Basic question KR 2, question 7 OWD

[carloslzn]

Hello guy

Can someone explain me the question of the new OWD KR 2 question 7 ?

7. My buddy and I are planning a shore dive. We’re descending
onto a very gradual slope that begins at 5 metres/15 feet, so our
descent and ascent will be a gradual part of swimming out and
back underwater. We have similar cylinders filled to 200 bar/3000
psi. We plan:
• 50 bar/500 psi reserve.
• 20 bar/300 psi for our safety stop.
• To turn the dive when we’ve used one-third of the air available
to use on the dive.
This means we should head back when either of our SPGs read
£ a. 70 bar/800 psi
£ b. 145 bar/1900 psi
n c. 157 bar/2270 psi
£ d. 170 bar/2500 psi

The answered is letter C. But why? Can someone give me an explanation to know how to get that number ?

thanks

 

[tbone1004]

500psi is nonusable as reserve, you have 2500 psi, if you're turning when you've used one third of that, it's 3000-(2500/3)=~2270

Unless you are on an AI computer or a technical gauge where the increments are in 100psi marks, you don't have the accuracy for that. Depending on the gauge it could be in 200 or 250psi increments, so you round to safety so you would actually turn at 160bar or 2400/2500 *though most would cheat that down to 2250psi* with that much of a reserve.

for gas planning the thirds decision I don't agree with the way they did that.
If you need 300psi for safety stop, you double that for safety stop with your buddy, and then do a third of gas remaining, which is 2400/3 so my answer if asking the question would have been 151bar/2200psi, but the math above is how the question was asked.

 

[Redshift]

You have 50 bar reserve, so it's not planned for use and you need 20 bar for the safety stop. So for the actual swimming part of the dive you have 200 - (50 + 20) = 130 bar available. If they say you should use 1/3 on the way out, that means you must turn when the pressure is at 200 - 130 / 3 = 157 bar.

 

[rongoodman]

I don't understand the use of thirds here either. You've already set aside 1/3 as a reserve, but they're starting off by stipulating an additional 500 psi reserve, so in their scenario, you're actually reserving over 1200 psi. Setting aside your 300 psi for the safety stop, thirds would have you turning at 2100 psi, but the way they have it set up, (c) is the answer.

 

[Redshift]

Me neither. They are mixing two methods, thirds and having a fixed reserve. But I guess the purpose is to follow what's being described and calculate the gas pressures.

 

[Searcaigh]

50 bar + 20 bar = 70 bar so that leaves available gas for breathing = 130 bar

One third is 43.3 bar so turn pressure is 70 + 86.6 bar = 156.6 bar (157 bar) concurring with the above replies.

However that makes no sense to me unless you just want to pose a mathematical question!

What I would do is breath half of the 130 bar on the way out (65 bar) so turn pressure is 65 + 70 = 135 bar.

 

[TSandM]

This approach would make perfect sense, if you were doing a boat dive onto something like a wreck. You'd need to figure thirds so you could get back to the anchor line, and then a reserve for your ascent up that line. For a shore dive, where it has been stated that your swim out and back IS your descent and ascent, you would not combine thirds with an additional reserve, because thirds has already built into it a significant amount of gas you do not intend to use, which is available for any emergency or delay in returning.

In practice, most people would plan a shore dive using a reserve and planning to use HALF of the available gas, since on most shore dives, the surface IS an option throughout the dive (although not always an attractive one). Thirds is used when you MUST return to a point certain, so it originated in overhead environments, but is sometimes appropriate for open water diving, as in the "must get back to the anchor line" example above. Any dive which requires thirds for planning is an advanced dive, in my personal opinion, and not appropriate for OW divers.

You've had an explanation of the math. The above is my conceptual explanation of the problem

 

[tbone1004]

agreed though I do think thirds is most appropriate for shore dive when the profile is more similar to a penetration dive than a true ocean dive. Dive out to a third, plan on diving back with a third, have the rest for deco/safety stop/oh sh!t moment.

For square ocean diving profiles though I usually do 500 reserve, 200 safety stop, then head back to the anchor line around 1000-1200psi assuming AL80