Long or short decompress dive?

[Jean eve]

For a decompress dive,a long deco dive is better or two short deco dive is better??
Example:Is better to dive for 120 min(21m) or two 60 min(21m)in one day???
Concerning about off gasing,nitrogen load up and etc....

 

[miketsp]

I don't mean to be rude but if you don't already have an opinion based on your training, then you shouldn't be considering either of the options.
Why don't you publish your thoughts and then we'll discuss them?
What table do you intend to use for the first option and what does your air planning look like?
What surface interval would you use for the 2nd option?

 

[Jean eve]

I don't mean to be rude but if you don't already have an opinion based on your training, then you shouldn't be considering either of the options.

It is not that I don't have an opinion based on my training.We can do a long 120 min (21m)dive and also can do two 60min(21m)dive(one in the morning and one in the afternoon).
For exemple,I'd dove to 180'(21m)plus 120 min deco.Total 300 min dive.But I also can also split the dive to half,one in the morning and the other one afternoon.But which one is better to our health?
I'm using a working deco table(normal air)design by a professor here.If you see the table carefully,it is more conservative than US navy table.This table is speacially used by all the pearl farm diver here and was approved by France.
Since you want more info,here it is.
Let say:
1.One 43min 36m dive plus deco 45min=88min dive

2.Morning,22min 36m dive plus deco 10min=32min dive
Afternoon,Surface interval 2hrs(have to plus 15 min).Dive for 22min 36m plus the 15 min(surface interval),is 37 min dive.
Deco time=40min
So,profile 1 dive is better or profile 2?
(All this was based on the Working table)

 

[miketsp]

I had a feeling it was something to do with working at the bottom but what made the question seem strange was the bottom time in your original question.

I have never seen standard agency tables that go to 120mins at 21m. Even the table you supplied only goes to 75mins at 21m. So I'm still curious what deco time you intended to use for 120mins. For this you will have to use one of the planning softwares mentioned in various threads on this board.

In all the examples you gave of a 36m dive you forgot the ascent time of 3min (36m at 12m/min by your table) when calculating total dive duration.
Similarly in the 2nd option for the 36m second dive you called the 15min addition "surface interval". The correct term is "residual nitrogen".

Anyway coming back to the original question, I would recommend two separate dives as you will saturate less and therefore run less risk if in an emergency you should have to surface or cut short deco stops.

Let's see what other opinions appear.

 

[Jean eve]

I have never seen standard agency tables that goes to 120mins at 21m. Even the table you supplied only goes to 75mins at 21m. So I'm still curious what deco time you intended to use for 120mins. For this you will have to use one of the planning softwares mentioned in various threads on this board.

Off course you never seen a standard agency tables that go to 120 min at 21m.The table I'm using is speacially invent for pearl farm diver and it's only used it here.You can't find it in the table I supplied is because it is very rare that a diver will stay for so long at 21m!So it is no point to put it in the table.(You can only can find it from the book of the author)
Also if they put it in the table,can you imagine that how big is the table if they put all the max. limit on each depth!!

In all the examples you gave of a 36m dive you forgot the ascent time of 3min (36m at 12m/min by your table) when calculating total dive duration.

The time I given was already included the ascent time of 3min.(40min+3min)

 

[miketsp]

I think you will find this report interesting:
http://www.fish.wa.gov.au/res/broc/frr/frr138/frr138.pdf

Especially the high O2 levels used for deco.

 

[padiscubapro]

Off course you never seen a standard agency tables that go to 120 min at 21m.The table I'm using is speacially invent for pearl farm diver and it's only used it here.You can't find it in the table I supplied is because it is very rare that a diver will stay for so long at 21m!So it is no point to put it in the table.(You can only can find it from the book of the author)
Also if they put it in the table,can you imagine that how big is the table if they put all the max. limit on each depth!!



The time I given was already included the ascent time of 3min.(40min+3min)

21m is not 180 fsw its approx 60 fsw.. 180fsw isabout 55m

2 hours at 21m is nothing difficult (I do 90-120 min levels at the end of many of my Deep multilevel dives).. in fact I can run 3hours at 21m on my CCR without ANY deco requirements.. This puts me just over the recommended exposure limit of 100% using a setpoint of 1.3bar

Decompression model: VPM-B

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

Dec to 21m (1) on Diluent Air, 0.70 SetPoint, 15m/min descent.
Level 21m 178:36 (180) on Diluent Air, 1.30 SetPoint, 13m ead
Asc to sfc. (182) on Diluent Air, 1.30 SetPoint, -9m/min ascent.

