PDA

View Full Version : Why can't I dive twice on the same tank?



littleleemur
12-02-2007, 04:52 PM
Most charter operators do not let me dive twice on the same tank even if I surface with more than 2/3 of air. However they're OK with my 2nd tank, which is smaller & has less air than what I came up with in the first one.

This would be the 2nd dive on the same wreck and with the same dive plan.

What's so annoying about this is that I still pay the same for the tank to be topped off at the end of the day.

Daddy-h2O
12-02-2007, 05:11 PM
My guess is becuase we live in fear of lawsuits, at least if something should happen, they can say you left the boat with a full tank, instead of a partial tank. Some non-diving jurors may not see the diffrence between a partially filled 80 vs a full 63. A tank is a tank thing.

Just my opinion.

lottie
12-02-2007, 05:29 PM
I agree with daddyh2o as well - lawsuits and liability.

down here, you can do a wreck dive, have an SI on the boat for an hour, then do another dive. They allow two tanks - one for each dive. at least that way they know that all divers are starting off with 3000psi in their tanks at the start of the dive. plus the added bonus of preventing a possible accident....

just my two cents :)

littleleemur
12-02-2007, 06:16 PM
That's what I've been told, but you wonder when there are a bunch of people trying to get your attention as you ascend to your safety stop....

At least I know that they won't forget me when it's time to go.

seasnake
12-03-2007, 04:33 PM
I would say it's a case of not understanding tank volume vs. psi ... It's amazing how many divers don't get that . . .

acelockco
12-05-2007, 03:18 AM
Most charter operators do not let me dive twice on the same tank even if I surface with more than 2/3 of air. However they're OK with my 2nd tank, which is smaller & has less air than what I came up with in the first one.

This would be the 2nd dive on the same wreck and with the same dive plan.

What's so annoying about this is that I still pay the same for the tank to be topped off at the end of the day.


Well, if you are a newbie, then just do as you are told and deal. Now if that is not the case and you really know what you are doing and have the dives to back that up, then I say, BE QUIET!!!

The only way they are even going to notice is if you say something. The captain is usually dealing with the boat, not watching you monkey with your gear. Keep what you are doing to yourself and no one will probablly even notice.

littleleemur
12-07-2007, 12:55 AM
I have decent air management, know what I am doing and have a large number of dives to back it up. And yes, I am quiet about it.

I don't even bother asking when on vacation. Why make things more complicated than they have to be? Also, if anything happens to a tank in the hot sun, well, I *may* have an extra tank :) See how considerate I am?

However, I got chewed out my a member of my usual dive group for being irresponsible when I use my tank twice because "everyone expects you to have 1/2 tank full if not the 2/3. You're the *go-to* person and you're endangering others when you don't"

Excuse me but ***? Endangering others?! Irresponsible?! When was I the designated self propelling spare air carrier? The people who are sucking their tanks dry are the dangerous and irresponsible ones. I'm not a DM so why would I carry air around for everyone else? Even when I use my tank twice, I have more than enough air for my buddy if he needs it.

seasnake
12-07-2007, 02:01 AM
That is a bit silly, because obviously your dive plan is going to incorporate having air to get your buddy home in case of a problem, regardless if you start with a full tank or 2/3 of a tank. It's crazy to think that others in your group feel they don't have to manage their air because you will be there to bail them out! yikes!

littleleemur
12-07-2007, 02:19 AM
You know it's a problem when Santa gives you a 360 swivel in your stocking for your Octo....& it's probably why the 60's I asked for 4 x'mases ago haven't materialized under the tree yet.

Daddy-h2O
12-07-2007, 02:31 AM
Why don't you use that extra air for more bottom time on the 1st dive? Sorry probly not helpful.

I dive with I guy I swear surfaces with more air than he started with. He has fantastic air management. But not a one of us expect him to be the "go-to "guy in an emergancy.

That is like saying you don't drink enough so you are the designated driver.

littleleemur
12-07-2007, 03:24 AM
I run out of time not air :)

bottlefish
12-07-2007, 08:39 AM
I think that member of your dive group needs to take a step down. Relying on one person, expecting one person to have all of your problem solutions up their sleeve can be dangerous in it self. What's the plan if it's you that had gas problems. Also, what if you weren't close enough to the problem to assist, had to bail early from the dive for whatever reason and so not even there etc.

