I agree, and you need to be able to do it without looking at the valves, ie, when you're wearing the tank(s).
Why isn't there a tank valve-open marking?
- Thread starter Javik
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I agree, and you need to be able to do it without looking at the valves, ie, when you're wearing the tank(s).
AfterDark
Contributor
I'm unclear on your test. Are you posting that with the valve opened all the way the needle didn't move much or with the valve not opened all the way? Even backed off 1/4 to 1/2 a turn may not show much at the surface.
As I stated in one of my posts this 1/4 or 1/2 turn is not correct. If manifold users are doing that then they asking for trouble. A few degrees back is all that it takes to back off the stop and the valve is wide open. There is absolutely nothing gained in air flow by leaving it against the stop except in the mind of the user.
Dan
Contributor
I watch his videos. He barely crack the valve open. As you see the valve curve above if you barely crack the valve open, it doesn't need much effort to move the needle. The pressure drop will be very sensitive with the gas flow rate, i.e., how hard you suck the reg.
Any valve design with profile less than linear (I don't know what the SCUBA tank air valve profile is) relative to the number of turns (stem opening), the pressure sensitivity with the flow rate would be more than that of the linear profile valve. It won't take much sucking up the reg to move the needle in the SPG when you barely crack open the valve, regardless the pressure reading.
The flow rate (how hard you suck the air out of the reg) is a function of square root of the pressure drop, not the pressure. So it is totally misleading to see a high pressure and equate it to having enough gas to inhale. The deeper you are in the water, the smaller the differential pressure between the tank pressure and the ambient pressure (meaning smaller pressure drop). Therefore, the worse the situation will be. You may get to the point of you suck & get no air, as if you are OOA, just because the smaller pressure drop you are experiencing down there at the deep end.
You simply would need to minimize the pressure drop, i.e., maximize the flow. To accomplish that, you want to open the valve all the way open (stem opening to 100%) period.
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A quarter turn on maybe... Not fully on and backing off a quarter
I've never ever had a DM or anyone mess with my tanks when on, if they did I'd feel it. But then I don't go on full service nanny boats
The only time I have nearly entered the water with a valve off is because I've turned it off again for some reason. My check have caught it, even if they didn't which just atmospheric pressure in my wing I stay at the surface (yup rolled without adding air too)
I am the reason that there is no such thing as idiot proof
Dan
Contributor
In any circumstances you don't want to see the needle moving as you inhale through the reg, especially at the deep end. If you see that, get the hell up, ascend safely & grab your buddy with you.
AfterDark
Contributor
There is absolutely nothing gained in air flow by leaving it against the stop except in the mind of the user.
Forty six years diving with over 2050 dives in addition to 20+ years using high pressure gas cylinders in an industrial setting proving the above statement to be correct. I've made dives to over 150FSW with my valves backed off the stop a degree or 3 with no ill effects. I don't do experiments, I'm not a scientist, I live in the real world where what works is the way to I do things. I do invite you to do whatever works for you.
I just cringe every time someone posts this 1/4 to 1/2 turn nonsense that is not correct and I agree could be dangerous in some situations.
Dan
Contributor
@Diving Dubai True. But forgetting to turn on the valve usually happen in the beginning, so might as well thumb the dive & fix it at the surface. If you are already spending a few minutes down there, you most likely already take care of opening the valve all the way at the beginning, so the problem could be something else, which causing the tank to lose pressure sooner than expected. I surely don't want to monkey around at the deep. Just thumb the dive.
Having your buddy to check your gear first is good though. I often see some bubbles leak though the bad tank o-ring that created high air usage. I get the habit of not only checking the tank pressure, but also listening any hissing sound coming from the reg before doning on the gear.
Having your buddy to check your gear first is good though. I often see some bubbles leak though the bad tank o-ring that created high air usage. I get the habit of not only checking the tank pressure, but also listening any hissing sound coming from the reg before doning on the gear.
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I don't think you guys understand. Nobody is arguing that closing the valve a 1/2 turn will impede airflow enough to bother anything. The problem is when it is just barely open because someone has closed it all the way and opened it a little. Someone helped my wife with her valve years ago and created a breathing problem when we went down. The valve needs to be closed "NO FLOW" or open "FULL FLOW' so the 2 are not confused. I was taught to do the back off from full open but gave that up and in cold and warm water open my valves fully with no negative consequences. If there is no downside why close the valve a little and leave the option for confusion?
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