Don't breathe tanks to zero?

[tr3a]

Actually breathing a cylinder to zero would be darn hard. Once the cylinder pressure approaches the intermediate setting of the first stage breathing becomes difficult. Older single stage regulators (at least in my experience) will work better at very low pressure.

 

[chrpai]

That's not what the transcript shows but whatever...

 

[The Chairman]

That's not what the transcript shows but whatever...

From post 5 in this thread:

I suggest to my students that they should start their ascent when their tank pressure equals 10 times their depth or 500 psi, whichever is greater.

Then again in post 30 I reiterated my post #5:

I did post that you should start your ascent with @ ten times your depth or 500 psi, whichever is greater.

Perhaps there was some other transcript you were referring to?

 

[chrpai]

You really want to go there? I will if you want me to. It's my day off.. I telephone call would be more enjoyable, faster and easier.

BTW, doing a "sunset" one tank boat trip tonight. I'm bringing the wife and kids so this'll probably be an occasion where I come back to the boat with more then 1000psi left as to not leave them alone too long.

 

[Tortuga68]

Me thinks that's a lazy gas blender. He doesn't want to take the time to analyze the starting FO2 and starting pressure and do the math of how much O2 to add. He just wants to start at zero every time and follow the same math each fill.

For the people that usally do my Nitrox blends, I much prefer them to use this method as it's more 'idiot proof'. I'm not saying they're idiots, just implying it

BTW, one situation in which tanks are intentionally taken to low pressures during a dive is one form of technical diving. In such diving, divers carry double tanks as their main gas supply, but they start the dive breathing from another tank that they carry on the side. When the pressure on that first tank gets low, they switch to the main tanks. They will be sure to keep a very generous reserve in the main tanks for purposes of safety.

Not to sidetrack this into a technical diving discussion, but I don't know anybody that dives like that and I don't think it's a sound plan

 

[The Chairman]

I'm not saying they're idiots, just implying it

That was righteous.

 

[TSandM]

Cave divers routinely breathe stages to zero (or to ambient, anyway). And people doing technical dives of sufficient magnitude also breathe their bottom stages down.

With respect to rock bottom, it's always important to remember that it's a dynamic number. If I do a dive to 100 feet, that doesn't mean I have to END the dive with 1300 psi in my tank -- it means if I get to 1300 psi when I'm at 100 feet, I have to start my ascent there. If I leave 100 feet and mosey up to 60, my reserve changes; if I'm ending the dive poking around in 20 feet of water, my reserve is 500 psi. I personally don't think it's terribly wise to make a habit of ending a dive with less than that, although I have done it . . . an epic dive with a Southern California friend in our local mudhole, with probably 100 feet of viz (yes, in Puget Sound!) ended with us chasing a jellyfish around in 6 feet of water, with about 300 psi showing on the gauge

 

[chrpai]

For the people that usally do my Nitrox blends, I much prefer them to use this method as it's more 'idiot proof'. I'm not saying they're idiots, just implying it

Well, if they really wanted to idiot proof it they would bank it. My life experiences tell me it's usually easier and more consistent to make (1) 50 units of something then it is to make (50) 1 unit of something.

---------- Post added July 5th, 2013 at 11:22 AM ----------

With respect to rock bottom, it's always important to remember that it's a dynamic number.

That's my point EXACTLY. "500" psi is just an arbitrary number. I can easily see circumstances where my rock bottom can be less without any appreciable decrease in safety. I can also see situations where "500" isn't nearly enough reserve. If I'm in 10' of water and some how go OOA because I was being a bottom time hog, I'll just swim it up and then orally inflate at the surface.

The only remaining risk is damaging gear due to loss of pressure as a result of an inaccurate guage. I don't see much difference between a tank running out of air and a pony bottle being charged, turned off and then accidentally being purged at depth. In fact I imagine a good number of people have forgotten to charge their regs and suffered no ill effects.

 

[scuba.mne]

Did drain my doubles everytime qfter helium dives ,shut the valve and thats it,they are good healthy me nice and smiley

sent from inner peace

 

[Tortuga68]

Well, if they really wanted to idiot proof it they would bank it. My life experiences tell me it's usually easier and more consistent to make (1) 50 units of something then it is to make (50) 1 unit of something

That's great if you always dive EAN28 or some other 'standard' mix