Average depth finder

[VR78]

What is the cheapest device out there (bottom timer or entry level computer) that will display average depth? Any suggestions?

+1 for Uwatec/Scubapro/Aladdin 330M.
The real time depth average indication is extremely useful to me and the reason why I am using the aforementioned timer.

Bought for $125 new.

 

[tbone1004]

Why is it so often the case on SB that wanting to know more about something is automatically presumed to mean that you are going to do it?

I have heard nothing of blind faith here. What I have heard is a pretty educated person seeking even more education. That includes quoting sources who promote a certain way of doing things, in order to solicit counterpoints. That seems way better than him giving us "his own" understanding of how RD works and then asking for a critique. Quoting sources does not mean he agrees with them. Asking questions that are predicated on stipulating that what someone else said is true, also does not mean he agrees with them. It just means he is seeking better understanding.

and he didn't have to explain it, all I asked were yes/no questions.

RD does not comply with the current recommendations on decompression
DAN | News
this is what they were talking about then. Please note the top disclaimer where DAN says it isn't current.
Alert Diver | Deep Stops
this is what they link to where they have a panel about deep stops. Please note especially where NEDU actually did studies to 170ft for 30 minutes and more divers got bent on the deep stop profiles than they did on normal profiles.
Review of Deep Stops - Shearwater Research
good stuff there
USN deep stops study | Rubicon Foundation
the actual deep stop study there

so, yes. I asked questions if he understood it. If he did, then fine, that is his choice. However, be advised, that this goes against the current thinking and studies that have been done in the last decade and are truly up to date

 

[CAPTAIN SINBAD]

ok, so what does Mark say about deep stops?

What does he recommend using for decompression algorithms and profiles?

What was the basis for this ratio deco based on average depths?

Sorry for the late reply Tbone but I was actually reading Mark Powell's chapter on Deep Stops. It has some great points in there. I do not have that book on me at the moment so I will skip that part for now and try to explain the reasoning behind UTD's Min-Deco as best as I have understood it.

Let us just begin by accepting that computers are bending certain people out there. This does not apply to technical computers as much as it applies to recreational computers. Technical diving computers allow people to adjust conservatism and add gradients factors etc. They also end up in the hands of someone who knows what he/she is doing so we will exclude technical diving and technical diving computers out of this discussion. Here, I am talking about recreational computers such as my Mares Puck and my Suunto Gecko that push everyone to a pre-programmed limit determined by the manufacturer. While I have never experienced any symptoms on any computer myself, within my circle of diving friends I have seen people get bent on these things. One of my close friends took a chamber ride from Millbrook while she was well within the computer limits. The same lady took another chamber ride in North Carolina while being within computer limits. DAN has a report of those cases because based on DANs recommendation she now decompresses on oxygen well within those computer generated NDL limits.

Another person that I previously mentioned is always knocking herself out after dives but she is unwilling to consider that it is a symptom of a Type 1 hit. She always looks at the computer and convinces herself that she is tired and headache is is just a headache etc. Then there are all those people that I frequently dive with who believe that nitrox gives them energy without realizing that they were bent on air previously and when higher percentage of oxygen does not bend them, they do not feel fatigued. In the end, computers bend people! This will not be surprising to the decompression researchers because people have been bent on tables while assuming square profiles. If precision devices are invented to chase those limits to their “undefined and scientifically disputed ends” then of course you will cause a community of bent individuals who do not realize that they have symptoms. In fact the link that your posted from DANs website says that computers have really not made anyone safe over all these years.

"Even the recent introduction of dive computers has not made a significant impact on dive injuries.

Decompression illness incidence remains consistent with the distribution for sex, age and training among divers, regardless of the computers or tables they use."

DAN | News

How does UTD solve this problem?

In a UTD open water class, do not be surprised if the instructor holds a dive computer and says to the students “This device will project a very precise NDL number that will not apply to any single one of you. It will be wrong on each and every one sitting in this class. On majority of you guys, it will err on the safe side and make you stay longer than you need to and that is why you will never experience symptoms. On some of you, the same number will err on the side of danger and you will experience symptoms like fatigue, headaches, joint pains all the way up to stroke. While looking at you right now, I can not say which one of you will be saved by this device and which one of you will get wrecked by it so we are going to throw this device out of the class.