Seasnake makes a valid point, we are all spare air carriers for each and every one of the other people we dive with, in case they have major problems with air supply and we're the ones that have to assist. So we should always have enough air left in our cylinder to get us and An Other to the surface safely in case of problems. Provided you have this, then obligation sorted... assuming you are running on a no deco schedule, then not too hard to manage.

acelockco
12-07-2007, 01:46 PM
FYI,

On New Jersey Dive boats, MANY of the divers use doubles, and do two seperate dives on the same cylinders. I thought it was strange as well, but even on their second dive, they start off with more air than I have in my al80.

seasnake
12-07-2007, 02:27 PM
FYI,

On New Jersey Dive boats, MANY of the divers use doubles, and do two seperate dives on the same cylinders. I thought it was strange as well, but even on their second dive, they start off with more air than I have in my al80.

I do that, but I think it's more out of laziness ... so I don't have to switch tanks between dives ... :D

littleleemur
12-07-2007, 04:53 PM
Even if I use my tank twice, I still have enough for my buddy, and more. So I'm fuming about the fact that someone is suggesting that I have to carry all that air to literally be an air-wetnurse for a bunch of potential idiots. In that line of logic: how long is my safety stop supposed to be exactly? 1hr until everyone is out of the water?

As for the doubles: how do you get out of the water with all that weight?!

bottlefish
12-07-2007, 05:20 PM
Hmm, re-educate, or find a new dive team :-)

Getting out of the water for doubles? Ah, well that sort of divings not for women really, they're too weak and fragile... OK, that was in bad taste (sorry!).

It does take a bit more care, but not as hard as you might imagine. Plus, if you're air is as good as you say, then there's no need for you to double on 12 litre cylinders, take a couple of tens or eights instead.

littleleemur
12-07-2007, 05:33 PM
My air management is ok so I haven't really thought about it until now. I average about 2/3 -1/2 an 80 for all of my dives so far. I haven't taken any of those clinics or courses but I'd like to.

acelockco
12-07-2007, 08:06 PM
Now I am confused?

It shoud be a rare time when you can reach your NDL on an AL80 and a non repetative dive. Is there something I am missing? Are you VERY VERY SMALL? Like 4'5" 70lbs?

What is your avg. dive depth and bottom time? I am just trying to figure this out.

littleleemur
12-07-2007, 08:58 PM
I'd like to be closer to 70lbs :) The closer you are to weighing 0lbs the better right? At least that's the impression I'm getting from the Victoria Secret Fashion Show :p

A sample dive would be: penetration would range from 90-70ft inside the wreck depending. So a 45/50-min dive (quick decent down the line and the requisite 3-min safety stop) I would be getting back on the boat with 2100-2250psi with a starting pressure of abt 3150psi. 2nd dive, same profile, same tank and surfaced just under 1000psi w/ a starting fill of 3150psi on an 80. Non-penetration, on the same wreck with quite a bit of swimming around, I would surface w/ abt 1600+, maybe more. I'm not quite sure b/c the non-penetration dives are done on a mixed boat and I'm always sharing my octo w/ someone. (Which is scary b/c they really suck the air back).

So if there's a lot of current or swimming I definitely would not do 2 dives on 1 tank.

acelockco
12-08-2007, 02:20 AM
Are you missing a lung?

Either you are pulling my chain or you might be the record holder for the least air consumption! At 90 feet, I doubt I would even get 45 minutes out of an 80 cubic foot cyl. You are saying you get 100 minutes with 1000 psi to spare, that does not compute to me.

Maybe I am missing something.

Now I have no idea what you are talking about "alyays sharing an octo", it is for an emergency, not for always use.

littleleemur
12-08-2007, 02:42 AM
No, my lungs are intact and quite healthy :) & the 45-mins is the total dive time (including quick descent and the 3-min safety stop), not 90ft at 45mins. Not sure if my computer will allow 45-min at 90ft.

In a wreck you just chill and do your thing so you don't silt out. So obviously the air consumption will be good. There's also no current inside that wreck. So for 2 penetration dives on that wreck I can get 2 45-min dives off the same tank while surfacing w/ abt 900 or so psi give or take a 100.

When swimming around, esp in current, it's not that good so I don't think I can do 2 dives while still surfacing with over 500psi.

As for the sharing octo part, I have more than enough for my buddy and I to reach the NDL together and surface with 1000psi each. Obviously we don't do the sharing bit while in the wreck (entaglement etc), but it's fine when we're out and going up the line & on the safety stop. We manage our air so that he will have at least 1200psi when exiting the wreck. Then we swim around for a bit to check out the outside of the wreck and head back to the line.

acelockco
12-08-2007, 02:11 PM
Who taught you this technique of sharing air without it being an emergency, or emergency practice?

