Ascending without a dive computer

[txgoose]

OK, since we disappointed you before, and since you asked so nicely, I'll take a shot at this. I could be wrong, I always questioned these "average depth" models, but they are pretty commonly used in teaching.

You are diving air to a bottom depth of 100 feet. You stay for 15 minutes. NDL of air at 100 feet is 20 minutes, so you are still within your recreational, no-stop limits when you decide to ascend.

You ascend at 3 FPM, so it takes 33 minutes to surface (ignoring the safety stop). Your average depth for this leg of the dive is approximately 50 feet (a bit deeper if you include a safety stop).

So for decompression calculation purposes, you can model this as a multi level dive: 15 minutes at 100 feet, and 33 minutes at 50 feet.

Here is that ascent on MultiDeco, assuming gradient factors of 30/70. Note that it uses a standard ascent rate between the levels:

Dec to 100ft (1) Air 60ft/min descent.
Level 100ft 13:20 (15) Air 0.85 ppO2, 100ft ead
Asc to 50ft (16) Air -30ft/min ascent.
Level 50ft 33:00 (49) Air 0.53 ppO2, 50ft ead
Asc to 20ft (50) Air -30ft/min ascent.
Stop at 20ft 21:20 (72) Air 0.34 ppO2, 20ft ead
Surface (72) Air -30ft/min ascent.


So you have gone from being 5 minutes away from your NDL at depth, to a 21 minute deco obligation at 20 feet, because of the ascent rate of 3 FPM.

I do appreciate your time. This is at least getting into some guts of the deal! This is cool stuff and definitely what I was probing at. And so maybe in reality, my question is actually beyond basic scuba. Maybe this is a tech question once a person were to go appreciably below a 30ft ascent rate? I am admittedly surprised by that. (You do realize that to completely cut me off at the knees you need to run it at 9ft/min and see it read right at the edge of NDL. )

Early on I wondered if this could possibly be a cool calculus problem. As you mention the average depth models seem to lack much granularity. And if that’s all we have at this time, I totally get it. In a bit of juxtaposition(?) we can only learn as people get bent, and if we’re getting smarter that will actually happen less and less. Our ability to study as you say, mildly goes away. But that said, it seems like a math guru could do an infinite number of average depths between the max depth and the surface. And in essence, this may be what DCs are doing under the covers. It seems like there is probably some sweet spot that says “If you go below x number of atmosphere of pressure, you can no longer puts about. When you need to go, you really do need to go.”

And to be mildly on topic, I think that we’ve at least proven that if you’re playing with anything other than square profiles, a DC can probably help you stay out of NDL.

I answered a poll that said I have no plans to go tech. I suspect that I’m going to end up there even if I never use it to dive.

 

[doctormike]

Maybe this is a tech question once a person were to go appreciably below a 30ft ascent rate? I am admittedly surprised by that. (You do realize that to completely cut me off at the knees you need to run it at 9ft/min and see it read right at the edge of NDL. )

Easy enough to do! 9 FPM from 100 feet is 11 minutes to surface, or 11 minutes at 50 feet.

Dec to 100ft (1) Air 60ft/min descent.
Level 100ft 13:20 (15) Air 0.85 ppO2, 100ft ead
Asc to 50ft (16) Air -30ft/min ascent.
Level 50ft 11:00 (27) Air 0.53 ppO2, 50ft ead
Asc to 20ft (28) Air -30ft/min ascent.
Stop at 20ft 5:20 (34) Air 0.34 ppO2, 20ft ead
Surface (34) Air -30ft/min ascent.

So that gives you 5 minutes of deco at 20 feet, which isn't much and might go away if you played around with conservatism or gradient factors.

Early on I wondered if this could possibly be a cool calculus problem. As you mention the average depth models seem to lack much granularity.

Yeah, but if you are talking about an ascent from depth at a fixed rate, they are probably accurate enough.

Remember, if you are trying to reverse engineer this by figuring out the ascent rate at which point you won't hit NDLs on the way up, that's really an unanswerable question, since that depends on the rest of the profile (mix, depth and time).

 

[JackD342]

Or, now here's a thought - maybe someone could move from the desktop to a wrist top, and actually dive an extra slow ascent while monitoring a computer or two (sample some different algorithms) to see when they go to deco, or how quickly dive time remaining lessens to where it will obviously do so.

Would take a little effort and a cooperative buddy, but sharing that profile download would be very instructive.

 

[KenGordon]

Yep. I was actually using maths, and theSSI tables with NDL times listed. I haven't seen you use maths yet.

Compare the ZHL16C compartments below. The normal ascent has highest saturation in the fastest tissue, the slow ascent has a peak centered around compartment 6 and that peak is closer to the m value line than in the normal case.

This first profile is using a 1m/minute ascent following 6 minutes as 40m

Run time 2.0 Depth 40.0 descent time 2.0 ceiling -5.1
Run time 8.0 Depth 40.0 level time 6.0 ceiling -1.5
Run time 48.0 Depth 0.0 ascent time 40.0 ceiling -0.4
compartment: ppInert (ratio) on/off gas depth GF m0 delta m m d= 0.0m
1: 5 1.3034 (1.8) 3.0 15.5 2.96 1.79 2.96
2: 8 1.5698 (2.1) 5.7 37.0 2.54 1.54 2.54
3: 13 1.7767 (2.4) 7.8 62.1 2.25 1.38 2.25
4: 19 1.8346 (2.5) 8.3 81.0 2.03 1.28 2.03
5: 27 1.7701 (2.4) 7.7 90.6 1.85 1.23 1.85
6: 38 1.6393 (2.2) 6.4 92.6 1.69 1.19 1.69
7: 54 1.4808 (2.0) 4.8 81.5 1.59 1.15 1.59
8: 77 1.3242 (1.8) 3.2 62.3 1.52 1.12 1.52
9: 109 1.1870 (1.6) 1.9 39.8 1.47 1.10 1.47
10: 146 1.0905 (1.5) 0.9 21.1 1.43 1.08 1.43
11: 187 1.0226 (1.4) 0.2 5.6 1.40 1.07 1.40
12: 239 0.9667 (1.3) -0.3 -9.0 1.37 1.06 1.37
13: 305 0.9212 (1.2) -0.8 -23.2 1.34 1.06 1.34
14: 390 0.8840 (1.2) -1.2 -37.4 1.31 1.05 1.31
15: 498 0.8542 (1.2) -1.5 -50.3 1.29 1.04 1.29
16: 635 0.8305 (1.1) -1.7 -62.8 1.27 1.04 1.27

