RedSeaDiver2
Contributor
Just wondering what the thinking is these days about riding trimix dils with say 45% helium all the way through deco? And who is switching/flushing to a nitrox dil part way up?
Who is riding trimix dil all the way to the surface and who is flushing to a nitrox dil?
- Thread starter RedSeaDiver2
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I don't see a lot of benefit to switching from a normoxic dil like 18/45. At the 20 ft stop averaging a PO2 of 1.3, the helium has already been diluted to about 10%. The time savings is only about 5 minutes on a 2 hr runtime (switching to EAN50).
On the plus side, it does allow a cell validation at higher PO2s, which is useful if you choose to periodically ramp up PO2 and let it decay down.
On the plus side, it does allow a cell validation at higher PO2s, which is useful if you choose to periodically ramp up PO2 and let it decay down.
@RedSeaDiver2 - not sure if I understand your question. What do you mean by "riding through deco?"
Ride tmx dil to 20'. At 20', I'll do an O2 flush, check the cells, and keep flushing the loop with O2 through the deco. Technically, the loop will contain nitrox as I'll off-gas into the loop. However, under normal circumstances this is as far as I'll go.
Ride tmx dil to 20'. At 20', I'll do an O2 flush, check the cells, and keep flushing the loop with O2 through the deco. Technically, the loop will contain nitrox as I'll off-gas into the loop. However, under normal circumstances this is as far as I'll go.
Dil switching from something as weak as 18/45 is rare. For 10/70 and 8/80 you'll find it more common BUT depending on the profile you can on on-gas more N2 than it's worth.
justinthedeeps
Contributor
We were doing dil switches/flushes to progressively lower He% during ascent in dive plans for 80-100(+) metres, based on results from the dive planners. It is optional, but it does affect how much deco time the planners prescribe.
It also means that at various stages you could bail out directly to a lower He% OC gas with lower risk of "ICD," in theory.
For dives on 45% He it probably doesn't matter much, some basic calcs:
At 60 metres on a 17/45 helium diluent, @ppo2 1.3, you have loop mix of ~19/44, on-gassing at ppHe of 7*0.44 = ~3.1 atm He
[ Math for loop He mix assuming (7 total atm - 1.3 ppO2 atm) / 7 atm * (0.45 He /0.83 inert) ]
During deco--say 12 metres--@ppo2 1.3 loop mix is now ~59/22, with ppHe ~2.2*0.22 = 0.48 atm. This may already be below many people's acceptable M-values; off-gassing of relevant supersaturated 'compartments' is happening.
By 6 metres, loop is 81/10, ppHe ~0.16 atm? And you can O2 flush by then (but probably don't need to)
People on CCRs with He sensors have extra fun with this?
It also means that at various stages you could bail out directly to a lower He% OC gas with lower risk of "ICD," in theory.
For dives on 45% He it probably doesn't matter much, some basic calcs:
At 60 metres on a 17/45 helium diluent, @ppo2 1.3, you have loop mix of ~19/44, on-gassing at ppHe of 7*0.44 = ~3.1 atm He
[ Math for loop He mix assuming (7 total atm - 1.3 ppO2 atm) / 7 atm * (0.45 He /0.83 inert) ]
During deco--say 12 metres--@ppo2 1.3 loop mix is now ~59/22, with ppHe ~2.2*0.22 = 0.48 atm. This may already be below many people's acceptable M-values; off-gassing of relevant supersaturated 'compartments' is happening.
By 6 metres, loop is 81/10, ppHe ~0.16 atm? And you can O2 flush by then (but probably don't need to)
People on CCRs with He sensors have extra fun with this?
J-Vo
Contributor
When you switch dill to Nitrox, your PDC immediately assumes you don't have He in the loop, but you are off gassing He into the loop. Therefore I believe it is most conservative to stay on your bottom dill. It also doesn't impact deco much.
If your switching dill and not switching your PDC, then your probably even more conservative, but you don't really no by how much. I would rather just reduce my GF High to increase conservatism.
If your switching dill and not switching your PDC, then your probably even more conservative, but you don't really no by how much. I would rather just reduce my GF High to increase conservatism.
OP
RedSeaDiver2
Contributor
Many thanks for all of the replies. My post is more about who is making a flush/switch to say 50% at 18 metres while staying on the deco plan for He dil in order to increase deco efficiency and therefore building in extra safety margin. In a way it follows on from this thread from three years ago - Dil switch off helium during deco
In our case we are looking at around +/- 2.5 hour run times, and frankly in the Red Sea being too warm before getting into the water is much more likely than being too cold towards the end of the dive so there is no push to shorten dives (baring the arrival of a grumpy OWT), but instead we want to do everything reasonably possible to reduce the chance of a DCS incident during the project.
In our case we are looking at around +/- 2.5 hour run times, and frankly in the Red Sea being too warm before getting into the water is much more likely than being too cold towards the end of the dive so there is no push to shorten dives (baring the arrival of a grumpy OWT), but instead we want to do everything reasonably possible to reduce the chance of a DCS incident during the project.
It depends on the dive.
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