isobaric decompression on CCR

[wedivebc]

I was reading an old thread about using high nitrogen mix switches to clear helium tissues and shorten decompression times in the Scubaboard archives.
http://www.scubaboard.com/forums/rebreather-diving/29113-trimix-gas-switching.html

Curt Bowen had written an article about it in ADM but he was using OC and switching basically from trimix to deep air at about 150ft half way through the bottom leg of dive to shorten decompression utilizing isobaric decompression.
I ran some CCR test dives on v-planner and came up with some surprising (at least for me) results:

Here is a typical CCR dive using 10/50 diluent and 1.2 PO2
Decompression model: VPM - B

DIVE PLAN
Surface interval = 1 day 0 hr 0 min.
Elevation = 0ft
Conservatism = + 2

Dec to 200ft (2) Diluent 10/50 1.00 SetPoint, 100ft/min descent.
Dec to 300ft (3) Diluent 10/50 1.00 SetPoint, 100ft/min descent.
Level 300ft 17:00 (20) Diluent 10/50 1.20 SetPoint, 132ft ead, 137ft end
Asc to 220ft (20) Diluent 10/50 1.20 SetPoint, -100ft/min ascent.
Stop at 220ft 0:12 (21) Diluent 10/50 1.20 SetPoint, 87ft ead, 101ft end
Stop at 210ft 1:00 (22) Diluent 10/50 1.20 SetPoint, 81ft ead, 97ft end
Stop at 200ft 1:00 (23) Diluent 10/50 1.20 SetPoint, 76ft ead, 93ft end
Stop at 190ft 1:00 (24) Diluent 10/50 1.20 SetPoint, 70ft ead, 88ft end
Stop at 180ft 1:00 (25) Diluent 10/50 1.20 SetPoint, 64ft ead, 84ft end
Stop at 170ft 1:00 (26) Diluent 10/50 1.20 SetPoint, 59ft ead, 79ft end
Stop at 160ft 1:00 (27) Diluent 10/50 1.20 SetPoint, 53ft ead, 75ft end
Stop at 150ft 1:00 (28) Diluent 10/50 1.20 SetPoint, 48ft ead, 70ft end
Stop at 140ft 1:00 (29) Diluent 10/50 1.20 SetPoint, 42ft ead, 66ft end
Stop at 130ft 2:00 (31) Diluent 10/50 1.20 SetPoint, 36ft ead, 61ft end
Stop at 120ft 1:00 (32) Diluent 10/50 1.20 SetPoint, 31ft ead, 57ft end
Stop at 110ft 3:00 (35) Diluent 10/50 1.20 SetPoint, 25ft ead, 53ft end
Stop at 100ft 2:00 (37) Diluent 10/50 1.20 SetPoint, 19ft ead, 48ft end
Stop at 90ft 3:00 (40) Diluent 10/50 1.20 SetPoint, 14ft ead, 44ft end
Stop at 80ft 3:00 (43) Diluent 10/50 1.20 SetPoint, 8ft ead, 39ft end
Stop at 70ft 4:00 (47) Diluent 10/50 1.20 SetPoint, 3ft ead, 35ft end
Stop at 60ft 5:00 (52) Diluent 10/50 1.20 SetPoint, 0ft ead, 30ft end
Stop at 50ft 5:00 (57) Diluent 10/50 1.20 SetPoint, 0ft ead, 26ft end
Stop at 40ft 8:00 (65) Diluent 10/50 1.20 SetPoint, 0ft ead, 21ft end
Stop at 30ft 10:00 (75) Diluent 10/50 1.20 SetPoint, 0ft ead, 17ft end
Stop at 20ft 38:00 (113) Diluent 10/50 1.20 SetPoint, 0ft ead, 13ft end
Surface (115) Diluent 10/50 -10ft/min ascent.

Off gassing starts at 252.1ft

OTU's this dive: 151
CNS Total: 53.6%



DIVE PLAN COMPLETE


Now if I do a diluent switch during the ascent phase I get a much shorter deco time.

Decompression model: VPM - B

DIVE PLAN
Surface interval = 1 day 0 hr 0 min.
Elevation = 0ft
Conservatism = + 2

Dec to 200ft (2) Diluent 10/50 1.00 SetPoint, 100ft/min descent.
Dec to 300ft (3) Diluent 10/50 1.00 SetPoint, 100ft/min descent.
Level 300ft 17:00 (20) Diluent 10/50 1.20 SetPoint, 132ft ead, 137ft end
Asc to 210ft (20) Diluent 10/50 1.20 SetPoint, -100ft/min ascent.
Stop at 210ft 1:06 (22) Diluent 10/50 1.20 SetPoint, 87ft ead, 91ft end
Stop at 200ft 1:00 (23) Diluent 10/50 1.20 SetPoint, 76ft ead, 93ft end
Stop at 190ft 1:00 (24) Diluent 10/50 1.20 SetPoint, 71ft ead, 87ft end
Stop at 180ft 1:00 (25) Diluent 10/50 1.20 SetPoint, 65ft ead, 83ft end
Stop at 170ft 1:00 (26) Diluent 10/50 1.20 SetPoint, 60ft ead, 78ft end
Stop at 160ft 1:00 (27) Diluent 10/50 1.20 SetPoint, 54ft ead, 74ft end
Level 150ft 1:00 (28) Diluent Air 1.20 SetPoint, 148ft ead
Asc to 140ft (28) Diluent Air 1.20 SetPoint, -30ft/min ascent.
Stop at 140ft 0:20 (29) Diluent Air 1.20 SetPoint, 136ft ead
Stop at 130ft 1:00 (30) Diluent Air 1.20 SetPoint, 123ft ead
Stop at 120ft 1:00 (31) Diluent Air 1.20 SetPoint, 110ft ead
Stop at 110ft 1:00 (32) Diluent Air 1.20 SetPoint, 98ft ead
Stop at 100ft 1:00 (33) Diluent Air 1.20 SetPoint, 85ft ead
Stop at 90ft 2:00 (35) Diluent Air 1.20 SetPoint, 72ft ead
Stop at 80ft 3:00 (38) Diluent Air 1.20 SetPoint, 60ft ead
Stop at 70ft 3:00 (41) Diluent Air 1.20 SetPoint, 47ft ead
Stop at 60ft 3:00 (44) Diluent Air 1.20 SetPoint, 35ft ead
Stop at 50ft 5:00 (49) Diluent Air 1.20 SetPoint, 22ft ead
Stop at 40ft 5:00 (54) Diluent Air 1.20 SetPoint, 9ft ead
Stop at 30ft 9:00 (63) Diluent Air 1.20 SetPoint, 0ft ead
Stop at 20ft 32:00 (95) Diluent Air 1.20 SetPoint, 0ft ead
Surface (97) Diluent Air -10ft/min ascent.

