clear.