Now that we have thrown the precious little computer out, we will perceive our limits of the dive, not as an exact number that everyone needs to calculate precisely and shoot for, but as a generalized grey area. Since we are now using a generalized shade of grey, the word “depth average” is not blasphemy in our world. After finishing our dives we will end up in that grey area where some of us will have symptoms upon surfacing (just like some of those computer NDL chasers) and some of us will not have symptoms (like many other computer NDL chasers.) Make no mistake! This is not a safe area to be in and that is why we call it the “grey area.” When you come into this grey area there is no way for us to know which one of you guys will be surfacing with symptoms and which ones will not have symptoms. We will not even try to make that distinction. We will make sure no one leaves the water until everyone follows a staged decompression schedule!

This is the UTD mindset. They all assume that they are bent. I normally do not dive this way and this way of perceiving limits of the dive is new to me. I was trained on tables and have used computers since. I do have some UTD trained dive buddies some of whom will say that they felt lethargic after a computer dive and they feel like they were “diving nitrox” after a UTD decompression ascent on air. One of them (an older fellow) started to dive this way because his computer dives were causing lethargy plus aches in different areas of the body. He thought that he was getting old for this kind of stuff and it never occurred to him that it could be DCS because computer was telling him that he was safe. Then he also had a cough attack upon surfacing from a deep dive once while being within computer limits. Ever since he stopped following the computer and started diving with UTD staged ascents he does not feel the extreme lethargy and aches and also did not get the cough.

While I have heard stories that people specially older folks immediately feeling better after UTD ascent, I have tried to dive both the conventional way (dive to your NDL with a computer generated single “stop” at the surface) and I have also dived with UTD mini decompression ascents. Personally I felt no difference between one or the other so I can not say that “UTD ascent strategy makes me feel like I am diving nitrox.” There are people out there who feel that such ascents suit them more and based on my limited conversations with the her, I believe Lynn (TsandM) was also one of them.

 

[tbone1004]

@CAPTAIN SINBAD
not higher O2 percentage, it's lower N2 percentage

What you are doing is making an unfair comparison. I have no idea what their ascent profile looks like, but I know GUE's is somewhere around Buhlmann 20/85 with the 85 being the important part. Why is this unfair? NOAA and PADI tables correlate to somewhere around 45/95 at least per Shearwater and I'm tempted to believe them. Similar profiles to what you'll see from most recreational computers.
You said yourself that you're removing technical computers from the discussion because you can adjust the gradient factors where you have control and have enough data to at least understand what the computer is doing and why, but then say that you're going to dive based on some non-scientific decompression profile based on trial and error where they straight up say some of you are going to get bent? That doesn't make sense at all. Get something like a Petrel, start diving it on 30/70. If you get bent on that, something seriously sideways happened. If you come out of those dives and feel good, then bump it up to 75, then 80. Find the point where you start to feel a little tired after the dive and then back off by 5. Experiment with the gf-lo if you're doing decompression diving to help with the stops, or do the stops if you want to. The Petrel isn't going to care since it is tracking real time, and then you control the curve of your ascent, but it will hold you until you are at a point where it is unlikely for you to get bent. Sounds like a much safer plan to me with a lot less risk.... With the limitations of 24/7 chambers in this country, I'm not willing to risk anything like that

 

[Kevrumbo]

oh, one other thing that comes up with the discussion of half depth. Half depth is a completely idiotic representation of a theoretically good idea for decompression. They do that because in the US, it is very difficult for us to comprehend the metric system in our head so instead of what is a good idea of "half pressure" they take that to something "easy" and use it as half depth.

300ft is 10ata. Half depth is 150ft or 5.5ata. Half pressure however is 130ft. Much close to what your ascent profile is likely to have for your first stop.

Following half depth, your stops would be 150, 75, 40, 20, 10, surface
Following half pressure, they would be 130, 50, 25 *usually done at 30ft, then 20ft to get on O2*, 12, surface

You can see that with half pressure, which if you think about it makes much more sense than half depth from a physics perspective. You double the bubble size, then let them stabilize, repeat as necessary. Adjust those stops a little bit if you want/need to for the standard deco gases if you want to use them, or adjust your deco gases to accommodate those depths, but no matter how you spin it, half depth stops make absolutely no sense. . .

