93 replies to this topic

[EASY]

Until the Rebreathers for Dummies book comes out, I will enviously be sitting on the sidelines!

Whoops... Time to get off the sidelines EASY...

Unbelievable! I'll see what I can do. Maybe after I reach 100 dives!

[EASY]

No worries... keep an eye out on the Cis Lunar/Poseidon Mark 6.... it is being positioned solely for the recreational market, modular, self calibrating and dummy proof...

Thanks Tim! My eyes and mind are wide open!

[Racer184]

What is used for diluent ? nitrogen? Or does it depend on the depth?

[shadragon]

My back and my knees have been very happy that I have made this decision.

OK, dumb Q's here SDM: Whatever weight you save on the unit has to go on your belt so how is this easier on the back? Are you referring to schlepping tanks back and forth? How much weight do you dive with on the Optima?

[diverdeb]

My back and my knees have been very happy that I have made this decision.

OK, dumb Q's here SDM: Whatever weight you save on the unit has to go on your belt so how is this easier on the back? Are you referring to schlepping tanks back and forth? How much weight do you dive with on the Optima?

I think he's referring to the weight compared to using steel doubles. My doubles weigh well over 100 lbs. and that doesn't include any deco bottles I might be carrying. Needless to say, I need no extra weight.

I could lift David's RB with no problems. I can only carry my dbl 100's on my back or if I lift them from a bench. I can't lift them up from the ground to a table by myself.

Anyway, I think the weight comparison applies to a dbls scenario, but SDM can correct me if I'm wrong.

[netmage]

What is used for diluent ? nitrogen? Or does it depend on the depth?

Well, it can depend on depth... there are a few circles of thought....
In the grand scheme of things, I'm a n00b and I havn't really pushed beyond recreational ranges in my rig just yet, so mostly my diluent is some 32% nitrox I had sitting in my set of cave doubles. When I progress into some deeper dives more regularaly, and after the tortugas trip, I'll likely dump the gas and standardize on 21/35.

This does two things. It introduces helium (to address narcosis, and faster offgasing), and pushes the O2 content in diluent lower.
As the name implies, diluent is intended to dilute the O2 content in the breathing loop. Say I'm at depth at i notice my pO2 rising, my response is to introduce diluent to lower the pO2, if I'm diving a high O2% mix, depending on the depth, it may not be sufficient to dilute the mix....

i.e. introducing 32% diluent at ~120' isn't going to lower a pO2 of 1.4, in fact, it will raise it ((120/33)+1)* .32 = 1.48
Whereas using 21/35, ((120/33)+1)* .21 = .97, when flushed through the loop is sufficient to drop the pO2 significantly.

[NJBerserker]

Thanks NetMage for giving me a great idea for a question. *DISCLAIMER* I am not a technical diver. . . yet, so this question is based purely on academic study and what I've learned from others. If I put anything the wrong way or say something that is incorrect please set me straight. Anywho, since the topic of adding helium to the loop has come up I have a question about how CCR will have an effect on deco times. On open circuit there is no free lunch. You can use nitrox to limit your narcosis but must then limit your depth appropriately. Trimix allows you to go both deep and lessens narcotic effects of N2 however it greatly increases decompression times as Helium is more easily absorbed and takes more time to come out of solution in human tissues. What is the difference in the average hang time for someone using advanced mixed gas on CCR as opposed to OC?

[SquattingRadishDM]

Yeah thats what I want to know too, can you give us a rough example of the deco procedures for OC and CCR for a given depth/time? Would be good to compare.

Thanks

[netmage]

You can use nitrox to limit your narcosis but must then limit your depth appropriately. Trimix allows you to go both deep and lessens narcotic effects of N2 however it greatly increases decompression times as Helium is more easily absorbed and takes more time to come out of solution in human tissues. What is the difference in the average hang time for someone using advanced mixed gas on CCR as opposed to OC?

