Nitrox Blending - Single Tank

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I'm not sure what you mean by min gas. Minimum gas as in starting a safety stop at 15 feet with 750 PSI and ending the dive around 500 PSI?
No. Min gas is the minimum amount of gas I need for a safe ascent from the depth I'm at, while donating to my OOG buddy, while we both have an increased SAC due to stress.

An example, min gas for me @ 30m:

Ascent rate is 10 m/min, so 3 minutes. Average depth during the ascent is 15m or 2.5 ATA. My relaxed SAC is some 15-20 SLM. I'll pick 20 for simple math. Assuming everything goes well, I need 20L/bar*min times 2.5 ATA times 3 minutes, or 150L for the ascent. Safety stop is 3 minutes at 1.5ATA, so another 90L, and I'm up to 240L. If I have to donate gas at 30m, we might need a minute or so to sort things out before starting our ascent, that's another 80L, and I'm up to 320L.

But that's just my gas. I'll be donating as well, so I have to double that to account for my buddy's consumption. Now I'm up to 640L. And if we're in an OOG situation, we're probably not at our relaxed SAC. Let's say we double our SAC due to stress, and I'm up to 1280L. I dive a 10L tank, so that's 130 bar. IOW, I have no business being at 30m when my tank pressure drops below 130 bar. That's my min gas at 30m, so when my SPG shows around 130bar it's time for me to explore the shallower parts of the site. Incidentally, I'll probably reach that pressure at around 20 minutes bottom time, which is the air NDL for 30m, so if my gas is air, I have no business being down there any longer anyway.

Now 20 SLM is a bit on the high side of relaxed for me, so perhaps I could have done the math using 15 SLM. Then my min gas reserve would be 960L, or some 100 bar. On the other hand, I'd really like to have at least a 10 bar reserve on surfacing to account for SPG error and to have my reg deliver gas all the way up, so I'll stick with 130.

And if I'm diving a site with a sloping bottom, I can stay within min gas during the whole dive and still be at the safety stop with 50 bar/750psi left. At e.g. 15m, my min gas is 600-800L or 60-80 bar, at 10m it's somewhere in the 400-600L range or 40-60 bar.

BTW, if you want it in feet and psi, you can check out @NWGratefulDiver 's article about Understanding Gas Management
 
Personally with Nitrox, I would always work on the "trust but verify" route. I have every confidence that the shop will provide the mix I want but, unless I have watched them check it, I will check it myself.

Even a weak nitrox mix such as 23 or 24 will give a longer NDL than air and it all helps with regards to Nitrogen loading. I would always set my computer according to the actual mix I am diving (it takes less than a minute with a Perdix to change it) - if I somehow get bent, I want an analysis of my computer to give the correct information about my dive such as theoretical loadings, SAC etc.

I would be interested to know what dive computer and algorithm you are using. This may have even more of an effect on your NDL than the nitrox blend as a lot of computers (virtually all that use bubble models such as RGBM or VPM AFAIK) will actively penalise any surface interval shorter than 1Hr. Personally I try to aim for 1Hr+ but will dive shorter if circumstances determine it (and accept the shorter NDL).

I do dive conservatively though staying well inside my NDL
 
I think most people would just figure nitrox for the first dive and air settings for subsequent dives. If you're pushing NDL's to the point that you're wanting to take advantage of a 23 mix you may be asking for trouble. Why not relax and enjoy the surface interval? I was taught to do a min 1hr surface interval with nitrox. You're doing less aren't you?
It's partially a time thing, but mainly a hunger thing. Lol.

We push the NDL with 3 tanks of air, but have plenty of room with nitrox on the first dive. The 23 mix isn't important for the second dive, I'm just wondering.

And no, we do right around an hour SI even when we aren't using nitrox. Maybe slightly less on the last dive when we're starving and are craving a big meal and margaritas :wink:. I was also taught to have at least a 1 hour surface interval when using nitrox for oxygen loading purposes. I was talking to a DM in Florida who talked about cumulative loading rather than absolutely needing 1 hour between nitrox dives. I still do 1 hour just to be safe. We basically dive from 10:30 until 3:30-4, and get 3 dives in during that timeframe.
 
Where does this come from?
I was also taught this in my PADI course for oxygen loading purposes and how to avoid oxygen tox.

I later learned it's very hard to get oxygen tox following the MOD and diving recreationally. I think it's just to be very conservative so you don't get toxed.
 
Personally with Nitrox, I would always work on the "trust but verify" route. I have every confidence that the shop will provide the mix I want but, unless I have watched them check it, I will check it myself.