This profile uses a 10m/minute ascent rate

Run time 2.0 Depth 40.0 descent time 2.0 ceiling -5.1
Run time 8.0 Depth 40.0 level time 6.0 ceiling -1.5
Run time 12.0 Depth 0.0 ascent time 4.0 ceiling -2.3
compartment: ppInert (ratio) on/off gas depth GF m0 delta m m d= 0.0m
1: 5 2.4762 (3.3) 14.8 75.3 2.96 1.79 2.96
2: 8 2.1911 (3.0) 11.9 77.3 2.54 1.54 2.54
3: 13 1.8566 (2.5) 8.6 68.5 2.25 1.38 2.25
4: 19 1.5819 (2.1) 5.8 56.5 2.03 1.28 2.03
5: 27 1.3608 (1.8) 3.6 42.4 1.85 1.23 1.85
6: 38 1.1991 (1.6) 2.0 28.9 1.69 1.19 1.69
7: 54 1.0750 (1.5) 0.8 12.7 1.59 1.15 1.59
8: 77 0.9821 (1.3) -0.2 -3.4 1.52 1.12 1.52
9: 109 0.9141 (1.2) -0.9 -18.3 1.47 1.10 1.47
10: 146 0.8714 (1.2) -1.3 -29.9 1.43 1.08 1.43
11: 187 0.8434 (1.1) -1.6 -39.1 1.40 1.07 1.40
12: 239 0.8214 (1.1) -1.8 -48.3 1.37 1.06 1.37
13: 305 0.8042 (1.1) -2.0 -57.6 1.34 1.06 1.34
14: 390 0.7904 (1.1) -2.1 -67.6 1.31 1.05 1.31
15: 498 0.7797 (1.1) -2.2 -76.0 1.29 1.04 1.29
16: 635 0.7713 (1.0) -2.3 -84.7 1.27 1.04 1.27

 

[KenGordon]

ps sorry about the formatting, if any one can tell me how to make tables I'll try to do that.

 

[BlueTrin]

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[KenGordon]

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Raw html? Ffs.

 

[BlueTrin]

Raw html? Ffs.

I tried, it does not work.

 

[txgoose]

Easy enough to do! 9 FPM from 100 feet is 11 minutes to surface, or 11 minutes at 50 feet.

Dec to 100ft (1) Air 60ft/min descent.
Level 100ft 13:20 (15) Air 0.85 ppO2, 100ft ead
Asc to 50ft (16) Air -30ft/min ascent.
Level 50ft 11:00 (27) Air 0.53 ppO2, 50ft ead
Asc to 20ft (28) Air -30ft/min ascent.
Stop at 20ft 5:20 (34) Air 0.34 ppO2, 20ft ead
Surface (34) Air -30ft/min ascent.

So that gives you 5 minutes of deco at 20 feet, which isn't much and might go away if you played around with conservatism or gradient factors.

Nine feet is close then. If we went to 130ft (gen. accepted rec limit) it could even be up in the 12-16 foot per minute range. Without a doubt higher than I would have guessed for something akin to a carte blanche rule of thumb.

I like @JackD342 ’s idea but I don’t have the resources currently and I’m not sure that I could ask anyone to do that. At this point, it really does boils down to a general idea that after 50/60 feet, when you need to go, you really do need to go.

Yeah, but if you are talking about an ascent from depth at a fixed rate, they are probably accurate enough.

Remember, if you are trying to reverse engineer this by figuring out the ascent rate at which point you won't hit NDLs on the way up, that's really an unanswerable question, since that depends on the rest of the profile (mix, depth and time).

Which is fair enough. I guess I was making the assumption of air, max rec depth, the short window of time for 120/130 feet. A pseudo worst case scenario for a rule of thumb to toss about, as it were. If it were to end up in the “teens” of feet per minute, we are functionally getting back close to the max ascent rate. And maybe one of my takeaways is how narrow that ascent rate window really is. That surprises me, but I am not wanting to argue the math. And as others have talked about, IMO it does put some fairly heavy weight on debunking deep stops. Granted that is way, way out of basic and into tech.

I honestly do appreciate you tolerating me long enough to do the math. I am a geek and I like to think we do more than wet our finger and judge the winds. (Understanding that as some level with tissue loading we are doing that to some degree.) You got us near an answer using the trusted info that we have.

 

[JackD342]

Or, now here's a thought - maybe someone could move from the desktop to a wrist top, and actually dive an extra slow ascent while monitoring a computer or two (sample some different algorithms) to see when they go to deco, or how quickly dive time remaining lessens to where it will obviously do so.

Would take a little effort and a cooperative buddy, but sharing that profile download would be very instructive.

I wonder if I can get my pressure pot to release the pressure slowly enough to simulate a deep dive with a slow ascent. Probably not, but I’ll try it.