how to dive with non-DIR divers?

[Phil K.]

I used 1 for his SAC...and 1 for ATA's

Last time I checked any descent from sea level must create avg ATA > 1 unless you can manage to be at depth and at altitude (above the surface) during the same dive. In other words, Minimum Gas is based on maximum depth of 45 fsw = 2.35 ATA. To average this, you subtract the atmosphere at the surface and divide by two (which equals 0.675 atmospheres or 22.50 fsw) then add back the atmosphere at the surface to get average ATAs = 1.675.

 

[Ben_ca]

Last time I checked any descent from sea level must create avg ATA > 1 unless you can manage to be at depth and at altitude (above the surface) during the same dive. In other words, Minimum Gas is based on maximum depth of 45 fsw = 2.35 ATA. To average this, you subtract the atmosphere at the surface and divide by two (which equals 0.675 atmospheres or 22.50 fsw) then add back the atmosphere at the surface to get average ATAs = 1.675.

You are right... I'll go back to remedial scuba math and not drag any newbies anymore

 

[rjack321]

I would skip the hundreths and thousands of an ATA
4 inches and about half an inch of depth here or there really aren't relevant.

I agree my calc would have been:
4 min ascent
combined SAC of 2 divers, 2 cf/min
average depth about 2 ATA (really a little less)

2 * 2 * 4 = 16 cf
16/3.5 = ~375 psi, round up to 500.

 

[Ben_ca]

I would skip the hundreths and thousands of an ATA
4 inches and about half an inch of depth here or there really aren't relevant.

I agree my calc would have been:
4 min ascent
combined SAC of 2 divers, 2 cf/min
average depth about 2 ATA (really a little less)

2 * 2 * 4 = 16 cf
16/3.5 = ~375 psi, round up to 500.

Agree I like scuba math....

 

[Phil K.]

Don't mean to harp on it. I'm sure you knew this, it just didn't come out that way in your post.

 

[MXGratefulDiver]

Frankly, we hold "newbie" dives here regularly ... and a lot of the mentors who show up are DIR-trained. For the most part, we don't sit down and make some newly-minted OW diver go through gas calculations before we take them diving. We keep the dive profile conservative, do a quick gas calculation based on conservative estimates, tell them at what pressure we should turn the dive, and take them diving. If, after the dive, they want to discuss how we came up with that number, we're more than happy to talk about it over refreshments.

Gets back to what I said at the beginning of this thread ... don't preach ... do. Most newbs got enough to think about with basic stuff ... like buoyancy control. So let 'em work on that. Set a good example, and they'll start asking questions quick enough.

... Bob (Grateful Diver)

 

[Ben_ca]

It's been a while since I've looked at the worksheets.... was doing this off the top of my head... the dive in question was at the local bunny slopes (OW training ground) and we usually use the standard rock bottoms...

it's good to refresh the scuba math skills

 

[Adobo]

Don't mean to harp on it. I'm sure you knew this, it just didn't come out that way in your post.

No, he's pretty much a moron.

Just kidding.

 

[MXGratefulDiver]

No, he's pretty much a moron.

I'm fairly sure you meant to say "dork" ...

... Bob (Grateful Diver)

 

[Phil K.]

I would skip the hundreths and thousands of an ATA.

What about the tenths of an ATA? If we round up to the nearest whole atmosphere as you suggest, we ignore the 3/ 10 difference between 1.7 to 2.0. That's a difference of 3 meters or 10 fsw. Minimum Gas based on avg ATA = 2.0 is appropriate for a dive with max depth = 66 fsw, which is more than 45% deeper than this dive and reduces our usable gas by 2.4 cf, or 17%, just due to rounding. That's the equivalent of rounding up to 1.7 only and using a SAC = 1.17. That's a lot of added conservatism, i.e. lost usable gas to gain the simplicity of multiplying by whole numbers.