Another Tables vs. Computers Thread

[limeyx]

I don't bring a computer along to run a deco planner on charter boats. The technique is pretty straightforward, providing that you can add numbers.

Take as an example the profile I mentioned a few posts back: 10@100', 10@70', 20@50', 20@40'.

If one doesn't bring a computer or palmtop onboard, what sort of methods are available to do a sanity check if someone proposes that profile to you?

One crude but effective method is to look at average depths. 10@100' + 10@70' is 20@85', so we are OK so far. Add in 20@50' and we are roughly 40 minutes at 70'. Probably OK, but pushing it a bit. Add in another 20 minutes at 40' and we are at ...... hmmmm. OK, I'll let someone else do the math.

Using a square profile table to approximate a multilevel dive is another potential method of doing a rough sanity check of a proposed dive profile. While the explaination using just the Internet makes it sound difficult, if you grab a PADI RDP and follow along, you might very well find it easy. Use the backside of the RDP since the numbers are bigger (the data on front and back are actually the same!) If you don't have an RDP handy, just use the rdp-like spreadsheet I've attached.

10@100' is column E. Go up to 70'. The entry there is 14. This simply means that, as far as the 60 minute halftime compartment is concerned, 10@100' is equivalent to 14@70'. Add your 10 minutes at 70' to that 14 minutes, and find that you are now in column K. Move up to 50' to find that the dive is so far equivalent to 36 minutes at 50'. Add your 20 minutes at 50' to get 56@50'. Move up to 40' to get the total dive equivalency so far of 79 minutes at 40'. Add the 20 minutes at 40' to get an equivalent of 100 minutes at 40'.

...

Anybody else willing to post how they plan multilevel dives ?????

I would do it similarly but a bit more simply, and during the dive, not on the boat.

(actually this is a profile that's cutting it pretty fine with air, and I'd use 32% or shorten the dive. I plugged it into deco planner and if you add even a few mins to the 50 or 70 depths, you start getting into some (very small admittedly) amounts of deco)

Since the two dive portions are at pretty different depths, you could say:
10@ 100 and 10 @ 70 = 20 @ 80 = 2/3 of my NDL

then 20 @ 50 and 20 @ 40 = 40 @ 45 or so, where my NDL is 55 mins. I have about 17 mins left on my NDL at 45 feet -- too close for my likings.

If i was actually diving, I'd say:
5 mins - avg 100 feet
10 mins - ave 100 feet
15 mins (now going up to 70) avg 90 feet
20 mins avg 80 feet
25 mins (now at 50) avg 75 feet
30 mins (now at 50) avg 75 feet
35 mins avg 70 feet
40 mins avg 70 feet
45 mins (now at 40) avg 65
50 avg 65
55 avg 60
60 avg 60

which gives me 60 mins @ 60 feet, which is over my NDL (maybe not the PADI one) of 50 mins.

So I'd cut down one of the sections, depending on the dive (maybe make the 40 portion 10 at 40 and a slow ascent).

The better (in my opinion) option would be use nitrox 32, ignore the 40 portion and ignore most of the 50 portion (as they fall into the < 40 feet 170 minute NDL portion)

I wouldn't bother planning if I had enough gas to do the dive or not on the boat since unless its a spot I do a lot, I'm not going to know how the depths are going to go anyway (which brings up another question -- honestly who ever says "do you want to do 10 @ 100, 10 @ 70 .... and so on.)

What I *would* plan is enough reserve gas to get an OOA diver to the surface and the gas plan (all usable, out and back or 1/3ds)

This is a profile that's (probably intentionally hard to average since the two portions of the dive are done at pretty different depths. More normal would be "Let's average 60 feet for 40 mins or 50 mins, and then stay between 80 and 40-50 for the dive. The 80 feet depth on the way out, converging up more to the 40 on the way back and then a slow ascent). One exception is that we dive the oil rigs sometimes where we go down to 100 but some of the best stuff is on platforms at 55 feet. So a profile there looks like 10-15 @ 100 (on 32%) and then 30-40 mins at 55 feet.

I think getting into 1/2 times and compartments during the dive or planning on the boat is un-necessary and just makes things overly complex. Those concepts have a place in deriving the tables or data that you base your deco/NDL on, but not when you're in or about to get in the water.

 

[limeyx]

Only if I took the RIGHT DIR-F. Some DIR-F graduates learned that surface intervals are not needed because a minute pause every 10' during the ascent leaves them bubble free and without residual nitrogen, and get taught that time-to-fly delays are optional. Others are taught differently.

I am pretty sure DIR-F is not going to teach people to not take a surface interval or fly right after diving. However, I think the effective water pressure of 8000 feet of air is not very much.

The min stops from 1/2 depth (simplification of a more complex deco profile) hopefully minimize bubbling, and give you a decent "safety" stop. Note that it's not 10 foot stops from max depth (which would make no sense I think).

I do take a surface interval (min 60 mins) and do not decrease bottom time om repetitive dives. If I get less than 90 min SI, then a slower ascent is done.

I am not recommending that anyone else follow what I do -- I am not a dive instructor, trainer etc. and probably never will be.

