Theory Vs Reality Will I Get Bent???

[@jdavis8610]

Another very informative video. More divers should be watching this channel!

[@LakeHickoryScuba]

Thank You for the kind words.

[@GaryRudd]

"Long time listener first time commenter!" Thanks Bryan for another awesome video... I think the issue you saw underwater was the 99 is the physical max display for that part of the screen (or the programming design is to show 99 when it's 99 or greater). In other words in the background that time was probably much higher but it can't display it. So when you hit 100 while it looked like you were over the limit, you actually we're not there yet. (Just my $0.02) Anyway thanks again for your series of videos. As somebody who enjoys the academics of diving as much as the physical part of it your channel is a diamond in the rough! You cover a lot of the academic stuff in ways most other channels don't. We all appreciate the time and effort you put into these short videos.

[@LakeHickoryScuba]

Hello Gary Rudd, you are most certainly correct. Thanks for being a long time subscriber, it truly means the world to us.

[@johnb2271]

Hi Bryan, Thanks for the follow-up on my question. Someone told me (and I thought it was you, awhile back) that when dive computers show "99" it means the number is something greater than 99 and can't be shown with just 2 digits. In the case of the 99 minutes of no decompression time on your dive computer, I think it was just agreeing with the dive tables that it could not compute a time because of the shallow depth, or, something greater than 99 minutes - like the 205 minutes you show from the PADI table at 35 feet. Again, thanks for another great thought provoking dive video.

[@Alejandro-vg7yt]

That's Right John! Dive computers either show 99+, 99, or --:-- when your NDL is greater than 100min. I have a Garmin descent dive computer and at the beginning used to show --:-- but after some diver's complained about the format they changed to 99+ like most of other computers. Even thought when going into the dive plan setting and inputing a depth of 20feet the computer shows the Maximum NDL of +999min, it only shows a value of 583.4min at 29ft this is with the conservatism set to low. In Bryan's experiment is really nice to see how he tries to see if going beyond the 99min mark will make him go into Deco. But as check at that depth his NDL would have been greater than 999min which even thought he passed the 100min wouldn't have put him nowhere near to the NDL values for that depth. It's also worth mentioning that this 99min way of displaying the NDL is reminiscent from 8 segments displays, with more modern computers using LED displays or dot matrix ones they could choose to show the value which would be around the 1000+min but for the sake of simplicity and ease of use to diver's that have become accustomed to a certain way of the readings to be displayed, manufacturer's have opted or been pushed by the customers (Garmin's case) to use that format. Cheers! PS: thanks Bryan for your videos and all the information you have been putting out here for all of us to share, learn and discuss. Keep it up!

[@LakeHickoryScuba]

You're welcome, and as stated by others, the general consensus is the screen can only display two digits. Thus, 99 minutes is the most it can display. This is another reason we highlighted the bar graph on the right side of the computer, to show the on gassing of Nitrogen versus the no decompression time remaining.

[@LakeHickoryScuba]

Thank You Alexjandro for the kind words. We appreciate everyone's comments here on our videos, and we enjoy learning from others as well. Happy Diving.

[@Alejandro-vg7yt]

Totally right! I missed mentioning that on my previous comment, your computer did not show any kind of reading when it came to N2 buildup, which demonstrates the point even further. Cheers!

[@Alejandro-vg7yt]

Happy and Safe Diving for you too!

[@frankdieber7090]

This is why I love this guy he doesn't just talk about stuff he does it and shows us him doing it. Brian your awesome keep up the great content!

[@LakeHickoryScuba]

Thank You for the kind words.

[@rotflol6666]

the ceiling for the safety stop on computer usually is 3 m (=10ft), so off-gassing is built into the models between 10 and 20 ft as you stated

[@LakeHickoryScuba]

Very True.

[@sapperstang]

I just found your channel and I think it's great. I'm a fairly new diver who does most of my dives in shallow rivers looking for fossils. I typically am in water less than 20', usually about half that. I was told and based on my research I was safe from decompression sickness so it's not something I've spent much time worrying about. Your video was very helpful in explaining this. My dive buddy did have an incident this summer that was supposedly communicated to DAN by folks at the hospital. My friend presented to the ER with bubbles in his neck. He also was found to have had a large bubble in his chest cavity. As it turns out, there was a preexisting condition that he was unaware of (hole in the heart) that may have contributed. I am unsure if this incident would be considered DCS or some other sort of dive injury. He admitted to popping back to the surface by pushing off the bottom multiple times. The water was about 8' deep. I don't worry much about nitrogen in my body from these shallow dives, but I do always surface slowly while exhaling!

