Rock bottom, 500 PSI, or something else?

[boat sju]

I believe Lamont's "rock bottom rule of thumb" was (depth x 10 ) + 300 . And the max depth for an 80 cu. ft. was 80 ft, 100 ft for a 100 cu.ft. tank, etc.

 

[Insta-Gator]

I'm thinking a lot of what has been said will be very confusing to new divers in this "Basic" forum. I recommend they discuss gas management with their instructor and with their DM.

 

[Lobzilla]

I'm thinking a lot of what has been said will be very confusing to new divers in this "Basic" forum. I recommend they discuss gas management with their instructor and with their DM.

Just want to add that if any new diver takes this topic seriously enough to discuss this with an instructor or DM and gets the answer "Don't worry, just be back with 500 psi" it's time to run, not just walk away from that teacher.

When one of your kids who just got his/her drivers license asks you whether there is enough gas in your car to make it to a destination and back, are you going to answer "Don't worry, just be back with 1/6"?

 

[w3dge]

The standard GUE min gas calculation. Because I enjoy kool-aid.

2 divers X 0.75 ft3 SAC X time needed to ascend x average ATA. = ft3

So for 100' it would be 2 x 0.75 x 11 x 2.5 = 41.25 ft3, rounded up to 42 ft3

(Time needed to ascend = 10ft/min + 1 min)

Then you would convert the ft3 to psi according to what tank you are using. (Xft3 ÷ tank factor x 100 = psi)

So an AL80 @100' = 42 ft3 / 2.5 X 100 = 1660, round up to 1700 psi.

HP100 = 42 ft3 / 3 X 100 = 1383, round up to 1400 psi.

I've never dove with someone, beside other GUE divers, that reserve that much...Which is why I got an AL40 pony.

Ascent speed of 10ft/min seems excessively slow, especially in an emergency situation on a no stop dive.

 

[boulderjohn]

Ascent speed of 10ft/min seems excessively slow, especially in an emergency situation on a no stop dive.

I have never heard 10 ft/min. The standard I was taught to use was 30 ft/min. For decades the world used 60 ft/min as a standard ascent rate--30 fpm is a relatively new standard.

I have heard some people argue this way:

For an NDL dive, I don't have to do a safety stop. Let's say it takes us a minute to make a regulator donation, after which we ascend to the surface at 60 ft/min. Let's assume that we will have a SAC rate of 1.0 for that ascent because at least one of us is a bit concerned. From 100 feet, that would mean we would need 2*4 = 8 cubic feet for the exchange. We would then ascend for about 2 minutes at an average pressure of about 2.5 ATA, so that would be 2*2*2.5 = 10 cubic feet for the ascent. Pad it for inaccuracies of the gauges, and I need to leave about 22 cubic feet for a true emergency. Anything more than that and I am adding layers of safety.
​

Now, I don't advocate that level of minimum safety, but it does indicate what you absolutely MUST have. Going beyond that is what you need to decide.

 

[tbone1004]

I'm thinking a lot of what has been said will be very confusing to new divers in this "Basic" forum. I recommend they discuss gas management with their instructor and with their DM.

what's wrong with discussing gas management with all of the instructors on here? Especially since the recreational agencies don't really teach anything about gas management....
anyone know of a recreational course taught by one of the large agencies where calculating your sac rate is actually required?

 

[boulderjohn]

In some locations, a rock bottom approach, even an especially conservative one, can lead you to a longer dive than the "back on board with 500 PSI" approach.

Let's say you are at 100 feet and reach the most conservative level advocated in this thread. You are diving a sloping reef, something like you would find in Bonaire. You ascend to 60 feet without incident, taking about a minute to do so. Time to recalculate. You find that you can now spend a pretty decent amount of time at that depth, using the same system. Then you go to 30 feet and recalculate. Then you go to 20 feet. At that point you have nearly all the gas left in your tank to finish off the dive.

 

[seeker242]

Ascent speed of 10ft/min seems excessively slow, especially in an emergency situation on a no stop dive.

From a GUE perspective, as soon as you have air, the emergency is over and because stops are planned for, there is no need to skip them. You just do a normal ascent just like any other dive. GUE does not advocate skipping stops. If you have air and are breathing fine, there is no longer any emergency. Of course, if you have your arm bit off by a shark, that's a different situation.

 

[boulderjohn]

From a GUE perspective, as soon as you have air, the emergency is over and because stops are planned for, there is no need to skip them. You just do a normal ascent just like any other dive. GUE does not advocate skipping stops. If you have air and are breathing fine, there is no longer any emergency. Of course, if you have your arm bit off by a shark, that's a different situation.

Is GUE advocating 10 ft/min as a normal ascent rate?

 

[Insta-Gator]

Just want to add that if any new diver takes this topic seriously enough to discuss this with an instructor or DM and gets the answer "Don't worry, just be back with 500 psi" it's time to run, not just walk away from that teacher.

When one of your kids who just got his/her drivers license asks you whether there is enough gas in your car to make it to a destination and back, are you going to answer "Don't worry, just be back with 1/6"?

True, but you have made some erroneous assumptions about my original statement.