Please, don't take any offense to this, but I think it may be time for some SCUBA classes. Your air management seems unrealistic and sharing your air for no reason sounds like not a good idea.

Good luck!

BamaCaveDiver
12-08-2007, 05:11 PM
Who taught you this technique of sharing air without it being an emergency, or emergency practice?

Please, don't take any offense to this, but I think it may be time for some SCUBA classes. Your air management seems unrealistic and sharing your air for no reason sounds like not a good idea.

Good luck!

A lot of us do this all the time Ace, we call it drills. If you wait until a true emergency, then you will have to waste a bit of time to think prior to reacting; if you practice these skills on a regular basis, it becomes second nature (muscle memory and all that garbage.) Even when I am diving solo, I will breathe my backup from time to time just to ensure its functionality.

As for banging the NDL's with an AL 80, it can be done quite easily. This is especially true for more experienced divers (the more you dive the more relaxed you should become and thus the better your SAC rate.) Women are notorius for having extremely low SAC rates to begin with, but I have met my share of male divers who I swear had to have gill implants.

acelockco
12-08-2007, 07:06 PM
Bamma,

He/she is not talking about doing a drill, or as I called it "emergency practice".

I also don't think the person asking this question would be of the level you are speaking about concerning NDL with an AL80. Of course it can be done, without too much difficulty, but what we are talking about is hitting your NDL TWO TIMES with 1000PSI to spare. I can't figure it out, even with unreasonablly low SAC rates.

littleleemur
12-09-2007, 07:51 AM
No offence taken acelockco. But I can't help being "unrealistic" about my air management. My air consumption has been the same since Day 1. So I guess I should take more SCUBA classes since the main indicator of skillz improvement is air consumption and it's been unchanged. Therefore I've obviously not improved as a diver.

No one has taught me this specific style of sharing air. I have a lot of it, more than I could "realistically" use. So I really don't see any harm in sharing any of it, regardless of whether it's a drill or not.

My original question was regarding "why" I was advised not to use the same tank twice -- logic and a basic understanding of psi vs. tank capacity didn't make sense to what they were telling me.

As for hitting the NDL, I have done it every time since my first checkout dive & can't currently see a reason where I won't continue to do so. As for a penetration dive for the dive profile I had used as an example, I have hit my NDL twice on the same tank multiple times with approximately 900psi (+/- 100) when allowed to do so.

amtrosie
12-09-2007, 03:26 PM
Bamma,

He/she is not talking about doing a drill, or as I called it "emergency practice".


.




What is a drill? Nothing more than simulating a scenario in which a specific procedure is required to mitigate the situation. Hence... an "emergency practice." It is most certainly a good idea to practice as often as possible. Heck, I have dives that are nothing more than one drill after another. What ever each member of the team can come up with. These may not be the most enjoyable, but they certainly are beneficial to the entire team. We always seem to manage to gain some insight to the scenario practiced.

BamaCaveDiver
12-09-2007, 04:29 PM
My original question was regarding "why" I was advised not to use the same tank twice -- logic and a basic understanding of psi vs. tank capacity didn't make sense to what they were telling me.


He who owns the boat gets to make the rules, and for most deck hands it is just easier to look at the pressure rather than attempt to convert that into cubic feet. It might help if you found some off time to walk the objecting party through the math and show them that it is the cuft that matters more than the psi, then again it might have the opposite effect. I just let the owners make their rules and if I can live with those rules I patronize their business and do the dives. If the rules seem to silly to deal with, I look elsewhere.

littleleemur
12-09-2007, 11:18 PM
It might help if you found some off time to walk the objecting party through the math and show them that it is the cuft that matters more than the psi, then again it might have the opposite effect.

That's what my rant is about:
1) I have to pay for a full fill even if I don't use a full tank.
2) I would rather have 1 less tank to schlep.
3) I can do it safely :D
4) I understand the physics, even if some don't :p



If the rules seem to silly to deal with, I look elsewhere.

I have no problem dealing with that :) I believe in always respecting the DMIC & the Boat Captain regardless of your level of training/specialty .

Anyways, thanks all for listening to my rant. :p

allisonfinch
12-26-2007, 11:15 AM
[QUOTE=bottlefish;8924]Hmm, re-educate, or find a new dive team :-)

Getting out of the water for doubles? Ah, well that sort of divings not for women really, they're too weak and fragile... OK, that was in bad taste (sorry!).