That's not a correct understanding or fair criticism of the motivation for deep stops in Ratio Deco. Here is the "sense" behind the half-depth min deco one minute stops, and general premise for deep stops in RD as well (and we all are aware the implications of the NEDU DeepStops Study with regards to slow tissue supersaturation from such a profile in technical staged decompression diving -a fair & valid criticism- so let's not rehash that aspect too much here). The basis and rationale seems sensible -explain then why the following is "an idiotic representation of a theoretically good idea for decompression":

From UTD's Student & Diver Procedures Manual v2.0 (abridged):

To understand our [Ratio Deco] Deep Stop strategy, we must understand MAX DECO, and to understand that we start by looking at the dissolved gas theory, or the Buhlmann Model.

The Buhlmann Model is a theoretical dissolved gas model in which you have 16 half-time compartments. From the fastest compartment @five minute half times, to the slowest @240 minute half-times. These compartments load and unload inert gases in an exponential manner, or in a "half-life" theory, meaning that they load or unload 50% in the first of the six time segments. Then 50% of the remaining 50%, so essentially an additional 25% in the next time segment, and then 50% of the remaining 25%, so an additional 12.5%, and so on until the tissues in that particular time compartment reach saturation or desaturation in the 6 half-lives. In the Ratio Deco Strategy, we consider the tissues to be saturated or desaturated when they reach 97%, so we use five half-times instead of six.

The compartments are named after their "half-times", so the fastest compartment is the five-minute compartment, and according to the Buhlmann Model it will take 30 minutes to saturate or desaturate that five minute compartment, 25 minutes according to Ratio Deco implementation of five half times. Then the next fastest compartment is the 10-minute compartment, then the 15-minute compartment, and so on.

So for our purposes of creating the Proper Ascent Profile we need to initially determine our "Deep Stop" protocols. In order to do this we will look at the first five tissue compartments: 5-minute, 10-minute, 15-minute, 20-minute, and 30-minute compartments; these are considered as the fast tissue compartments, the ones we start to unload first. In the deep part of the deco these compartments will be used to determine deep stop depths and time to ensure they are allowed to off-gas properly.

For desaturation we will look at the fastest compartment first, the 5-minute compartment, and then the next fastest compartment and so on. For saturation purposes, we will look at the slowest compartment. The slowest of these fast tissue compartments is the 30-minute compartment, which will take 150 minutes to saturate and desaturate according to our Ratio Deco method. . .

Now these five fast tissue half-time compartments will also have a "Max Stop" depth. This is considered to be the depth at which the compartment will first start to unload or desaturate. In other words, when you are on the bottom, you are saturating the compartments at different rates and they are reaching different levels of saturation. For example, after a 10 minute bottom time, the 5-minute compartment will have gone through two half-lives and will be saturated to 75% of ambient pressure. The 10 minute compartment will have gone through one half-life and therefore it will be at 50% saturation of ambient pressure, and the 15-minute compartment will be less than 50% of ambient pressure.

When you start your ascent, you will not start unloading these compartments immediately, as the ambient pressure will be greater than the dissolved gas pressure in each of the compartments. However, at some point during the ascent the ambient pressure becomes less than the dissolved pressure in a particular compartment, and that compartment starts to unload, or desaturate, or decompress. We call this the "Max Stop" depth of that compartment.

As you continue to ascend, the difference in pressure between the saturated gas in that tissue compartment and the ambient pressure is called the driving gradient. The greater the difference, the greater the gradient, and the more you off-gas.

However, at some point the gradient becomes too great and the inert gas no longer comes out of the tissues as molecules, but it starts to bubble. This point is called the Critical Tension or M-Value in Buhlmann's Model.

The idea of the Buhlmann Model is that you can ascend all the way until you reach the M-Value, so you would ascend past the Max Stop depth, start off-gassing, and continue ascending, driving the gradient steeper, therefore maximizing the off-gas speed until you reached the M-value, and then you would stop, not crossing the M-value depth, and therefore not bubbling. Theoretically this maximizes the difference in pressure between the dissolved gas in the tissue and the ambient pressure, "driving the gradient&" and maximizing the off-gas rate without bubbling.

