Rock Bottom

[Blackwood]

I know of a few shops that post a $$$ fee for rental tanks brought back with less than 500 psi.
They claim that it is for required VIS inspection on such a tank.
Some will even try to charge you for this if you bring your tank in to get a fill.

Hilarious.

 

[Doc Harry]

Not to try to learn to dive in the internet but, when would you recommend to your students to use thirds of usable gas when diving recreational profiles?

"Thirds" of usable gas would be used when it is necessary to descend and ascend at a specific point. Just to be specific, we're talking about usable gas, not total gas.

For example, diving the Speigel Grove in a ripping current. Gotta go down and back up the mooring line.

 

[AndyNZ]

Not to try to learn to dive in the internet but, when would you recommend to your students to use thirds of usable gas when diving recreational profiles?

Shore diving with a surf entry, for example - not uncommon around here.

It's conservative, but for new divers what's wrong with that? Ultimately, it's their call - we go through the situations of all usable, half usable and thirds. Then they can make their own decision.

 

[AndyNZ]

Er ... PADI does not cover it, NAUI Instructors are free to do so if they choose to. Many so choose

Sorry, Thal... whilst it's not an explicit concept in any PADI courses it is also not precluded from being covered.

Many PADI instructors do not teach, simply because they've never seen it, heard of it or understood it. But it can be taught in the PADI system - even at OW if you so choose. I know plenty of PADI instructors who do teach it.

 

[Thalassamania]

But within PADI you can't test for an understanding of it or deny certification to anyone who, for whatever reason, fails to understand it. All you can do is add it in a sort of, "be nice if you knew this," sort of way. Correct?

 

[Coldwater_Canuck]

It's conservative, but for new divers what's wrong with that?

I always thought the rule of thirds, in a situation where it is necessary (you have to get back to the starting point before ascending) wasn't that conservative. Granted, it accounts for the possibility your buddy runs out of air at your maximum point, but it doesn't take increased SAC rates into account. It would also need to assume that either you and your buddy have the same setup and are both following the rule or that your buddy's SAC is the same or lower than yours.

 

[Doc Harry]

I always thought the rule of thirds, in a situation where it is necessary (you have to get back to the starting point before ascending) wasn't that conservative. Granted, it accounts for the possibility your buddy runs out of air at your maximum point, but it doesn't take increased SAC rates into account. It would also need to assume that either you and your buddy have the same setup and are both following the rule or that your buddy's SAC is the same or lower than yours.

Yes and no to all of the above. There are a lot of assumptions that affect your decisions.

For instance, we often plan on using the gas savings riding a stiff current back to the mooring line to give us the gas buffer we need in the case of "thirds." At other times, when the weather topside is really bad with heavy seas, we may use a more convervative "thirds."

...are both following the rule....

I would hope that everyone on the team is following the same game plan as far as gas management is concerned.....

...your buddy's SAC is the same or lower than yours...

Plan you usable gas based on the diver with least amount of gas and then calculate your thirds independently. In most cases, this method automatically accounts for differences in SAC. You can get tables that use SAC to calculate turn pressures, too.

 

[AndyNZ]

But within PADI you can't test for an understanding of it or deny certification to anyone who, for whatever reason, fails to understand it. All you can do is add it in a sort of, "be nice if you knew this," sort of way. Correct?

Mumble, mumble, mumble.... kind of, depends on how you interpret the standards.

There are standards that require a student to surface with a safe reserve. As far as I am concerned, that means to me that they need to be able to work at pressure they should leave the bottom.

I'd have to dig out my instructor manual to work out which standards apply to which courses - I'm not sad enough to have memorised them all. But there are standards at OW that I would quite happily use to deny certification if the student couldn't do some basic gas management. Unfortunately, the same standards can (and often are) interpreted as "be back on the surface with 50 bar".

I always thought the rule of thirds, in a situation where it is necessary (you have to get back to the starting point before ascending) wasn't that conservative. Granted, it accounts for the possibility your buddy runs out of air at your maximum point, but it doesn't take increased SAC rates into account.

Straight thirds can either be conservative or aggressive depending on cirumstance... "thirds of usable" is a bit different. Firstly, work out the minimum amount of gas you need to make a safe ascent with two divers breathing of the same tank. Deduct that amount from your fill pressure - - this is your usable gas, then do thirds on that.

How you manage usable gas depends on the dive - so if you *must* return to the start point then you do thirds, if it's kinda nice but you can cope if you don't then turn pressure is half of usable. If it really doesn't matter and you can finish the dive anywhere, then your turn pressure is your minimum gas reserve.

I tend to encourage new divers to use thirds where I would use halves, and halves where I would use all usable.

 

[hudson]

Andy, nice explanation. I would also add that on a multi-level dive I use the average depth on the way back to calculate my reserve. So the gas I need for the return and emergency ascent would be considerably less on the way back than on the way out, which means I can turn around later. Deducting the reserve gas and considering it off limits before calculating thirds or halves is really best only for square profiles.

 

[boulderjohn]

But within PADI you can't test for an understanding of it or deny certification to anyone who, for whatever reason, fails to understand it. All you can do is add it in a sort of, "be nice if you knew this," sort of way. Correct?

This all depends upon one's definition of the word "test." In education, the word "assessment" covers a broad range of methods by which the instructor is aware of the status of student learning. A formal test is only one measure.

In education-speak, the informal monitoring of students as they learn is called formative assessment, and the formal one they have to pass is called a summative assessment. If you have done a good job with formative assessment, a summative assessment is really just a red tape affirmation of what you already know. I don't let the students take the final exam in the OW class (summative assessment) until I am pretty darn sure (formative assessment) they will do well. For that reason, I have never had a student come close to failing.

In a typical small class, it is easy to teach something like basic gas management and see if students are understanding it by their responses to the instruction. If they are not getting it, you keep teaching it until they do. (This assumes, of course, that your instruction is interactive and not mere lecture.)