 

[TheRedHead]

Actually, in most cases depth averaging works. Just like the simplistic zero SI method of using tables, there are some profiles that look ok when averaged but are a bit outside the model limits, but it is generally close enough to use.

Say you had 2 wrecks that were close to each other: one was at 122 feet and the other was at 40 feet. You want to spend 15 minutes on the first wreck and you want to ascend to 40 feet and head out on a compass heading where you'll find the 2nd wreck and spend 20 minutes including travel time. That would be 35 minutes at 150/2=75 feet. Doesn't work on a table. How can you plan this dive?

 

[anakin]

i just got a wheel and a computer. makes it ten times faster than all this gimcrack.

 

[H2Andy]

Say you had 2 wrecks that were close to each other: one was at 122 feet and the other was at 40 feet. You want to spend 15 minutes on the first wreck and you want to ascend to 40 feet and head out on a compass heading where you'll find the 2nd wreck and spend 20 minutes including travel time. That would be 35 minutes at 150/2=75 feet. Doesn't work on a table. How can you plan this dive?

these are just speculations, i don't know if it can be done this way:

well, the max i could spend at 122 (130) is 10 minutes = (H)

then ascend to 40 feet and spend 20 minutes there = (C) = +3 pressure groups

that leaves me at H+3 = K at the end of the dive


another way would be to take the first 10 minutes at 130 and walk the
column back to the 40-foot row. that is 37 minutes. then, add 20 minutes
to 37, which gives 57. 60 is the closest, which gives you an ending
pressure group of N.

does anyone know if these would work? they seem to be the simplest ways

 

[TheRedHead]

I'm not talking about ratio deco. We are just talking about recreational dives here.

I just want to know where those funky compartments are that everyone is talking about.

Why not talk about ratio deco because it works for these kinds of dives too? Or profiling with averages.

Everyone knows the compartments are theoretical constructs. Even GUE's Deco Planner software spits out MVal%.

I'll confess that I like computers and planning sotware because I do dives somewhat outside of recreational dive limits and plan accordingly.

 

[Scuba]

If you back up a bit, you'll see that we got on this tangent in response to a comment I made along the lines of "If you are within table limits, then you are within the model limits. OTOH, since table are crude representations of the model, you can be within model limits and be outside of the table limits".

All that fuzz about nothing, as I suspected. That is what you meant. This is what you wrote:

Originally Posted by Charlie99
However, the reverse is NOT true. I frequently dive profiles that violate the table limits but are well within the limits of the model. Multilevel dives with a maximum depth of 100', and a total dive time greater than 60 minutes are pretty common in my dive log. A dive profile like 10minutes@100', 10@70', 20@50', 20@ 40' would clearly be within the model limits and within the NDL of my computer.

One can't say on the one hand that per table rules the profile you illustrated must be applied:

OTOH, a 100', 60 minute dive is clearly beyond the limits of all air tables, and that is how that multilevel dive must be treated on square profile tables

while ignoring other square profile dive tables confining parameters, rules and representations, that are not intended to be applicable to multilevel profiles, such as a 60 min at 55-60' average depth dive profile. This is outside of, in violation of table parameters.

I was unsuccesfully trying to explain the fine but important distinction between violation, and - in violation of. As in the former fits within the model's parameters but violates one of its derivative tenets, demonstrating a flaw. The latter does not fit within parameters, therefore it does not demonstrate table flaws.

So: "If you are within table limits, then you are within the model limits. OTOH, since tables are a limited representation of the model, you can be within model limits and be outside of the tables more limited parameters"

Look where this mess got us into now. LOL

 

[Charlie99]

well, the max i could spend at 122 (130) is 10 minutes = (H)

then ascend to 40 feet and spend 20 minutes there = (C) = +3 pressure groups

that leaves me at H+3 = K at the end of the dive


does anyone know if this will work? it seems to be the least complicated way

It doesn't always work because the ongassing is exponential and you won't always move the same number of groups for 20 minutes. The other reason is because of a quirk in how PADI put the table together. If my reverse engineering was correct, "A" corresponds to 12% of M value, "B" is 21%, and "C" is 26% of M value. After that it goes up linearly at 3% of M-value per pressure group. IIRC, the USN tables go up 2fsw ppN2 per letter group.

The way that actually does work in all situations (and is actually how the PADI Wheel works):

10@ 130' is H. H at 40' is 37 minutes. Spend 20 minutes there. 20+37=57 =N .

 

[H2Andy]

dude, that's awesome!

i actually thought of the secod way and edited my post accordingly!

ok, sorry... didn't mean to get so exited, but this is cool

 

[TheRedHead]

does anyone know if these would work? they seem to be the simplest ways

The simpliest way to do it is run through Deco Planner and it tells me how I can do the dive as stated AND it gives me a gas plan including gas reserved for my buddy AND it even suggests the appropriate sized cylinder for this dive and it took about 15 seconds. I can print the sucker out, laminate it, and take it with me. Or I can write it on my slate. I can come up with a couple of other contingency tables in case I overstay my 15 minutes. Then I can rethink my cylinder size. All this table stuff is lame.