But now we know this is not the case. Bubbles are formed much earlier in the ascent, and this is a shortcoming of Buhlmann's Model (it penalizes for deep stops for which current dual phase models like VPM and RGBM factor them in to reduce the driving gradient, and prevent bubble nuclei/seed growth). What is important is not to get confused between Max Stop depth and M-Value: Max Stop is when you first start to off-gas, and M-Value is when you first theoretically start to bubble.

So once again, in designing our Proper Ascent Profile for our Ratio Deco ascent profile, let's figure out where we want to make our first stop. We already know that stopping at the M-Value line is not good, as we bubble much sooner than the M-Value line predicts, and therefore those bubbles grow in size and frequency requiring us to stop much longer and shallower; it is much harder to decompress from a bigger and more frequent bubbles than from a smaller and less frequent bubble. This harder decompression is what we colloquially call the "Bend and Treat" method.

(continued below)

 

[Kevrumbo]

(continued from above):

Looking at the Buhlmann Model, we see the first compartment to saturate and desaturate is the 5-minute compartment. Using a rule of thumb derived from this compartment we can predict our first stop depth. That rule of thumb is 75% of the depth for the 5-minute comparment. The 10-minute compartment rule of thumb is 50% of the depth, and the 15-minute compartment is 25% of the depth.

Now that we know our first stop depth is at Max Stop depth, we need to figure out the time we need to spend at that first stop depth to maximize the off-gas efficiency. We know the maximum time should be five minutes, as this is the half-life of the compartment which maximizes the off-gas of that compartment depth. We also know that if we stayed longer than five minutes, we would still be off-gassing, but not nearly as efficiently as the first half-life time, and we may still be on-gassing some of the slower compartments. If we stayed for 25 minutes at the max stop depth, 5 minute compartment X 5 half-lives = 25 minutes, then theoretically that 5-minute compartment would be 97% equalized to the ambient pressure, however it would not be the most efficient use of that 25 minutes, and we would continue to be saturating the slower compartments at that ambient pressure.

But if we stayed for five minutes at max stop depth for that compartment then continued to ascend, lowering the ambient pressure, creating a new gradient and maximizing new 5-minute half-lives for each of the shallower stops, we would create a more efficient off-gassing of that compartment. This would occur at each of the depths shallower than that first stop depth and would maximize efficiency until we reach the 10-minute compartment Max Stop depth at 50% of the bottom depth.

So we create a strategy in which we stop at the max stop depth, stay for five minutes and then ascend, stopping for five minutes and then ascend, stopping for five minutes at each 10'/3m depth above that until we reached the max stop depth of the following compartment -the 10 minute compartment. Theoretically we will have maximized both our 25-minute total deco time frame of off-gas for the 5-minute compartment, and have done the least on-gas of the slower compartments, still without compromising or bubbling. So now we are ready to tackle the 10-minute compartment.

So, how do we determine how much time for each of these stops is needed if you have not reached the max bottom time or max saturation of the slowest of the fast tissue compartments (which is the 30-minute compartment)? Remember, we would need to have a 150 minute bottom time (or more) to reach a 97% saturation, or 5 half-lives of that 30-minute compartment. So we consider a bottom time of 150 minutes or more as full saturation of the five fast tissue compartments. In order to decompress them you would not need to do any more than 5 minute stops from 75% of your depth to decompress the fastest of the fast tissue compartments, and then 10 minutes for each stop for the 10-minute compartment from its max stop depth, and so on. This then becomes MAX DECO Strategy.

But what if we don't do a 150 minute bottom time dive, but something far less? Then we do not need to do five minutes each stop for the 5-minute compartment because it's not fully saturated. So, we could essentially break that five minutes down into individual minutes such as 1 or 2; 3 or 4, or 5 minutes. This time includes the ascent to the next 10'/3m stop. So based on this 150-minute bottom time and 5-minute deco per stops, we could simply divide the bottom time of 150 by the deco time of 5 minutes to get a requisite minute value per stop strategy, per amount of Bottom Time.

So, 150 divided-by 5 = 30 minute Bottom Time for each minute of deep stop. In other words, we have now figured out that for every 30 minutes of bottom time over NDL, you will need to conduct at least one minute of deep stop deco, starting at the 75% of max depth or average depth depending if you are deeper or shallower than the average depth.