OTU's this dive: 268
CNS Total: 101.5%


lets have you use 42% nitrox here is the plan

Decompression model: VPM-B

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

Dec to 21m (1) on Nitrox 42.0, 15m/min descent.
Level 21m 178:36 (180) on Nitrox 42.0, 1.30 ppO2, 13m ead
Asc to sfc. (182) on Nitrox 42.0, -9m/min ascent.

OTU's this dive: 266
CNS Total: 99.3%

390.9 cu ft Nitrox 42.0
390.9 cu ft TOTAL

Either of these is far safer than using AIR, using AIR is stupid. The CNS is slightly lower because the PO2 drops for the open circuit diver when ascending.. In either case I would still do a safety stop at 6m

if you decide to use a "standard" nitrox mix like 36% you'll only have limited deco

Decompression model: VPM-B

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

Dec to 21m (1) on Nitrox 36.0, 15m/min descent.
Level 21m 178:36 (180) on Nitrox 36.0, 1.11 ppO2, 15m ead
Asc to 3m (182) on Nitrox 36.0, -9m/min ascent.
Stop at 3m 17:00 (199) on Nitrox 36.0, 0.47 ppO2, 1m ead
Asc to sfc. (199) on Nitrox 36.0, -9m/min ascent.

Off gassing starts at 10.7m

OTU's this dive: 213
CNS Total: 77.6%

406.3 cu ft Nitrox 36.0
406.3 cu ft TOTAL

 

[Jean eve]

Originally posted by padiscubapro
2 hours at 21m is nothing difficult (I do 90-120 min levels at the end of many of my Deep multilevel dives).. in fact I can run 3hours at 21m on my CCR without ANY deco requirements.. This puts me just over the recommended exposure limit of 100% using a setpoint of 1.3bar

If using ccr of course there is nothing difficult!But I'm using normal air, open circuit.With this,I'd dove for 180 min at 21m and Deco for 120min(using my dive table).
Because of this kind of dive profile(open circuit,normal air),that is why I'm asking which one is better.One long dive or seperate it to two dives. :06:

 

[padiscubapro]

If using ccr of course there is nothing difficult!But I'm using normal air, open circuit.With this,I'd dove for 180 min at 21m and Deco for 120min(using my dive table).
Because of this kind of dive profile(open circuit,normal air),that is why I'm asking which one is better.One long dive or seperate it to two dives. :06:

My whole point is that AIR is stupid... For dives at shallow working depths (21m qualifies as shallow) using a rich nitrox mixture is the proper way to conduct the dive.. (use a target po2 of 1.3 and it allows a 3 hour exposure, if you need longer back the po2 down a bit) You can get much longer usable time in the water.. Even NASA uses a NITROX exclusively in the zero g training pool. Theytypically do exposures even longer than you are suggesting.

at 21m my RMV is about 36lpm with an average work load thats only 6480 lfg, in the states I would have to use twin 19s slighty overfilled to cover the dive (2 hours) with some reserve, in europe you could opt for twin 13 300 bar cylinders and still have more gas than me...

If you follow current deco theory, repetive exposures cause micro nuclei generation. your first ascent will create bubbles that will get crushed on the second exposure, and will grow much easier on the second ascent than not starting with a seed already.. repetitve deco dives can be done safely assuming you are using the right gasses to limit bubble growth and ongassing of the slower tissues.. AIR is not a very good gas to deco with.... while your faster tissues are offgassing you are still loading up your slower tissues.

WHen I run deco training Its normal practice for me to do 2 or 3 deco dives a day, but I am using proper gases. If I plan long exposures I end up using Helium in the mix, on short dives He based dives have longer requirements but once you reach a certain point for a specific depth the helium actually requires shorter decos.

 

[Jean eve]

Originally posted by padiscubapro
My whole point is that AIR is stupid... For dives at shallow working depths (21m qualifies as shallow) using a rich nitrox mixture is the proper way to conduct the dive..

It is true that AIR is stupid,unfortunately there is none of the pearl farm here have a mix gas station yet.That is why we are still using normal air.
Hope that in the future we'll have our own mix air station.

Thanks for your answer.