<a href=showthread.php?s=&postid=15075821#post15075821 target=_blank>Originally posted</a> by yousmellsfishy
:thumbsup:
Awesome thread
Yeah, man... Sure is.
As I had to state once before, that's too far beyond my knowledge or understanding at this point.
Yeah, sorry, man... Just checking.
See, a lot of the things you've said and the concepts that you seem to understand pretty well are also beyond your training... Yet you seem to have a pretty good grasp on them already.
You must be one of them "natuals" I keep hearin' 'bout.
The "ceiling" is the point at which you can't go above, for risk of DCS.
In the example above, the ceiling is at 80'. One minute later, the ceiling lifts to the next decompression stop... Which in this case would be 40'.
Therefore, the ceiling is a moving entity, but you are always to stay below it.
Conversely, the "floor" is the point at which you begin ongassing... That is, taking minutes off of your NDL or INCREASING your decompression obligation.
This is a simple concept, I'm sure you'll agree... A very simple and practical way of explaining what goes on in your body physiologically with ongassing (your body taking on gasses that need to be expelled as you ascend) and offgassing (these gasses coming out of your body tissues). But it's a very powerful concept that will do you very well later in your training. If you think that way now, you'll be dramatically better off when you begin diving more aggressive profiles.
This is a concept, Tim, that I have worked for four pages of this thread to teach you.
A few posts back I asked you if you had noticed that your PADI tables counted ALL of your dive time as "bottom time," while I did my calculations on "bottom time only counts as bottom time."
This is a basic shortcoming of dive tables. Here's the reality of what's reallly going on in your body:
At less than 8 feet, there is enough of an increase in ambient pressure to measure ongassing. This means that in the beginning of your dive, the floor is close to zero feet.
As you dive deeper, you continue to ongas. The deeper you go and the longer you stay there, the more gas becomes dissolved in your body tissues. (You're below the floor, which is constantly moving downward to the depth where you are.)
As you ascend, there is a point at which you rise above the floor - depending on your depth and time spent there, there is a point at which you are no longer ongassing... But the opposite happens - you offgas... You decompress. Interestingly, the floor reverses it's direction of travel - and begins to rise to meet your new depth. The floor moving in this direction - upwards - represents your offgassing.
This happens on EVERY dive you make. Therefore, EVERY dive is a decompression dive. There is no such thing as a no-decompression dive.
However, if you stay down deep enough and long enough, there is a point where the offgassing is so violent that it causes injury - DCS. This point is called the "ceiling."
When coming up from a dive, you need to rise above the floor to begin offgassing, but if you rise above the ceiling, you're going to get hurt or die.
Thus... The times that are listed on your tables as NDLs aren't really "total dive time" times... They're times at which you have to be above the floor, offgassing... But not exceeding the ceiling.
Example: You finish your advanced open water certification and dive to 120 feet. There, whatever gas you're breathing dictates that you can spend 8 minutes there before having to come up. After 8 minutes, your floor may be at 60 feet... In other words, if you stay below 60 feet, you continue to ongas and have more decompression obligation. However, at 60 feet and above, you offgas - your floor. If you then hang out at 20 feet for 20 minutes, (let's say that you've got enough gas to do that), then that's actually good for your body, not bad. And while your tables and the idea of "120 feet, eight minutes" may now consider you dead or headed to a chamber, the reality is that you've done a very safe dive and had a great decompression, and are now loaded very little.
...What's considerably worse is telling a student that he's got 8 minutes at 120 feet, and expecting some of that 8 minutes to be used up in descent/ascent time. This will tend to make students rush to ascend and get out of the water in 8 minutes or less... A much less safe profile than taking your time to ascend and doing a long deco stop near the surface.
I don't know exactly what your tables tell you in terms of time at depth... 8 at 120 feet may not be correct... It all depends on the tables. I only used those numbers because that's what I used in one of the examples above.
...My point is that these published times are not from splash in to getting out... Yet that's how they're being taught, which is a dangerous practice because not only is it incorrect, but it significantly rushes the diver.
...What's a much more truthful teaching is that these times are times to get above the floor, which is constantly moving... Yet never exceed the ceiling, which is also contantly moving.
Dive with the philosophy of floor/ceiling instead of hard and fast tables, and you'll get a much better understanding of physiology and what's really happening in your body. You'll also stay longer, see more, be more relaxed on your dives, and ascend and descend the way that the science actually calls for.
Later in your traiing, you'll also have a very significant grasp on the properties of gasses in your body, and all of the new material will make a whole lot more sense to you.