Referring to the attached table link below (see page 6 & 7), you can see that if bottom time is up to 30 minutes more than NDL, then do one minute per stop. If BT is between 30 minutes and 60 minutes over NDL, then do two minutes per stop, and if BT is more than 60 and less than 90 minutes, then do three minutes per stop. Remember that this time is for each stop, starting at the 5-minute compartment Max stop depth and then for each 10'/3m above until you reach the 10-minute compartment max stop depth. . .

Now to get stop time for each depth above the 10-minute compartment max stop depth or 50% of your max depth or average depth, you would take the max BT of 150 minutes and divide it by 10 minutes (10 minute half-times), 150/10 = 15 minutes, or each 15 minutes of additional bottom time would require one additional minute of stop time at each of the 10'/3m deco stops above the max stop depth for the 10-minute compartment. Basically you would do one minute per 15-minute bottom time.

Keep in mind that this is for all bottom times even if you are within NDL. So that means to create a proper ascent profile when recreational diving and doing bottom times that keep you within NDL, you simply do one minute stops (including the ascent time) from 50% of your recreational dive depth. So for example, if depth is 80'/24m and your bottom time = 25 minutes, you are within NDL. You would do one minute for each stop depth (10'/3m) starting at 50% of your depth until you reached the surface.

So a proper NDL ascent profile would be to leave 80'/34m and ascend at a rate of 33'/10m per minute until you reach 50% of the depth. Then at 40'/12m, it is one minute. Then ascend to 30'/9m (counting the travel time as part of the 30'/9m stop) and do one minute, then up to 20'/6m and another minute, then up to 10'/3m to do your final minute before surfacing. This is a Proper NDL Ascent Profile and is always conducted regardless of how much time you spent on the bottom as long as it is LESS THAN NDL.

However, if your bottom time was more than NDL, for example 25 minutes over NDL, and you're doing a decompression dive, your stop times would be one minute for each 10'/3m starting at 75% of your bottom depth and then three minutes per 10'/3m stop starting at 50% of your depth.

As a side note, if your bottom time was less than 15 minutes over NDL, then you would do nothing for 75% stop depth and start 1-minute stops at 50% of depth. There is no need for the 75% stop depth and start 1-minute stops at 50% of depth. There is no need for the 75% stops in this case, as the tissue loading is so minimal.

Take a look at the rule of thumb tables (WKPP applied deep stop theory) on p. 6-7 of the attached file link below:

http://www.ultimatedivelog.com/articles/8.pdf

 

[tbone1004]

@Kevrumbo so you wrote a novel about ratio deco theory, in response to a bolded fragment from me about why half depth is dumb. However, nothing in there said anything about why they think depth is better than pressure. In contrast, you are arguing about tissue loading and bubble formation, which is caused by pressure, not depth, so if you actually believe in that, you would adjust those stop profiles to account for half pressure instead of half depth and you wouldn't be loading the slow tissues. You would also realize that you can adjust the buhlmann gf-lo in order to force ascent profiles for deep stops which has been studied since that article was written and there were more DCS incidents with deep stops than with shallow stops, and there have been a lot more reports of DCS from guys diving VPM than Buhlmann lately

 

[stuartv]

What tbone said 2 posts ago.

Why throw the computer out instead of adjusting settings to make it more conservative?

 

[tursiops]

Nothing in Kevin Rumbo''s novel about average depth as an indicator.

 

[Kevrumbo]

Nothing in Kevin Rumbo''s novel about average depth as an indicator.

@stuartv claims:
I'm also fascinated by the tech divers who use average depth for ANYTHING when it seems like all the science agrees that on- and off-gassing is not a linear process. Example: 15:00 at 90 followed by 15:00 at 130 yields an average depth of 110. 15:00 at 130 followed by 15:00 at 90 yields the same average depth. And there are certified tech divers who, apparently, think the same ascent plan is appropriate in either case!

PfcAJ replies:
That's because it *is* appropriate.

Your first depth/ time profile with 30/30 as a bottom gas and oxygen for deco, GF 40/85 produces a 7min oxygen stop.
Your second profile produces a 5min oxygen stop.
A 110ft avg depth produces a 6min oxygen stop. Literally within 1 minute of your example dives.

60 seconds dude. Come on.

Doolette's Alert Diver Interview

Simply take the average of the particular bottom depth segment (in this instance 90' and 130'), and then weight that average larger if you spent time deeper (example: 120'), or lesser if time spent shallower (100') over that bottom segment.