I would restate the above as....
You can use nitrox to limit/extend your decompression/NDL but must then limit your depth appropriately. Trimix allows you to go lessens narcotic effects of N2 however it decreases NDL times as Helium is more easily absorbed and comes out of solution in human tissues faster.

Introducing nitrox vs. air has no tangible effect on narcosis.

Helium is a smaller gas molecule, as such moves into and out of solution rapidly compared to Nitrogen. As such, the NDL will be shorter, but in theory the resulting deco should be shorter as well - theorhetically. Some algorithims do tend to penalize you for introducing helium into the equation, or show no appreciable difference in resulting profiles, i.e. a few minutes when talking about a 90-120 min dive....

One should also consider the resulting set point, OC MOD pO2 is based on 1.4, CCR MOD we drop down to 1.2 since we're constant.

Yeah thats what I want to know too, can you give us a rough example of the deco procedures for OC and CCR for a given depth/time? Would be good to compare.

sure.... here's 120' for 30...

Decompression model: VPM - B

DIVE PLAN
Surface interval = 5 day 0 hr 0 min.
Elevation = 0ft
Conservatism = + 4

Dec to 120ft (2) Nitrox 32 50ft/min descent.
Level 120ft 27:36 (30) Nitrox 32 1.48 ppO2, 99ft ead
Asc to 50ft (37) Nitrox 32 -10ft/min ascent.
Stop at 30ft 4:00 (42) Nitrox 32 0.61 ppO2, 21ft ead
Stop at 20ft 3:00 (45) Nitrox 32 0.51 ppO2, 13ft ead
Stop at 15ft 19:00 (64) Nitrox 32 0.47 ppO2, 8ft ead
Surface (65) Nitrox 32 -10ft/min ascent.

Off gassing starts at 62ft, OTU's this dive: 61, CNS Total: 26.8%

138.4 cu ft Nitrox 32
138.4 cu ft TOTAL

Dec to 120ft (2) Diluent Air 0.70 SetPoint, 50ft/min descent.
Level 120ft 27:36 (30) Diluent Air 1.20 SetPoint, 111ft ead
Asc to 50ft (37) Diluent Air 1.20 SetPoint, -10ft/min ascent.
Stop at 30ft 3:00 (41) Diluent Air 1.20 SetPoint, 0ft ead
Stop at 20ft 2:00 (43) Diluent Air 1.20 SetPoint, 0ft ead
Stop at 15ft 11:00 (54) Diluent Air 1.20 SetPoint, 0ft ead
Surface (55) Diluent Air -10ft/min ascent.

Off gassing starts at 69.4ft, OTU's this dive: 71, CNS Total: 25.4%

And here's 170' for 30 using 21/35 & 50% same conservative/SI parameters as above... only this time the CCR bumps to 1.4 for deco.

Dec to 170ft (3) Triox 21/35 50ft/min descent.
Level 170ft 26:36 (30) Triox 21/35 1.29 ppO2, 80ft ead, 99ft end
Asc to 100ft (37) Triox 21/35 -10ft/min ascent.
Stop at 80ft 2:00 (40) Triox 21/35 0.72 ppO2, 30ft ead, 40ft end
Stop at 70ft 2:00 (42) Nitrox 50 1.56 ppO2, 32ft ead
Stop at 60ft 2:00 (44) Nitrox 50 1.41 ppO2, 26ft ead
Stop at 50ft 3:00 (47) Nitrox 50 1.26 ppO2, 20ft ead
Stop at 40ft 4:00 (51) Nitrox 50 1.10 ppO2, 13ft ead
Stop at 30ft 7:00 (58) Nitrox 50 0.95 ppO2, 7ft ead
Stop at 20ft 5:00 (63) Nitrox 50 0.80 ppO2, 1ft ead
Stop at 15ft 27:00 (90) Nitrox 50 0.73 ppO2, 0ft ead
Surface (91) Nitrox 50 -10ft/min ascent.