[@LakeHickoryScuba]

Hello sapperstang, we are so happy to have you as a subscriber. Though it is very unlikely that a diver will find themselves developing Decompression Sickness at such a shallow depth, the risk is still there. As you stated, the ascent rate is a huge determining factor. I will state this though, Silent Bubbles are very real and can occur on land as well. We can never be too safe when it comes to diving. I know of several cases where divers made 1-2 hour dives in less than 20 feet of water, and upon surfacing decided to take a very hot shower to warm up. This caused Silent Bubbles to form due to the fact as their bodies heated up, the residual Nitrogen expanded, thus causing the Silent Bubbles to manifest, and then developed into skin bends. Thankfully though, this is the most common type of Decompression Sickness, that is rarely fatal.

[@bikeon143]

Yes, a hole in the heart can be DSC problem, the lung can normally handle some of the air bubbles generated but if it is too much and goes from the right side to the left side of the heart, then it can cause a bubble to go to the brain, not good.

[@gee4526]

Hey Bryan, My theory on this topic, and (correct me if I am wrong), is that at 20' depth is that you are only a little over ATM, and not under enough additional pressure to bring Nitrogen out of solution, thus making surface air perfectly safe for an unlimited time.

[@LakeHickoryScuba]

From a theoretical stance, you are correct. The problem though is, no two humans are identical. We all have fast and slow tissues with different decompression rates. From 10 meters of depth (33ftsw, 34ftfw) we find the biggest pressure differential. The good news though, this is the least amount of on gassing of any gas. So once again, theoretically speaking, yes you would have an unlimited amount of no decompression time.

[@DanBowkley]

If the max bottom time at 20' is infinity, taking a break for a while and going back will only add bottom time to your infinity. So it'll end up like when we were in elementary school, your bottom time becomes infinity many plus three.

[@LakeHickoryScuba]

Very True LOL

[@jjohn8989]

Another great presentation, only thing I could recommend is use the infinity symbol (sideways 8) for the "no limit listed" section of table. Stay safe.

[@LakeHickoryScuba]

That could work.

[@chadryther8710]

Bryan, I think you messed up on your presentation of how your dive computer works...Mares simply shows the "no deco" time remaining at your current depth. In your case since you remained very shallow (20-25 feet) the "no deco" time did not decrement. If you were to go deeper that time would count down and when it gets to zero it will in fact go into decompression. If it gets close to zero and you swim up a bit it will add time back on the clock. Bottom line, the 100 minutes of current dive time being greater than the 99 minutes of "no deco" time remaining means nothing on the model dive computer you show. What you want to see is when the 99 minutes "no deco" time begins to count down and get close to zero. When that happens you are close to entering decompression. I believe the 99 minutes shown on the screen is simply an a design choice made to limit "screen real estate" to 2 characters.

[@rotflol6666]

Not a Mares exclusive (2 digit limit to the no deco time), my suuntos, Uwatec all do the same, doesn't mean the model internally is keeping more digits (it is THE difference between dive computers, rec vs tec

[@LakeHickoryScuba]

Hey Chad, you are 100% correct and we discussed that before shooting this video. Primarily, we believe that the screen real estate is one of the biggest reasons 99 feet is the maximum for this depth (based off what the computer shows in the plan function), (10:07 mark in the video). Even comparing this to the SSI Tables, the maximum allowable time at that depth is 130 minutes, well beyond a 99 minute bottom time. Also you would be correct as well with the remaining time at 20 feet. The 99 minutes on the computer (dive time remaining) has not been met, even though the dive time has reached the 100 minute mark. This is another reason we outlined the bar graph on the right side of the computer, to show the viewers that we are no where close to super saturation of Nitrogen. We have discussed doing a follow up to this video by placing a computer at a depth of 20 feet, for 24 hours, just to see what would happen. We are curious to see if the 99 minutes remaining changes, or if it stays the same though out the 24 hour time frame. Given the depth of 20 feet being the test depth, and the history of historical dives made by previous divers, my guess would be that the computer would show a diver going into decompression, both on the bar graph and on the time remaining, but the diver himself would be just fine.

[@rotflol6666]

You could try to simulate with a dive planner like Suunto SDM, has the same models as in the computers and you might not lock the computer beyond repair (suunto cobra for 24 hrs at 12m locked forever, not even factory reset possible)

[@LakeHickoryScuba]

Good idea. I will probably call Mares first to see what they say about the Mares Smart. To my knowledge it only locks up for 24 hours after a DECO Dive, without DECO obligations being met.