Whew! I don't trust myself to reply to this!

I always get a chuckle when I see guys show up on a strictly rec dive with doubles. I use them for tec dives but don't feel the need to try to impress the reccies when they are not necessary. I think it's a waste of energy. Gee, how hard is it to move a BC and reg?


I also have really good air consumption rates. It is rare to come up with less than 1400 lbs of air in my tanks. I try to arrange small tanks from the dive operation, if I can. Otherwise, I just bring back the air. I use it to practise many air rings during the safety stops. I feel secure that I would have plenty to spare, just in case.....

acelockco
12-26-2007, 02:46 PM
Gee, how hard is it to move a BC and reg?



It is very easy to move a BC and reg, but to change over from a single cylinder to a double cylinder is a major change that usually requires tools and adaptors. Plus you will need an additional cylinder or a replacement valve and someone to inspect and fill your cylinder.

seasnake
12-27-2007, 11:07 PM
I wear my doubles mainly because I need the practice in them!! :p Shallow dive or not! Plus I don't have a single tank wing, but the smurf is going to hook me up there ... ;)

invalidlink
12-27-2007, 11:27 PM
It is very easy to move a BC and reg, but to change over from a single cylinder to a double cylinder is a major change that usually requires tools and adaptors. Plus you will need an additional cylinder or a replacement valve and someone to inspect and fill your cylinder.

I guess we are pretty spoiled here in Anilao. After doing doubles with 2 side mounts. We return to the resort eat and rest, by the time we are ready to go back in the water, our twins and deco bottles are topped off and ready to go. No need to remove the bladder from the twins.

But I agree having to move your bladder to another twin tank setup is a pain!

Doing two tech dives in a single day is really tiring.

Alison, a female friend who is 5'3 manages her twin tank setup quite nicely! And she is so slim.

bottlefish
12-31-2007, 12:00 PM
QUOTE=bottlefish;8924]Hmm, re-educate, or find a new dive team :-)

Getting out of the water for doubles? Ah, well that sort of divings not for women really, they're too weak and fragile... OK, that was in bad taste (sorry!).

Whew! I don't trust myself to reply to this!


Just thought I should clarify, my comment was totally tongue in cheek, not at all serious and totally opposite to my true opinion; just a gentle reference to another post by LittleLemur, "Why are scuba chix ridiculed/not taken seriously by guys" that was running at the same time

http://www.scubamagazine.net/showthread.php?t=1289&page=2

Apologies for any elevated blood pressures caused :)

Re the original question, how much money would be returned if the dive centre/boat in question refunded you for that second air fill and tank hire, or put another way, how much would they charge you if you rocked up with your own tanks? Surely you're only talking about a couple dollars for the fill, the majority of the money paid goes towards the space on the boat, and the cost of running the boat?

Tigerbeach
12-31-2007, 12:19 PM
So I'm fuming about the fact that someone is suggesting that I have to carry all that air to literally be an air-wetnurse for a bunch of potential idiots.


So tell us how you really feel about this; don't be shy and hold back...

scuba smurf
12-31-2007, 04:58 PM
I wear my doubles mainly because I need the practice in them!! :p Shallow dive or not! Plus I don't have a single tank wing, but the smurf is going to hook me up there ... ;)


first I gotta teach you how to use the doubles:p

littleleemur
01-02-2008, 10:12 PM
So tell us how you really feel about this; don't be shy and hold back...

I'm sure what I'd like to *really* say would be censored :p

I never deny air to anyone or yell at them later for coming to me underwater; I'm just glad that I was there when they needed it and so avoided the unthinkable. Just the attitude that annoys me. Plus the additional comments on how they saved $ on air b/c they could always use mine! Once or twice is funny, more & it gets old. In that light, it seems fair that fills should be charged on what you need.