NOTE: This Multi Level dive requires intermediate deco stops between levels.

OTU's this dive: 127
CNS Total: 45.1%



DIVE PLAN COMPLETE


So I guess my questions are, is anyone doing this? Are IBCD hits a significant risk using this?
How often would the loop need to be flushed to keep the helium levels down?

 

[rjack321]

I really have no idea but that type of profile wasn't that uncommon on OC 15yrs ago.

What about using 12/60 then switching Dils a little later in the profile, like 100ft? And maybe switch to something not quite so abrupt, like 25/25?

Is vplanner giving you IBCD warnings out the wazzu?

 

[wedivebc]

I really have no idea but that type of profile wasn't that uncommon on OC 15yrs ago.

What about using 12/60 then switching Dils a little later in the profile, like 100ft? And maybe switch to something not quite so abrupt, like 25/25?

Is vplanner giving you IBCD warnings out the wazzu?

I have asked Ross to explain where he derived the ratio used in the v-planner ICD warnings and he really has no science to back it up. Mark Ellyatt seems to have a few opinions but they are based on crazy deep dives. I am unaware of any bona-fide IBCD hits occurring above 100m.
Remember I am talking about CCR diluent so the actual O2 content of the dil makes little difference.

 

[rjack321]

I have asked Ross to explain where he derived the ratio used in the v-planner ICD warnings and he really has no science to back it up. Mark Ellyatt seems to have a few opinions but they are based on crazy deep dives. I am unaware of any bona-fide IBCD hits occurring above 100m.
Remember I am talking about CCR diluent so the actual O2 content of the dil makes little difference.

True I was just thinking it might as well be mixable and suitable for flushes. Even higher He to start but then not such a pronounced slam as the air switch at 150ft (not something I would want to try). You had a 50% drop, while X/60 to Y/25 is a 35% drop. You could even drop again to air at say 30ft where there's little N2 ongassing risk.

IBCD's rarity seems to give it a mythical boogy man status.

 

[Doppler]

Whoops... I cannot read apparently!

 

[rjack321]

Seems like Rich Pyle is doing exactly what you wondered about:

On most decompression dives involving helium during the deep phase of the dive, the diver will want to flush the helium out of the loop and replace it with nitrogen. I usually do this during an ascent at a depth of about 130-150 ft/40-45 m, and start the flush by venting gas from the loop until the loop volume is at Vmin. I then inflate the loop to Vmax with air, and repeat this cycle at least three times. The partial pressure of any remaining helium in the loop is negligible, and will continue to drop as more gas is vented from the loop during the remainder of the ascent. When I reach the 20-ft /6-m decompression stop, I shut the diluent input supply, and flush the loop with oxygen until the loop PO2 reaches set-point. I will generally remain at this depth until the decompression ceiling has been cleared. If I ascend shallower, I reduce the PO2 set-point to 1.0 atm.

From: LGRB

 

[SFLDiver3445]

I really have no idea but that type of profile wasn't that uncommon on OC 15yrs ago.

yep, I remember completing OC dives in 95/96 in the 270' to 300' range, where we would switch off of the heavier helium mix to air, starting at 218'.

Can't tell you what my bottom mix was, because back then we didn't use standard mixes, but instead would custom fill for the specific dive. However, since we did a lot of deep air diving back then, we were pretty accustomed to the heavy nitrox, so we would generally not worry about overdoing it on the helium, unless it was a special dive and we wanted to ensure we were VERY clear headed.

If I had to guess, on one of our special 300' dives, which we filled with heavy helium, I probably switched 15/55 to air at 218'.

Never even heard of IBCD hits back then.

That was just the way we were taught to dive back then.

 

[rossh]

I have asked Ross to explain where he derived the ratio used in the v-planner ICD warnings and he really has no science to back it up.

Here is a diagram that might help readers understand the problem of IBCD. IBCD diagram

Regards

 

[wedivebc]

Here is a diagram that might help readers understand the problem of IBCD. IBCD diagram

Regards

It's a pretty chart, no argument there. I have asked on several occasions where the data came from that caused you to choose those warning limits in v-planner. Your reply was if I don't like the warning I could shut them off.

 

[rjack321]

Seems like there are alot of undefined conditions on that page:
"changes greatly"
"excessive and prolonged"
"significant decompression obligations"
"where the deco obligation is small / none"
"stops are short, the divers rises up through the levels quickly"

I have never considered IBCD on any of my dives, then again if SFLDiver3445 was not experiencing the phenomenon at 218ft air switches I am much less likely to experience it keeping ENDs throughout the deco in the 70-100ft window or less.