Off gassing starts at 112.9ft, OTU's this dive: 90, CNS Total: 33.3%

156.2 cu ft Triox 21/35
53.0 cu ft Nitrox 50
209.3 cu ft TOTAL

Dec to 170ft (3) Diluent 21/35 0.70 SetPoint, 50ft/min descent.
Level 170ft 26:36 (30) Diluent 21/35 1.20 (1.29), 82ft ead, 97ft end
Asc to 110ft (36) Diluent 21/35 1.20 SetPoint, -10ft/min ascent.
Stop at 70ft 2:00 (41) Diluent 21/35 1.40 SetPoint, 7ft ead, 45ft end
Stop at 60ft 2:00 (43) Diluent 21/35 1.40 SetPoint, 0ft ead, 39ft end
Stop at 50ft 4:00 (47) Diluent 21/35 1.40 SetPoint, 0ft ead, 33ft end
Stop at 40ft 4:00 (51) Diluent 21/35 1.40 SetPoint, 0ft ead, 28ft end
Stop at 30ft 6:00 (57) Diluent 21/35 1.40 SetPoint, 0ft ead, 22ft end
Stop at 20ft 4:00 (61) Diluent 21/35 1.40 SetPoint, 0ft ead, 17ft end
Stop at 15ft 18:00 (79) Diluent 21/35 1.40 SetPoint, 0ft ead, 14ft end
Surface (80) Diluent 21/35 -10ft/min ascent.

Off gassing starts at 111.5ft, OTU's this dive: 116, CNS Total: 43.9%

Now on this one, I changed to 10/50, and introduced more helium, notice the slight penality...
Dec to 170ft (3) Diluent 10/50 0.70 SetPoint, 50ft/min descent.
Level 170ft 26:36 (30) Diluent 10/50 1.20 SetPoint, 59ft ead, 79ft end
Asc to 110ft (36) Diluent 10/50 1.20 SetPoint, -10ft/min ascent.
Stop at 70ft 2:00 (41) Diluent 10/50 1.40 SetPoint, 0ft ead, 38ft end
Stop at 60ft 3:00 (44) Diluent 10/50 1.40 SetPoint, 0ft ead, 34ft end
Stop at 50ft 3:00 (47) Diluent 10/50 1.40 SetPoint, 0ft ead, 29ft end
Stop at 40ft 5:00 (52) Diluent 10/50 1.40 SetPoint, 0ft ead, 25ft end
Stop at 30ft 6:00 (58) Diluent 10/50 1.40 SetPoint, 0ft ead, 20ft end
Stop at 20ft 4:00 (62) Diluent 10/50 1.40 SetPoint, 0ft ead, 16ft end
Stop at 15ft 20:00 (82) Diluent 10/50 1.40 SetPoint, 0ft ead, 14ft end
Surface (83) Diluent 10/50 -10ft/min ascent.

Off gassing starts at 111.2ft, OTU's this dive: 121, CNS Total: 45.9%

[NJBerserker]

Excellent answer Mage thanks. And good catch on my not including the extended NDL in my nitrox line there. The effect on narcosis is there, but very limited.

[WreckWench]

You can use nitrox to limit your narcosis but must then limit your depth appropriately. Trimix allows you to go both deep and lessens narcotic effects of N2 however it greatly increases decompression times as Helium is more easily absorbed and takes more time to come out of solution in human tissues. What is the difference in the average hang time for someone using advanced mixed gas on CCR as opposed to OC?

I would restate the above as....
You can use nitrox to limit/extend your decompression/NDL but must then limit your depth appropriately. Trimix allows you to go lessens narcotic effects of N2 however it decreases NDL times as Helium is more easily absorbed and comes out of solution in human tissues faster.

Introducing nitrox vs. air has no tangible effect on narcosis.

Helium is a smaller gas molecule, as such moves into and out of solution rapidly compared to Nitrogen. As such, the NDL will be shorter, but in theory the resulting deco should be shorter as well - theorhetically. Some algorithims do tend to penalize you for introducing helium into the equation, or show no appreciable difference in resulting profiles, i.e. a few minutes when talking about a 90-120 min dive....