& the Charter operator isn't always the one who does the fills. I'm happy to tip the Charter guys :)

Papa Bear
01-31-2008, 02:31 PM
First, you are talking about diving in a overhead environment! You should always have a topped off tank and should never dive a tank smaller than your buddy! In Cave Diving that can get both of you killed! When you are in an overhead situation you have to think of your buddy team as one unit. You have to carry enough air to get both of you to safety! It is the rule of thirds! And you are going in a third down to start! If your Caribbean diving in open water I could care less! But if your doing a 63cft in wreck dives you won't be my buddy or I wouldn't let you in the water on my boat! Take a wreck course or a cave course! You can't make more are at 80ft! For Some people a 63cft is a bailout bottle! you can always dive a bigger tank than your buddy but never smaller!

lars2923
01-31-2008, 10:37 PM
First, you are talking about diving in a overhead environment! You should always have a topped off tank and should never dive a tank smaller than your buddy! In Cave Diving that can get both of you killed! When you are in an overhead situation you have to think of your buddy team as one unit. You have to carry enough air to get both of you to safety! It is the rule of thirds! And you are going in a third down to start! If your Caribbean diving in open water I could care less! But if your doing a 63cft in wreck dives you won't be my buddy or I wouldn't let you in the water on my boat! Take a wreck course or a cave course! You can't make more are at 80ft! For Some people a 63cft is a bailout bottle! you can always dive a bigger tank than your buddy but never smaller!

Well, if you dive with a bigger tank than your buddy, than your buddy is diving a smaller tank then their buddy, and you never dive on a smaller tank than your buddy... lol... Just messin with ya...

Papa Bear
01-31-2008, 10:58 PM
I am only talking about overhead environments on days that end in "Y"!;) :rolleyes: :eek: Otherwise go for it!

seasnake
02-01-2008, 02:27 AM
Wow ... that is a LOT of exclamation points!!!


First, you are talking about diving in a overhead environment! You should always have a topped off tank and should never dive a tank smaller than your buddy! In Cave Diving that can get both of you killed! When you are in an overhead situation you have to think of your buddy team as one unit. You have to carry enough air to get both of you to safety! It is the rule of thirds! And you are going in a third down to start! If your Caribbean diving in open water I could care less! But if your doing a 63cft in wreck dives you won't be my buddy or I wouldn't let you in the water on my boat! Take a wreck course or a cave course! You can't make more are at 80ft! For Some people a 63cft is a bailout bottle! you can always dive a bigger tank than your buddy but never smaller!

Papa Bear
02-01-2008, 02:32 AM
I think of them as passion points!!!! And I am very passionate about my diving;)

scuba smurf
02-01-2008, 01:12 PM
First, you are talking about diving in a overhead environment! You should always have a topped off tank and should never dive a tank smaller than your buddy! In Cave Diving that can get both of you killed! When you are in an overhead situation you have to think of your buddy team as one unit. You have to carry enough air to get both of you to safety! It is the rule of thirds! And you are going in a third down to start! If your Caribbean diving in open water I could care less! But if your doing a 63cft in wreck dives you won't be my buddy or I wouldn't let you in the water on my boat! Take a wreck course or a cave course! You can't make more are at 80ft! For Some people a 63cft is a bailout bottle! you can always dive a bigger tank than your buddy but never smaller!

I definitley agree with getting the proper training portion of your post papa. However, in a perfect world everyone would be diving the same tanks, this isn't the case however. When planning ANY dive turn pressures/volumes should be calculated based on the person with the smallest amount of breathing gas and/or the highest sac rate.
I'm not a cave diver..........yet, but I have heard of cave divers re-calculating 3'rds/6'ths and heading back in for a little jaunt in the cave so I'd say these folks are heading in with a less than full gas supply. For any overhead diving, the team should be configured the same with redundant gas supplies, proper training.

Papa Bear
02-01-2008, 03:03 PM
If I am in A OH environment with a diver with a 63 and their sac rate is great and they get into trouble we have no problem, unless I screw up, but if have a problem and have to put my life and theirs on whats left in their tank we have a big problem. It happen in Caving where the team is H & W or B & G and one sets up with twin 72s or 80s and the the hog sets up with 105s or 120s! The trouble is on the horizon, what happens if you just before turn around and the hog has a major problem with the air supply? Is 2/3rds of a set of 72s enough to get both of you out? Maybe, and I am not putting my life on the line for a maybe! Never go in an Overhead environment unless planned and with the amount of air you planned on! So its not about her or her sac rate, its about the dive, and where your going and what your doing!

scuba smurf
02-01-2008, 03:19 PM
from the sounds of it maybe some proper overead/dive planning training would be in order for you;)

Papa Bear
02-01-2008, 03:48 PM
Really? Why do you say that? I was trained by Bill Rennaker and John Orlowski can you think of any two that are better? If you have some constructive criticism of my explanation then post it! Otherwise I don't see the reason for your statement? Even with a wink? You lost me some where deep in the cave I guess? Or maybe its your rant?

