Midnight Star
Contributor
myth: Buying diving gear online is ALWAYS a bad idea and potentially life threatening due to poor quality or refurbished equipment.
Misconceptions and Fallacies
- Thread starter Charlie99
- Start date
Please register or login
Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.
Benefits of registering include
- Ability to post and comment on topics and discussions.
- A Free photo gallery to share your dive photos with the world.
- You can make this box go away
Actually, the first sentence is true. It's the second that's the myth.awap:
Codyjp
Contributor
Myth:
All discussions on scuba board end peacefully with everybody coming to a collective agreement.
All discussions on scuba board end peacefully with everybody coming to a collective agreement.
miketsp
Contributor
Sharks will attack me while diving.
OP
Charlie99
Contributor
- Messages
- 7,966
- Reaction score
- 166
- # of dives
- 500 - 999
No fair. We may have to wait 50 years or more to find out if that statement is a myth or is accurate. :1poke:miketsp:Sharks will attack me while diving.
I like this one.
Sharks are attracted to human blood.
Sharks are attracted to human blood.
matt_unique:
Sory matt, I didn't realise you were referring specifically to weight, I thought you were talking about drag as a comlete factor. My apologies
Sorry about being snide, but I get tired about arguing the little stuff when there’s much, no make that much, much bigger contributors…
Little stuff? How about this difference that you point out, I assume because you think it amounts to some significance:
I’m about to spend the next ½ an hour typing to show you just how insignificant this is. It’s not rocket science, it’s a calculation that anyone can do, and hopefully will illustrate what the phrase “insignificant difference” means.
80 CF cylinder at 3000psi.
Diver has a SAC of 0.5CF/min
10 lbs of extra weight
Water weighs 60lbs/CF (to make the math easier)
Dive to 66 feet (3 ATA)
Assuming no other air usage, how long a dive can the diver make at 3ATA (assume we empty the cylinder):
80 cf / (0.5 cf/min * 3 ATA) = 80 / 1.5 = 53 minutes and 20 seconds.
Now, how much gas does the diver “lose” to arrest his descent at 3ATA carrying 10 extra pounds?
10 lbs / 60lbs/cf = 1/6 of a cubic foot of air at 1 ATA. At 3 ATA that’s 3/6 or 0.5 CF of air consumed to offset the extra 10 lbs.
That amounts to 20 seconds of dive time for our diver at 3 ATA. So his 53 minute, 20 second dive is “shortened” to “just” 53 minutes. But wait, there’s more! Let’s say the diver yo-yos to the surface and back six times. 6 * 0.5CF = 3cf. Even that amounts to only two minutes of lost bottom time for our diver at 3ATA.
No one pictures a delta of a mere two minutes at sixty feet as significant when they’re talking about “increased air consumption” – that’s a 50 minute dive being shortened to 25, or maybe 35 minutes. Anything less is noise – the tiny difference if offsetting 10 lbs with back gas pales when compared to the more common contributors to dive times like short fills, a rough surf entry, stress, cold, etc. Far more significant than inflating your BC with 1/6 of a cubic foot of gas.
As for “huffing and puffing” after carrying the extra 10 lbs to the dive site contributing to increased air consumption, using that reasoning I can make the argument that if I carry my buddies (overweighed) weight belt to the site and he carries mine, if follows that your buddy being overweighed can increase your air consumption. Strictly speaking yes it can, but of course, that’s not what we’re talking about, is it?
When a discussion like this starts, an unwritten assumption is “all other factors being equal.” Maybe we need to start writing it…
Roak
Ps.
Pps. A good thread ruined by arguing minutiae.
Little stuff? How about this difference that you point out, I assume because you think it amounts to some significance:
Ohmygoodness! 50%!!! Sounds like a lot, doesn’t it? How much is this really?Temple of Doom:
I’m about to spend the next ½ an hour typing to show you just how insignificant this is. It’s not rocket science, it’s a calculation that anyone can do, and hopefully will illustrate what the phrase “insignificant difference” means.
80 CF cylinder at 3000psi.
Diver has a SAC of 0.5CF/min
10 lbs of extra weight
Water weighs 60lbs/CF (to make the math easier)
Dive to 66 feet (3 ATA)
Assuming no other air usage, how long a dive can the diver make at 3ATA (assume we empty the cylinder):
80 cf / (0.5 cf/min * 3 ATA) = 80 / 1.5 = 53 minutes and 20 seconds.
Now, how much gas does the diver “lose” to arrest his descent at 3ATA carrying 10 extra pounds?
10 lbs / 60lbs/cf = 1/6 of a cubic foot of air at 1 ATA. At 3 ATA that’s 3/6 or 0.5 CF of air consumed to offset the extra 10 lbs.
That amounts to 20 seconds of dive time for our diver at 3 ATA. So his 53 minute, 20 second dive is “shortened” to “just” 53 minutes. But wait, there’s more! Let’s say the diver yo-yos to the surface and back six times. 6 * 0.5CF = 3cf. Even that amounts to only two minutes of lost bottom time for our diver at 3ATA.
No one pictures a delta of a mere two minutes at sixty feet as significant when they’re talking about “increased air consumption” – that’s a 50 minute dive being shortened to 25, or maybe 35 minutes. Anything less is noise – the tiny difference if offsetting 10 lbs with back gas pales when compared to the more common contributors to dive times like short fills, a rough surf entry, stress, cold, etc. Far more significant than inflating your BC with 1/6 of a cubic foot of gas.
As for “huffing and puffing” after carrying the extra 10 lbs to the dive site contributing to increased air consumption, using that reasoning I can make the argument that if I carry my buddies (overweighed) weight belt to the site and he carries mine, if follows that your buddy being overweighed can increase your air consumption. Strictly speaking yes it can, but of course, that’s not what we’re talking about, is it?
When a discussion like this starts, an unwritten assumption is “all other factors being equal.” Maybe we need to start writing it…
Roak
Ps.
Does the above help?Temple of Doom:
Pps. A good thread ruined by arguing minutiae.
Temple of Doom
Contributor
roakey:
Now who is in 'the laboratory'?
I think we're crossing our wires here. The conversation began when one person said being overweighted does not increase consumption. That's certainly not true, and it was pointed out by a few people that there is a correllation. Everybody agrees that there are cases where the effect is very little, but it seems not everybody is agreeing that there are plenty of cases where the effect is noticeable.
Can I ask the following...
- the slight increase in air needed in your BC
- the increased drag through the water (even more so on the surface)
- the added exertion of packing around and swimming the extra weight
- the cumulative effects of all this on multiple dives
All of that amounts to nothing?
What exactly are you arguing? That sometimes all that amounts to very little? Scroll back several dozen posts and you'll see that was settled long ago.
Craig
wettek:
Can't let that one sneak by.
It's HUNDREDS of times denser....Similar threads
