Ending Dive with 500psi

[Donnah]

When I dove with my kids, I always followed the "rules" religiously. Just as a matter of principle. However, now that they are almost middle-aged and I dive with my husband, I'm a little less strict. If my husband is getting low on air, and I'm not, he'll go up and I'll buddy up with another pair to look around under the boat. We usually dive with a small group of divers that are experienced.

I have noticed that good DMs check people out the first dive and learn quickly who they have to watch out for. Last time in Cozumel, we had planned a dive the day before and the dive shop put a few "new" divers on the boat. The DM came over to us and apologized that we were not going to be able to do that dive because he didn't know the new people on the boat.

 

[toonworld]

I'm gonna go out on the limb and ask a really simple question: Why can't your son do his flips at 5' or even on the surface while snorkeling?

It's all about managing the risks. Being 15' away from your son adds an extra risk factor in case something goes wrong. Other risk factors to consider are:
- The bottom around him. (If it's ~20', not that bad. If the bottom goes to 100', not sure it's a great idea).
- The current and swells.
- the dive profile you've both had

Personally, I don't like being more than an 1.5 arms' length away from my buddy. I want to be there for my buddy in case something goes wrong. Since I do take pictures while diving, there will be the odd time when I will be a bit further away from my buddy while I finish taking the picture of whatever amazing thing I found, but I get right back to my buddy after I am done and this wouldn't last long at all.

The DM should not have made a scene about it. If he had something to say to you, he should have taken you aside and discuss with you calmly. If he made a scene, it just makes him look bad.

I think the real solution was already mentioned. If he absolutely must stay at 15' to do his flips, then get a bigger tank or a steel tank.

Plan the dive, dive the plan.

 

[scubadada]

Interesting, long discussion of a simple concept, surfacing with a reasonable amount of gas reserve. I've never had my end pressure checked, by anyone. Most of my solo diving is drift diving. Since ascent is perfectily predictable, I do not feel compelled to surface with an excessive amount of gas. Most of my buddy diving is with my family in a controlled environment. My son and I are well matched, we surface at the same time. I'm better at air consumption than my wife, she ascends and I burn the rest of gas under the boat, or in some other way I judge as appropriate. Nobody has balked at this strategy, including Grand and Little Cayman.

In your case, either your son comes up with you or you figure out an acceptable way to monitor him. Maintaining a reasonable reserve and then snorkeling above him would seem acceptable. The former seems simplest to me. Maybe your SRMV will improve to more closely match your son's over time.

Good diving, Craig

 

[geoff w]

I always thought the reasons dive operators want divers to return with at least 500 psi in their tanks was 1) to have a reasonable safety margin, and 2) to prevent tanks from being sucked dry and having divers run out of air, and/or getting water in the tank, which is a real pain for the operators, especially in out of the way places. If DMs told divers "return with some air" instead of a specific amount, like 500 (which most SPGs will show in the "red" zone), invariably bozo divers who only dive on vacation every so often will regularly suck tanks dry, do an emergency ascent, get water in the tank requiring taking the tank out of service, and potentially screw up everyone's enjoyment while CPR is performed or oxygen administered. I've returned from plenty of dives with half a tank because someone I'm diving with is out of air. I think of air in a tank like food at an all you can eat buffet - just because there's some left doesn't mean you have to consume it all. DMs generally cut us a lot of slack on our vacation dives and tell us we can go down earlier and stay longer if we want. But we also don't give them any cause to worry, abide by their rules, and try to make their lives easier. They have no way to tell if you're really paying attention to your air, or if you're just lucky enough to come up before running out. Kind of like driving around until the fuel light comes on in your car instead of paying attention of your gas gauge and planning around varying circumstances; at some point you're more likely to coast to a stop on empty that you would like

The DM was totally out of line chewing you out, but DMs are also human, responsible for everyone on the boat and do get stressed. I'd give the guy a second chance, but nothing beyond that. Think about how you looked from his perspective

If someone borrowed one of my tanks and sucked it down to 200 psi, I'd be really pissed, because you need a certain pressure difference above ambient to ensure no moisture gets in the tank. Just because a pressure gauge reads 200 psi doesn't mean it's right, only that it's somewhere in the vicinity of 200 psi. Mechanical gauges are inherently less accurate at ends of their ranges. I like to err on the conservative side when it comes to remaining air in a tank. I only use my own gear, never rental gear, and am conservative with how much faith I put in my gauges. I shudder to think how accurate rental gear like an SPG is (or isn't), give how it's used and abused by divers

 

[redacted]

I always thought the reasons dive operators want divers to return with at least 500 psi in their tanks was 1) to have a reasonable safety margin, and 2) to prevent tanks from being sucked dry and having divers run out of air, and/or getting water in the tank, which is a real pain for the operators, especially in out of the way places. If DMs told divers "return with some air" instead of a specific amount, like 500 (which most SPGs will show in the "red" zone), invariably bozo divers who only dive on vacation every so often will regularly suck tanks dry, do an emergency ascent, get water in the tank requiring taking the tank out of service, and potentially screw up everyone's enjoyment while CPR is performed or oxygen administered. I've returned from plenty of dives with half a tank because someone I'm diving with is out of air. I think of air in a tank like food at an all you can eat buffet - just because there's some left doesn't mean you have to consume it all. DMs generally cut us a lot of slack on our vacation dives and tell us we can go down earlier and stay longer if we want. But we also don't give them any cause to worry, abide by their rules, and try to make their lives easier. They have no way to tell if you're really paying attention to your air, or if you're just lucky enough to come up before running out. Kind of like driving around until the fuel light comes on in your car instead of paying attention of your gas gauge and planning around varying circumstances; at some point you're more likely to coast to a stop on empty that you would like

