Safety Pressure

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netmage

Rest in Peace
Rest in Peace
Messages
1,818
Reaction score
58
Location
Coral Springs, FL
# of dives
500 - 999
Ok - I've been taught and had engraved in my head "back on the boat with no less than 500psi".... For all of these experiences - I was using 80cuft. aluminum's at 3000psi. I went diving today with my brother who had borrowed an LP80 steel from his LDS - this was being filled to somewhere around 2400-2600psi.

So all day long - I'm starting at 3000psi - he's starting at 24-2600psi and his computer is going nuts when I've got 1000psi left in my tank and he's approaching 500psi. (Which is the default on both of our computers.)

My question is this - should we continue sticking with the 500psi rule when diving these lower pressure tanks - or is there another number/formula for working with these steel tanks. Thanks.
 
500 psi is really fairly arbitrary. 500 psi in an Al 80 is 12.9 cf, 500 psi in an LP80 is 15.15 cf.

A better gas management plan is to begin your ascent when you hit your rock bottom pressure. I have a bit of an explanation of the concept of rock bottom at http://jonnythan.com/rockbottom.html

This is using a fairly conservative SAC of 1 cf/min. You can increase this if you feel it's necessary. Basically you want to figure out how much gas it would take you and your buddy to ascend from the max depth for your dive, then begin your ascent as you reach this pressure.

If you're doing a dive where you want to be back at the anchor point before ascending, subtract rock bottom then divide the gas in two. For example, on a 90 foot boat dive, your rock bottom would probably be about 14 cf x 2 = 28 cf (1100 psi in an Al 80 and 950 psi in an LP80). So, for the Al 80, assuming you start with 3000 psi, that leaves 1900 psi for the dive. Once you've gone through 950 psi (your SPG reads 2050 psi), turn and head back for the anchor at 2050 psi or your buddy, on the LP80, has 1800. You should reach the anchor before you hit rock bottom, at which point you can begin your ascent.

You can see how it simplifies things if everyone is on the same type of tank. Rock bottom and turn pressures are good things to write down and go over before the dive..
 
Netmage, what's wrong with a simple analog gauge? It won't beep at you and it is easy to read. Single 80's are mostly used for no decom dives on head boats. The 80's don't hold much air-- so, to require somebody to surface with 500 psi is unrealistic sometimes. When you're on a charter, you never know if the wind will blow and cancel out the remaining dives for the day or week. Consequently, the diver may prefer to get the most out of that particular dive. If the dive is at typical sport depths, like 60 feet, the ascent can be started safely at 500 psi. There is enough air for a 5-10 minute stop if one really feels this is necessary. However, the present day computers are really conservative and the diver should use his/her judgement as to whether this is necessary. If you're really bushed and cold, or overweight, a 'safety' stop might be advisable.

Forget all those complicated formulas being billed as 'simple'. Simple is keeping an eye on things and surfacing when air is low. The only time you need to calculate air consumption is when swimming in current. At the beginning, proceed up current and then slowly proceed in the reverse direction when air is at 2/3. Make sure you can see the anchor line when air is no less than 1/4. Again, surface when air is low. For a std aluminum tank allow 500 psi for a slow ascent and short stop.

Don't ration your air with the assumption that you can swim back on the surface using tank air. carry a snorkel.

Even if your tank has only 50 psi, that is enough to blow water out of the valve orifice (if any) before refilling.
 
If any machine beeps at me while I'm diving I'll smash it wit a hammer.
 
MikeFerrara once bubbled...
If any machine beeps at me while I'm diving I'll smash it wit a hammer.

Do you bring a hammer with you just for that purpose ;-)

BTW, my mechanic says he'll remove the "beep" module from any car for free. Can you imagine working on a car for 1 hour with the door open, key in the ignition, and having it go beep beep the whole time?

Down with all beeps! :)
 
most computers have user adjustable alarms... it sure beats having someone sucking on an empty hose!

Never skip a safety stop unless you are completely out of air. In fact you should also do at least one deep stop at half of your max depth for one minute. You should then finish with a 3-5 minute stop at 15-20 fsw. You won't get a rebate on the air you don't use, so don't sweat it... just do the stop!

The gas you ascend with depends not only on your depth, but also the tank volume. You are not going to see the whole ocean in one dive, so don't try. Don't dramatically push the limits of your experience. If you want to stay down longer, work on your fitness before diving and becoming more comfortable in the water... and lazier too!!!
 
Braunbehrens once bubbled...


Do you bring a hammer with you just for that purpose ;-)

BTW, my mechanic says he'll remove the "beep" module from any car for free. Can you imagine working on a car for 1 hour with the door open, key in the ignition, and having it go beep beep the whole time?

Down with all beeps! :)

LOL. Not but I don't take anything that beeps on a dive. I always have a hammer in the truck just in case though.

Really though, I think hat's a great idea a beeper to tell you you're out of air. That togetther with a beeper to tell you you're bent and you won't have a thing to worry about because you'll always know when you're out of air and bent.

I heard there was going to be a dive computer that would call 911 for you.
 
most computers have user adjustable alarms... it sure beats having someone sucking on an empty hose!

Never skip a safety stop unless you are completely out of air. In fact you should also do at least one deep stop at half of your max depth for one minute. You should then finish with a 3-5 minute stop at 15-20 fsw. You won't get a rebate on the air you don't use, so don't sweat it... just do the stop!

The gas you ascend with depends not only on your depth, but also the tank volume. You are not going to see the whole ocean in one dive, so don't try. Don't dramatically push the limits of your experience. If you want to stay down longer, work on your fitness before diving and becoming more comfortable in the water... and lazier too!!!
 
In all seriousness...

The problem with all these crutches, like a computer that beeps to let you know you are running low on air, is that eventually people rely on them. Then the crutch breaks, you fall down, you break your neck.

To avoid running out of air, check your spg. It really doesn't get any simpler than that....(and I'm being nice here).

Most indicators such as lights and beeps have a basic flaw....their failur mode is identical to the "everything is OK mode".

There is a great scifi story about a guy who has to have this test every morning. The machine responds with either green light or red light. Every morning the light is green, then one morning there is no light. The guy assumes the green light has burned out, and figures he'll deal with it later...but doesn't get around to it for a few days. You guessed of course that the light that was out wasn't the green light, it was the red light.

An spg is easy to check. If it hasn't moved after 10 mn and you're breathing from the tank it's broken.
 
Braunbehrens once bubbled...
In all seriousness...

Most indicators such as lights and beeps have a basic flaw....their failur mode is identical to the "everything is OK mode".



In all seriousness and from an engineering point of view you're right. No beep means you have air or a broken computer. If they reversed the logic and made it beep to mean you had air you would have a valid test. It would be "Fail safe" in other words.

Since at most it should be considered a backup it might be forgivable, though still useless and just an iritating noise in a place where I don't want it. IMO, it's another useless frill to sell pretty toys. They would be better off putting a couple of d-rings on it or something.

BTW, I'm not what you would call technically challanged. On the contrary I design such test, monitoring and warning systems for manufacturing processes. I still say an awful lot of dive equipment designed by non-diving pranksters.
 
https://www.shearwater.com/products/perdix-ai/

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