Even a weak nitrox mix such as 23 or 24 will give a longer NDL than air and it all helps with regards to Nitrogen loading. I would always set my computer according to the actual mix I am diving (it takes less than a minute with a Perdix to change it) - if I somehow get bent, I want an analysis of my computer to give the correct information about my dive such as theoretical loadings, SAC etc.

I would be interested to know what dive computer and algorithm you are using. This may have even more of an effect on your NDL than the nitrox blend as a lot of computers (virtually all that use bubble models such as RGBM or VPM AFAIK) will actively penalise any surface interval shorter than 1Hr. Personally I try to aim for 1Hr+ but will dive shorter if circumstances determine it (and accept the shorter NDL).

I do dive conservatively though staying well inside my NDL
Puck Pro. I think running the RGBM algorithm. I think it's pretty conservative as is.

It's easy to change my nitrox setting. Takes 30 seconds between dives.
 
I was also taught this in my PADI course for oxygen loading purposes and how to avoid oxygen tox.

I later learned it's very hard to get oxygen tox following the MOD and diving recreationally. I think it's just to be very conservative so you don't get toxed.
Actually, even the original PADI nitrox class (the one with all the theory, equations, and caveats) did not suggest an hour SI to help with O2 exposure, because it assumed a rolling 24h window for O2 uptake, with no "oxygen offgassing" over a surface interval. Longer SIs to help with N2, yes, but not O2.
 
No. Min gas is the minimum amount of gas I need for a safe ascent from the depth I'm at, while donating to my OOG buddy, while we both have an increased SAC due to stress.

An example, min gas for me @ 30m:

Ascent rate is 10 m/min, so 3 minutes. Average depth during the ascent is 15m or 2.5 ATA. My relaxed SAC is some 15-20 SLM. I'll pick 20 for simple math. Assuming everything goes well, I need 20L/bar*min times 2.5 ATA times 3 minutes, or 150L for the ascent. Safety stop is 3 minutes at 1.5ATA, so another 90L, and I'm up to 240L. If I have to donate gas at 30m, we might need a minute or so to sort things out before starting our ascent, that's another 80L, and I'm up to 320L.

But that's just my gas. I'll be donating as well, so I have to double that to account for my buddy's consumption. Now I'm up to 640L. And if we're in an OOG situation, we're probably not at our relaxed SAC. Let's say we double our SAC due to stress, and I'm up to 1280L. I dive a 10L tank, so that's 130 bar. IOW, I have no business being at 30m when my tank pressure drops below 130 bar. That's my min gas at 30m, so when my SPG shows around 130bar it's time for me to explore the shallower parts of the site. Incidentally, I'll probably reach that pressure at around 20 minutes bottom time, which is the air NDL for 30m, so if my gas is air, I have no business being down there any longer anyway.

Now 20 SLM is a bit on the high side of relaxed for me, so perhaps I could have done the math using 15 SLM. Then my min gas reserve would be 960L, or some 100 bar. On the other hand, I'd really like to have at least a 10 bar reserve on surfacing to account for SPG error and to have my reg deliver gas all the way up, so I'll stick with 130.

And if I'm diving a site with a sloping bottom, I can stay within min gas during the whole dive and still be at the safety stop with 50 bar/750psi left. At e.g. 15m, my min gas is 600-800L or 60-80 bar, at 10m it's somewhere in the 400-600L range or 40-60 bar.

BTW, if you want it in feet and psi, you can check out @NWGratefulDiver 's article about Understanding Gas Management
I see. I've actually never calculated min gas in that kind of detail. I also don't even know my average SAC. I'll look into this.

Although I dive a lot in the non snowboarding months (at least 2 dives per week), I am a fish diver and mainly do very safe dives. Fish are my favorite animal so I like swimming and interacting with them. This almost always takes place in 20-50 feet of water on easy to navigate reefs here in SoCal. Catalina is the exception where I'm deeper, although it's still a very easy dive. We just get back to the shallows with about 1/3 of our air and are under 20 feet not far after that.

I'll look into this for our vacation dives where we do deeper, unfamiliar dives.
 
tursiops,

Back in the day a couple of goats did less than an hour SI and got bent.

Seriously, "I was taught" is just another factor of safety. I know it's not really based on anything but it's not a bad idea.
And this : "Longer SIs to help with N2, yes, but not O2."
 
tursiops,

Back in the day a couple of goats did less than an hour SI and got bent.

Seriously, "I was taught" is just another factor of safety. I know it's not really based on anything but it's not a bad idea.
And this : "Longer SIs to help with N2, yes, but not O2."
I'm not questioning DCS. I'm questioning O2 toxicity.
 
https://www.shearwater.com/products/peregrine/

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