Deco Gasses
- Thread starter Hetland
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You are going to need to add one or two more deco gases beyond 50% before you need to worry about ICD. Most people adding HE to there deco gases is more b/c we just feel better after big dives. Well some do worry about ICD for the most part in non commercial diving it is a non issue.
I'd say it depends on your profile. There are times that I prefer to use just a 50% gas because it allows me to start my off gassing sooner and I can stay a bit deeper and look around as I slowly make my way back to the top.
Depending on the profile, you may only gain a few extra minutes using 100% vs. 50%.
Edit- crap. Didn't realize there were this many replies already. Back to catch up.
It depends on the dive. More often than not... 50% is just as fast to get out of the water.
I use 50% for almost all my accelerated deco. Virtually all EANx in the UK is produced using partial pressure blending, so any nitrox cylinder has to be O2 clean. The cylinder and valve needs O2 cleaning every 15 months, but different countries have different regulations, your instructor will be able to tell you what the rules are where you are.
I use Apeks regs which the manufacturer rates as suitable for up to 40% EANx, but I have never had them specifcally O2 cleaned even for use with 100% oxygen. They get stripped, cleaned and lubricated using oxygen compatible grease once a year.
OP
Hetland
Contributor
Thanks to everyone for the great responses.
There are a great many reasons for choosing one deco gas over another, but in terms of handling equipment, as has already been said, anything over 40% requires O2 cleaning, and 80% and 100% are essentially identical in the risks involved in filling and storing them. I have my regs serviced once a year and my tanks get VIPed once a year (and they O2 clean them when they do that). I do buy all my gas from a facility that pumps Nitrox, so their gases have to be O2 clean as well. Facilities that do not provide Nitrox do not have to filter their air to the same standards, so if you are filling tanks from a place like that, you need to know whether they are using O2 standards or not.
And you have to be careful with the O2 . . . I suspect I damaged a regulator earlier this year by turning the gas on too fast. The person who repaired it didn't find any char or anything in it, but something in the first stage just blew when I pressurized it, and I've never had that happen with anything but O2.
And you have to be careful with the O2 . . . I suspect I damaged a regulator earlier this year by turning the gas on too fast. The person who repaired it didn't find any char or anything in it, but something in the first stage just blew when I pressurized it, and I've never had that happen with anything but O2.
At the advanced nitrox level you will be diving to a maximum of 150' and in that depth range both 100% and 50% work ok and which one is ideal may depend on the dive and where you are diving.
Consider a 150' dive on 25% nitrox:
With 50% for a deco gas you will have 35 minutes of deco (65 min run time) and with average bottom and deco RMVs you will use around 116 cu ft of back gas and 31 cu ft of deco gas.
With 100% for a deco gas you will have 33 minutes of deco (63 min run time) and with average bottom and deco RMVs you will use around 136 cu ft of back gas but you will use only 11 cu ft of deco gas.
The run time differences are minor, but the difference in back gas used with 100% can be significant, especially if you push the run time out another 5 minutes. In that case you have a 76 min run time with O2 and use 160 cu ft of back gas compared to an 80 minute run time and only 138 cu ft of back gas with 50%. So while the run time differences are minor, the need to do all the stops below 20' on back gas will mean you use a lot more back gas and that can begin to become a limiting factor before it would with 50%.
Another serious consideration can be the CNS percentage incurred on the dive. The comparatively long stops at 10 and 20 feet on O2 will use result is as much CNS impact as the entire bottom portion of the dive and the CNS percentages will be significantly greater than with 50%.
So, for offshore diving in the 100-150' range, 50% for deco makes a great deal of sense.
On the other hand, in N Florida cave country O2 is very common as deco is more likely after something like a 70 ft for 90 minute profile or a 100 ft for 60 minute profile where the total deco may be only 15 minutes with most of it at shallow stops - and where deeper stops won't really fit the cave profile and may not allow you to use 50% efficiently.
-----
Once you go beyond the 150' limit, the picture will start to look different. Consider a 200' dive for 30 minutes on 18/35 where RMVs etc are otherwise equal to the 150' dives above.
With 100% only, you would have a 100 minute runtime and use 223 cu ft of back gas - not a viable option if you have only 260 cu ft of back gas as it does not leave an adequate reserve - and 31 cu ft of O2.
With 50% only you have a 108 minute run time and use only 165 cu ft of back gas, but 62 cu ft of 50%. In this case you are cutting the deco gas close in terms of a reserve.
So again, the run time differences are minor but the gas planning implications are significant.
There are also lost gas considerations and that, along with longer single gas run times, argues heavily for two deco gases.
On the same dive, to 200' for 30 minutes but with both 50% and 100%, you will have a 92 minute run time, use only 165 cu ft of back gas, 25 cu ft of 50% and 23 cu ft of 100%. So an AL 80 of 50% and an AL 40 of 100% would be all you need to cover the dive plus the loss of a single deco gas.
That is also where it starts getting interesting and controversial. Consider the same dive but with 32% for a travel/deco gas along with 80% for a deco gas. You will get a 90 minute run time, use only 148 cu ft of back gas, 29 cu ft of 32% and 32 cu ft of 80%. It works well when everything goes as planned. However you are then right on the edge with the lost single lost gas scenarios unless your deco RMV is pretty good (.4 cfm or below). The advantage is that 80% has a significantly smaller CNS percentage than 100% O2 on the same dive and with a MOD of a 33' it is much friendlier offshore where you may prefer to do the last stop at 20'.