scuba smurf
02-01-2008, 03:56 PM
now who needs to lighten up ROFL

Papa Bear
02-01-2008, 04:02 PM
I have been working on that! Yo Momma! :p

BamaCaveDiver
02-01-2008, 04:56 PM
I definitley agree with getting the proper training portion of your post papa. However, in a perfect world everyone would be diving the same tanks, this isn't the case however. When planning ANY dive turn pressures/volumes should be calculated based on the person with the smallest amount of breathing gas and/or the highest sac rate.
I'm not a cave diver..........yet, but I have heard of cave divers re-calculating 3'rds/6'ths and heading back in for a little jaunt in the cave so I'd say these folks are heading in with a less than full gas supply. For any overhead diving, the team should be configured the same with redundant gas supplies, proper training.

SAC rates have no affect here, you simply calculate your turn pressure based upon the minimum capacity within the group. If diver A starts the dive with 90 cuft and diver B starts with 250 cuft, diver A's supply would dictate turn pressure, in this case 30 cuft. Convert that back to PSI and note your turn pressure. Thus both divers are limited to using 30 cuft of gas before turning the dive. It makes no difference who has the highest SAC rate, as when the number is hit by either diver, the dive gets turned. Nothing at all wrong with recalculating as long as you adhere to your rule of thirds (the dives will be shorter in duration, but just as safe.) :)

You make use of your SAC rate to determine the minimum gas requirements for your dive. If you want to dive to x feet for y minutes, you would make use of your SAC rate to determine how much gas you will need to accomplish the dive. You would then calculate thirds as usual, with no reference to individual SAC rates.

scuba smurf
02-01-2008, 08:28 PM
Thanks Bama, my bad, you explained it much better

Papa Bear
02-02-2008, 01:24 AM
Bama, you did make it simple! You did a better job of explaining it than I did! Thats for sure, but the out come is the same! ;)

littleleemur
02-04-2008, 07:35 PM
When you are in an overhead situation you have to think of your buddy team as one unit. You have to carry enough air to get both of you to safety! It is the rule of thirds!

Of course I carry enough air for my buddy! & yes, we follow the rule of 3rds & dive our plan. Whoever first uses 1/3 of their starting supply & we turn back regardless. I do agree with you about the pitfalls of H&W, B&G buddies, so that is why planning is important. In a perfect world, we would be buddied-up with those who have the same dive-style, SAC rates, air supply & gear. [I think we're starting to get into DIR territory here....]


I am not putting my life on the line for a maybe! Never go in an Overhead environment unless planned and with the amount of air you planned on! So its not about her or her sac rate, its about the dive, and where your going and what your doing!

& for an overhead environment, I would not buddy with a known hog, or potential hog (aka he/she-who-becomes-a-hog-when-conditions-are-less-than-ideal). In my limited experience, I've seen such individuals not follow basic rules/common sense much less the rule of 3rds b/c they are so focused on trying to entend their time/depth/range with little thought of the consequences. These are the same people that routinely ask for air at the bottom of the line & surface at barely 500psi. These are the people I'm kvetching about.

& as for diving in an overhead environment with a 60cuft remaining air supply & a buddy with 120cuft - now what is the point in that?

hbh2oguard
02-04-2008, 09:05 PM
These are the same people that routinely ask for air at the bottom of the line & surface at barely 500psi. These are the people I'm kvetching about.


Now there is nothing wrong with surfacing with less than 500psi if the site and conditions allow.

Papa Bear
02-04-2008, 09:25 PM
Two points, one overhead and the other open water! In open water my feeling as long as your breathing when you hit the surface you have enough air! Overheads a lot different! Requires planning and understanding of all the factors!
If a woman has a sixty and her buddy (male) goes in on 120 if stuff hits the fan with her no problem, but with him big problem because she has to have enough to get both out! She won't! In overhead you have to carry enough for your buddy otherwise go solo! If she has 120 and he has a problem then there would be plenty of air to get both out! A miss match in overhead does spell potential disaster! If you share usable are on the bottom I see no problem at all and if they end with 500psi its 500psi they didn't need!

DrBill
03-25-2008, 02:49 PM
Note the OP is from Canada. I'm not sure how litigious that country is compared to the US.

Unless I planned a very short dive to retrieve something, I always prefer descending with a full tank. Over the 47 years I've been diving, I've had enough situations where a partially emptied tank might have made the dive less safe. Having said that, I used to do two, sometimes three dives off one HP120 tank.