One should also consider the resulting set point, OC MOD pO2 is based on 1.4, CCR MOD we drop down to 1.2 since we're constant.

Yeah thats what I want to know too, can you give us a rough example of the deco procedures for OC and CCR for a given depth/time? Would be good to compare.

sure.... here's 120' for 30...

Decompression model: VPM - B

DIVE PLAN
Surface interval = 5 day 0 hr 0 min.
Elevation = 0ft
Conservatism = + 4

Dec to 120ft (2) Nitrox 32 50ft/min descent.
Level 120ft 27:36 (30) Nitrox 32 1.48 ppO2, 99ft ead
Asc to 50ft (37) Nitrox 32 -10ft/min ascent.
Stop at 30ft 4:00 (42) Nitrox 32 0.61 ppO2, 21ft ead
Stop at 20ft 3:00 (45) Nitrox 32 0.51 ppO2, 13ft ead
Stop at 15ft 19:00 (64) Nitrox 32 0.47 ppO2, 8ft ead
Surface (65) Nitrox 32 -10ft/min ascent.

Off gassing starts at 62ft, OTU's this dive: 61, CNS Total: 26.8%

138.4 cu ft Nitrox 32
138.4 cu ft TOTAL

Dec to 120ft (2) Diluent Air 0.70 SetPoint, 50ft/min descent.
Level 120ft 27:36 (30) Diluent Air 1.20 SetPoint, 111ft ead
Asc to 50ft (37) Diluent Air 1.20 SetPoint, -10ft/min ascent.
Stop at 30ft 3:00 (41) Diluent Air 1.20 SetPoint, 0ft ead
Stop at 20ft 2:00 (43) Diluent Air 1.20 SetPoint, 0ft ead
Stop at 15ft 11:00 (54) Diluent Air 1.20 SetPoint, 0ft ead
Surface (55) Diluent Air -10ft/min ascent.

Off gassing starts at 69.4ft, OTU's this dive: 71, CNS Total: 25.4%

And here's 170' for 30 using 21/35 & 50% same conservative/SI parameters as above... only this time the CCR bumps to 1.4 for deco.

Dec to 170ft (3) Triox 21/35 50ft/min descent.
Level 170ft 26:36 (30) Triox 21/35 1.29 ppO2, 80ft ead, 99ft end
Asc to 100ft (37) Triox 21/35 -10ft/min ascent.
Stop at 80ft 2:00 (40) Triox 21/35 0.72 ppO2, 30ft ead, 40ft end
Stop at 70ft 2:00 (42) Nitrox 50 1.56 ppO2, 32ft ead
Stop at 60ft 2:00 (44) Nitrox 50 1.41 ppO2, 26ft ead
Stop at 50ft 3:00 (47) Nitrox 50 1.26 ppO2, 20ft ead
Stop at 40ft 4:00 (51) Nitrox 50 1.10 ppO2, 13ft ead
Stop at 30ft 7:00 (58) Nitrox 50 0.95 ppO2, 7ft ead
Stop at 20ft 5:00 (63) Nitrox 50 0.80 ppO2, 1ft ead
Stop at 15ft 27:00 (90) Nitrox 50 0.73 ppO2, 0ft ead
Surface (91) Nitrox 50 -10ft/min ascent.