Once you get deeper and use mixes with greater amounts of helium, isobaric counter diffusion begins to be an issue, so the travel gas/first deco gas will need to have some helimum in it and a single deco gas is just not an option.
------
Back in the day when I took advanced Nitrox and deco procedures, TDI taught the use of two gasses as well as deco mixes up to 100% while IANTD limited you to one gas and 50%. With that in mind the TDI course allowed the diver to start getting comfortable with 2 deco bottles or a travel gas and deco gas as well as allowing the diver tyo use 100% for 02. In my opinion, the greater flexibility of the TDI left the diver better prepared for both offshore and cave diving in the 100-150' depth range and makes the transition to normoxic trimix a bit easier. But - check the current standards as things do change.
In any event, buying an AL 80 and an AL 40 with regs suitable for high percentage O2 use will leave you well prepared for pretty much anything you will encounter down to 200' or so.
Consider a 150' dive on 25% nitrox:
With 50% for a deco gas you will have 35 minutes of deco (65 min run time) and with average bottom and deco RMVs you will use around 116 cu ft of back gas and 31 cu ft of deco gas.
With 100% for a deco gas you will have 33 minutes of deco (63 min run time) and with average bottom and deco RMVs you will use around 136 cu ft of back gas but you will use only 11 cu ft of deco gas.
The run time differences are minor, but the difference in back gas used with 100% can be significant, especially if you push the run time out another 5 minutes. In that case you have a 76 min run time with O2 and use 160 cu ft of back gas compared to an 80 minute run time and only 138 cu ft of back gas with 50%. So while the run time differences are minor, the need to do all the stops below 20' on back gas will mean you use a lot more back gas and that can begin to become a limiting factor before it would with 50%.
Another serious consideration can be the CNS percentage incurred on the dive. The comparatively long stops at 10 and 20 feet on O2 will use result is as much CNS impact as the entire bottom portion of the dive and the CNS percentages will be significantly greater than with 50%.
So, for offshore diving in the 100-150' range, 50% for deco makes a great deal of sense.
On the other hand, in N Florida cave country O2 is very common as deco is more likely after something like a 70 ft for 90 minute profile or a 100 ft for 60 minute profile where the total deco may be only 15 minutes with most of it at shallow stops - and where deeper stops won't really fit the cave profile and may not allow you to use 50% efficiently.
-----
Once you go beyond the 150' limit, the picture will start to look different. Consider a 200' dive for 30 minutes on 18/35 where RMVs etc are otherwise equal to the 150' dives above.
With 100% only, you would have a 100 minute runtime and use 223 cu ft of back gas - not a viable option if you have only 260 cu ft of back gas as it does not leave an adequate reserve - and 31 cu ft of O2.
With 50% only you have a 108 minute run time and use only 165 cu ft of back gas, but 62 cu ft of 50%. In this case you are cutting the deco gas close in terms of a reserve.
So again, the run time differences are minor but the gas planning implications are significant.
There are also lost gas considerations and that, along with longer single gas run times, argues heavily for two deco gases.
On the same dive, to 200' for 30 minutes but with both 50% and 100%, you will have a 92 minute run time, use only 165 cu ft of back gas, 25 cu ft of 50% and 23 cu ft of 100%. So an AL 80 of 50% and an AL 40 of 100% would be all you need to cover the dive plus the loss of a single deco gas.
That is also where it starts getting interesting and controversial. Consider the same dive but with 32% for a travel/deco gas along with 80% for a deco gas. You will get a 90 minute run time, use only 148 cu ft of back gas, 29 cu ft of 32% and 32 cu ft of 80%. It works well when everything goes as planned. However you are then right on the edge with the lost single lost gas scenarios unless your deco RMV is pretty good (.4 cfm or below). The advantage is that 80% has a significantly smaller CNS percentage than 100% O2 on the same dive and with a MOD of a 33' it is much friendlier offshore where you may prefer to do the last stop at 20'.
Once you get deeper and use mixes with greater amounts of helium, isobaric counter diffusion begins to be an issue, so the travel gas/first deco gas will need to have some helimum in it and a single deco gas is just not an option.
------
Back in the day when I took advanced Nitrox and deco procedures, TDI taught the use of two gasses as well as deco mixes up to 100% while IANTD limited you to one gas and 50%. With that in mind the TDI course allowed the diver to start getting comfortable with 2 deco bottles or a travel gas and deco gas as well as allowing the diver tyo use 100% for 02. In my opinion, the greater flexibility of the TDI left the diver better prepared for both offshore and cave diving in the 100-150' depth range and makes the transition to normoxic trimix a bit easier. But - check the current standards as things do change.
In any event, buying an AL 80 and an AL 40 with regs suitable for high percentage O2 use will leave you well prepared for pretty much anything you will encounter down to 200' or so.
OP
Hetland
Contributor
Thanks Aquamaster. Just one question.
You are saying that the plan you described would incur a heavier CNS percentage, and that the same dive would have less of a CNS load because with 50% you start breathing a leaner deco gas earlier and deeper?
You are saying that the plan you described would incur a heavier CNS percentage, and that the same dive would have less of a CNS load because with 50% you start breathing a leaner deco gas earlier and deeper?
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