500 psi on the boat?

[LeadTurn_SD]

Okay, back to the original subject of 500 psi back on the boat......as a new diver, I can completely respect that guideline. However, I use a LP 95 that starts at only 2400 psi. So what should a diver return with given that scenario? My first boat dive during AOW was the deep dive. I returned with 240 psi and 2 of the other 4 students (using AL 80s) had to share air!!! Don't ask....I'm still pissed about that dive!

I think as others have stated, 500 psi is picked many dive operators because it's an easy number to remember and ensures a least a bit of a safety margin... and the thought of displeasing the boat captain or dive master by getting back aboard with <500 psi might ensure that the "lowest common denominator" diver will actually remember to pay attention to their gas supply the dive!!

In your case with a LP 95, I see your point, but you probably just have to live with the 500 psi rule if you are on a dive boat, unless you have a sympathetic DM.

Getting away from the "500 psi Rule" and just looking purely at safety.... I look at the dive as a whole. How deep, difficult, etc. the dive I am planning determines how big a "safety buffer" of reserve gas is needed. Do a search on this board for "Rock Bottom" and you'll get the idea. I often finish a dive with quite a bit more than 500 psi (shore diving, so nobody is "telling" me what to do, but I'm giving myself a big safety buffer).

Safe Diving!

 

[on_two_wheels]

Thanks guys!!

 

[Bob Vincent]

My personal feeling is that it is almost ridiculously dangerous to be traveling on a small boat for the purposes of diving, if you are someone who would panic in the water without a fully inflated BC and a snorkel. What happens if you fall overboard?

I agree that in rough seas, having a regulator is a nice thing. If I surface with 500 psi in my 95s, I've got over 30 minutes of gas on the surface. If I'm in really heavy seas and it takes the boat longer than 30 minutes to pick me up, somebody has made a significant judgment error (probably me).

I have read a story about an experienced diver and instructor who got blown away from the boat in heavy current in the Galapagos, and surfaced where two currents intersected, and the seas were six feet or more. He breathed his tank absolutely empty, and then tried to use his snorkel to help. He was a couple of hours in the water before they picked him up by his EPIRB. That's a pretty terrifying situation, but no new diver should be in the water in anything close to those conditions.

I'm a believer in reserves and gas management/planning, but telling new divers to keep a third of their tank in case they have to breathe it on the surface is a little overkill in my opinion. Learning to swim, however, is NOT.

Yes what she said! When she talks ... it's always been pretty good stuff I've read.

 

[The Logic Theorist]

swimming back down to recover your missing wife

Why on earth would anyone want to do that?

 

[Scared Silly]

Cause she has the keys to the room


As many have mentioned the 500 psi back on the boat is a guideline that most operators use to keep the one a year vacation diver out of trouble. It is based on the Al80 @3000psi. If less the operator feels uncomfortable, any more the diver feels like the operator is shorting them on dive time.

As for the snorkel. It is usually on the boat. If I think conditions warrant it then I take it with me. There are many dives where weak/poor swimmers should be left on land. You do not need to be a strong swimmer but being a competent swimmer is important and to me makes the diving more enjoyable.

 

[Zieg]

OK. So the boat asks for you to return with at least 500 psi. How many of you have ever had someone from the boat go around and check the gauges? Have any of you seen someone denied a second dive because they allowed their tank to get below the magic number? Besides, if they are my tanks, what do you care if I suck them dry?

 

[Scared Silly]

No but I have had "arguments" with DM who want to end the dive after 40 minutes I still have 1000 psi in my cylinder.

 

[Mr Carcharodon]

Okay, back to the original subject of 500 psi back on the boat......as a new diver, I can completely respect that guideline. However, I use a LP 95 that starts at only 2400 psi. So what should a diver return with given that scenario? My first boat dive during AOW was the deep dive. I returned with 240 psi and 2 of the other 4 students (using AL 80s) had to share air!!! Don't ask....I'm still pissed about that dive!

On two wheels,

Rock bottom is the amount of gas reserve needed to get both you and you buddy to the surface while doing all required stops. So it is the pressure you need to leave the bottom at, not the pressure back on the boat. The amount of reserve in psi will depend on the larger of your, and your buddies, surface consumption rate, your depth and tank characteristics as you have already figured out. So assuming between you and your buddy the higher SAC is 0.6 cu ft/min, and you both have LP95s, then the rock bottom for 120, 100, 80 and 60 fsw would be 1135, 920, 724 and 546 psi. At least that is what I would plan on, others may make somewhat different assumptions. Also this is leaving you with zero psi at the surface and you will have to decide if that is a problem or not.

The other imbedded assumption is that you are doing a no deco dive with a GUE like min deco ascent to the surface. That is a relatively slow ascent compared to older 30 or 60 fpm direct ascents to the surface. The faster ascent to the surface would allow for significantly less gas reserve but carries with it a higher risk of DCS. Still the risk is probably below 1%. Personally I would rather get back on the boat with <<500 psi and have a lower risk of DCS.

 

[String]

We have to teach (and in theory use) rule of thirds now to trainees. Ridiculous overkill for no stop shallow open water dives but the agency has mandated it. So for a typical dive now we have to be back on the boat with 77 bar (1100psi) assuming they had a full tank to start with.
Silly idea for that sort of diving but at least it is based in logic (at least slightly).

 

[Blackwood]

Rock bottom is ... leaving you with zero psi at the surface and you will have to decide if that is a problem or not.

Only if you were forced to share from the deepest portion of the dive. I don't think there are many people (even amongst the most vehement supporters of arbitrary rules such as 'back with 500') who would get on your case for coming up with less gas than mandated because you had an emergency.

Though, depending on why, they may get on your case for having to share in the first place