Off gassing starts at 112.9ft, OTU's this dive: 90, CNS Total: 33.3%

156.2 cu ft Triox 21/35
53.0 cu ft Nitrox 50
209.3 cu ft TOTAL

Dec to 170ft (3) Diluent 21/35 0.70 SetPoint, 50ft/min descent.
Level 170ft 26:36 (30) Diluent 21/35 1.20 (1.29), 82ft ead, 97ft end
Asc to 110ft (36) Diluent 21/35 1.20 SetPoint, -10ft/min ascent.
Stop at 70ft 2:00 (41) Diluent 21/35 1.40 SetPoint, 7ft ead, 45ft end
Stop at 60ft 2:00 (43) Diluent 21/35 1.40 SetPoint, 0ft ead, 39ft end
Stop at 50ft 4:00 (47) Diluent 21/35 1.40 SetPoint, 0ft ead, 33ft end
Stop at 40ft 4:00 (51) Diluent 21/35 1.40 SetPoint, 0ft ead, 28ft end
Stop at 30ft 6:00 (57) Diluent 21/35 1.40 SetPoint, 0ft ead, 22ft end
Stop at 20ft 4:00 (61) Diluent 21/35 1.40 SetPoint, 0ft ead, 17ft end
Stop at 15ft 18:00 (79) Diluent 21/35 1.40 SetPoint, 0ft ead, 14ft end
Surface (80) Diluent 21/35 -10ft/min ascent.

Off gassing starts at 111.5ft, OTU's this dive: 116, CNS Total: 43.9%

Now on this one, I changed to 10/50, and introduced more helium, notice the slight penality...
Dec to 170ft (3) Diluent 10/50 0.70 SetPoint, 50ft/min descent.
Level 170ft 26:36 (30) Diluent 10/50 1.20 SetPoint, 59ft ead, 79ft end
Asc to 110ft (36) Diluent 10/50 1.20 SetPoint, -10ft/min ascent.
Stop at 70ft 2:00 (41) Diluent 10/50 1.40 SetPoint, 0ft ead, 38ft end
Stop at 60ft 3:00 (44) Diluent 10/50 1.40 SetPoint, 0ft ead, 34ft end
Stop at 50ft 3:00 (47) Diluent 10/50 1.40 SetPoint, 0ft ead, 29ft end
Stop at 40ft 5:00 (52) Diluent 10/50 1.40 SetPoint, 0ft ead, 25ft end
Stop at 30ft 6:00 (58) Diluent 10/50 1.40 SetPoint, 0ft ead, 20ft end
Stop at 20ft 4:00 (62) Diluent 10/50 1.40 SetPoint, 0ft ead, 16ft end
Stop at 15ft 20:00 (82) Diluent 10/50 1.40 SetPoint, 0ft ead, 14ft end
Surface (83) Diluent 10/50 -10ft/min ascent.

Off gassing starts at 111.2ft, OTU's this dive: 121, CNS Total: 45.9%

Now before you guys give me too much grief...I think this one post alone explains why MANY of us will never be a ccr diver. Trying to follow this for the majority of us makes the grey matter hurt...and diving is not supposed to hurt!

[netmage]

Now before you guys give me too much grief...I think this one post alone explains why MANY of us will never be a ccr diver. Trying to follow this for the majority of us makes the grey matter hurt...and diving is not supposed to hurt!

For you Marine's out there...

On both the 120' and 170' for 30 dive, the CCR gets you out ~10 minutes faster...

[finGrabber]

Now before you guys give me too much grief...I think this one post alone explains why MANY of us will never be a ccr diver. Trying to follow this for the majority of us makes the grey matter hurt...and diving is not supposed to hurt!

For you Marine's out there...

On both the 120' and 170' for 30 dive, the CCR gets you out ~10 minutes faster...

Looks like the biggest difference is at 15', otherwise the runtimes are about the same

[scubafanatic]

.....I think I'll wait for the 'Cliff Notes' to come out......

Karl

[ScubaDadMiami]

Overall, decompression can become a complicated subject, and there are competing theories of decompression. A good instructor can break these down in a meaningful way to the student for the given level of diving.

This being said, the short version of helium diving is that helium adds time to required decompression for shorter, shallow dives. It can shorten decompression on very long shallow dives.

Helium starts to make sense even if it contributes to longer decompression when the diver gets to a point where narcosis starts to be a factor, somewhere starting as shallow as 80 feet for some individuals. By the time the diver starts to dive to 150 feet and beyond, Helium can be a tremendous asset in terms of narcosis abatement and for decompression efficiency.

Whether Helium is a benefit for decompression depends on the depth and the time of exposure. For rebreather divers making long dives, it can shorten the overall decompression required. More importantly, it reduces the work of breathing required on deeper dives.