The DM was totally out of line chewing you out, but DMs are also human, responsible for everyone on the boat and do get stressed. I'd give the guy a second chance, but nothing beyond that. Think about how you looked from his perspective

If someone borrowed one of my tanks and sucked it down to 200 psi, I'd be really pissed, because you need a certain pressure difference above ambient to ensure no moisture gets in the tank. Just because a pressure gauge reads 200 psi doesn't mean it's right, only that it's somewhere in the vicinity of 200 psi. Mechanical gauges are inherently less accurate at ends of their ranges. I like to err on the conservative side when it comes to remaining air in a tank. I only use my own gear, never rental gear, and am conservative with how much faith I put in my gauges. I shudder to think how accurate rental gear like an SPG is (or isn't), give how it's used and abused by divers

How much pressure do you think you need to keep air from flowing into the tank?

 

[Doc Harry]

Jim why don't you just get a bigger tank so that you two can stay together for the entire dive??????

 

[geoff w]

How much pressure do you think you need to keep air from flowing into the tank?

To answer your question, anything pressurized above ambient will keep air from flowing into the tank

In reality, or what the gauge reads? When I fill my tanks at my LDS, the compressor and handheld pressure gauge read differently by about 300 PSI. If I put my own regulator with SPG on (before the tank has started to cool off), that's typically 200 PSI different from either reading. As far as I can tell, none of these gauges are calibrated, they are all reading somewhere in the mid-range of their measurement for an air fill (which is typically most accurate for mechanical gauges), so I have no idea what a pressure gauge reading 200 PSI really means. Maybe it's 200 PSI, maybe it's 450 PSI, maybe it's 50 PSI. At a true 50 PSI in the water, it's doesn't take much for a slightly freeflowing regulator in the water to empty a tank, like when you drop the regulator out of your mouth and waves slosh the second stage back and forth. Unfiltered air entering a tank isn't great (since it has all the moisture in the air), salt water is a lot worse. I generally dive with high pressure steel tanks, which are much more susceptible to corrosion than any other tank I've seen

Just for grins I searched ScubaBoard for SPG calibrate accuracy and the first match with a likely sounding title was http://www.scubaboard.com/forums/computers-gauges-watches-analyzers/424833-testing-spg.html which more or less confirms there is variance in SPG readings. http://www.scubaboard.com/forums/computers-gauges-watches-analyzers/303723-spg-calibration.html is another good read. Has anyone ever heard of replacing an SPG because it was not accurate? Or just because it got flooded and stopped working? I take all my gauge readings (air pressure, depth, water temperature, compass heading) with a grain of salt. You can read gauges as accurately as possible, but that doesn't make the reading more accurate

I'm talking about my own personal gear here, including tanks, so I have the luxury of making whatever rules I want for a friend that wants to borrow one of my tanks, at least more than once. Obviously I can't speak for the dive operator the OP was asking about

 

[waterpirate]

A thinking diver will eventuallly get to " redundant gas is needed" when they come to understand that while a CESA is possible from 100 fsw, it is not a good option. Slinging a pony early and often will only improve your trim and bouyancy in the long run, not impair it. Just a random thought.

As for the statement " knowone ever checked my tank post dive". I doubt that statement is true if you are on a charter. We crew are pretty stealthy creatures and we see and notice alot of stuff that seems benign to casual observers from the monent you get out of your car till the time you get back in it.
Eric

 

[DivemasterDennis]

I agree with Steve_C that this is a buddy issue, not an air issue. Buddy teams stay together if they are competent and safe divers. If your son thinks dad is a drag on his dive time, then he should buddy with someone who is going to dive his profile. But remember, in dive planning, air consumption is only one factor in dive planning, and NDL's are another. My wife has smaller lungs than I do and invariably surfaces with more air than I do. Yet she and I dive the same profile and both know when the dive is over. As to the air question, if Golfer38 is at the surface with 200 psi and observes his son to have a problem necessitating his descent to assist the son, he endangers himself and is not likely to be an effective rescuer. So I have a problem there as well. Now if he and his son surface together, neither dad's air or buddy separation are a problem. So go in the water together and come out together.
DivemasterDennis

 

[DBailey]

Just scanned through the posts, I suggest you have a good look at "rock bottom". A realistic way of gas planning and not as arbitrary as return with 500 psi.


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Even with the Rock Bottom planning, if the boat says get back on the boat with at least 500 psi, then that is not arbitrary. Your gas planning has to account for at least 500 psi in your tank when you exit the water in the worst case scenario. That last 500 belongs to the boat, not you. Their boat, their rules.

This thread isn't about the topic of "is 500 psi enough of a reserve". It is more about the the boat's rules and the verbal lashing from the DM.