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?
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?