[@rotflol6666]

Many moons ago (2002) we had a Cobra permanently go into locked mode, never recovered and was replaced by Suunto, had been at 12m for 24 hrs ("lost" in a rescue course, found the next day :-( )

[@grahambirch3715]

Firstly, thank you for creating these videos, they are always well thought out, practical and helpful. I know that this video was posted a year ago, but the title grabbed my attention so I watched it & just wanted to comment on one aspect: ** Warning. This post became longer than I expected. If you don't like reading, skip the to "CONCLUSION" ** Decompression sickness occurs when the partial pressure of Nitrogen in the diver's tissues exceeds the ambient partial pressure of nitrogen to the point where nitrogen can no longer stay in solution. The question is, how large can that difference be before it becomes a problem. According to Haldane it was a 2:1 ratio. This essentially meant that a person could be saturated to 2 atmospheres (33 fsw) and return to one atmosphere (the surface) and have a low probability of experiencing DCS. This was later modified down by Workman to a ratio of 1.58 (Interesting footnote to all of you SSI divers that this question comes up in the SSO Open Water Diver class. Don't ask me why on earth such a question made it into an Open Water level class, but I'm sure that there is a story behind that somewhere!). According to Workman's ratio, that depth would now be 0.58*33 = 19 ft. Decompression theory moved on and we now think in terms of "M-Values" (also attributable to Workman). The "M" in M-value stands for "Maximum." For a given ambient pressure, an M-value is defined as the maximum value of inert gas pressure (absolute) that a hypothetical "tissue" compartment can "tolerate" without presenting overt symptoms of decompression sickness (DCS). In the Buhlmann ZH0L16 decompression model, the tissue compartment with the lowest M0 value is tissue 16 at 41.8 fsw. Taking our 20 ft dive that equates to (33+20)*.79 = 41.87 fsw CONCLUSION: Although I would not like to test it and I'd certainly not advocate it (always dive your computer). The reason why this 20 fsw dive does not show on tables or computers is because you can stay at 20 fsw, become fully saturated (i.e. stay as long as you wish) and could still make a (slow) direct ascent to the surface with a low chance of DCS because your tissues can take this level of supersaturation (at least in theory :) )

[@LakeHickoryScuba]

Hello Graham, thank you for the very methodical comment. In regards to tissue saturation, this is how it has been explained to me over the last 31 years as a diver. In short, Decompression Theory is Theoretical at Best. Simply meaning, there are not two human beings on the face of the earth with the exact physiological make up. We all have fast and slow tissues, but both mine and your fast tissues will on gas and off gas Nitrogen at two different rates. The same applies with slow tissues. When a formula is developed (Workman's, Buhlmann, etc., etc.) training agencies and gear manufactures will usually consult with attorneys, and before they produce a product (computers and tables) and then make a claim that this is concrete evidence a diver will become bent, they will always distort the numbers for safety and liability. I relate the Scuba Industry to the Food Industry. When the FDA decides to release a new ingredient or edible product to consumers, they will take the maximum amount that is safe for consumption and cut it in half. They will then take that amount and cut it in half again. Lastly, they take that number and cut it in half one more time, and this is the number they release for safe consumption. When we go tech diving, we are purposely putting ourselves into decompression (according to the decompression models), but yet through accelerated decompression we bring ourselves out of decompression at a controlled rate. But even at this level, we still are allowed to choose what algorithm we want to use, thus proving the fact that Decompression Theory is Theoretical at Best. We did a video a while back showing the differences in several of the agency's dive tables, and the numbers were astounding. I can say for a fact, that as a dive professional for 5 different training agencies, including SSI, PADI, PDIC, SEI, and CMAS, I routinely violate one or more of the agencies dive tables and limitations, but at the same time still well with in the limits of another agencies decompression model (it all depends on which agency I am teaching for day by day). By theory alone, I would have developed Decompression Sickness many times over. My suggestion to all divers is to simply, don't over think the physics in Scuba Diving. Pick a formula or model that you feel safe following, and simply go diving. At the end of the day, a diver can do a simple bounce dive, well with in recreational limits with little saturation and still get bent simply because he exceeded a 30 feet per minute ascent rate. Hopefully this makes sense.

[@bloodymarvelous4790]

@@LakeHickoryScuba But you can't compare dive tables to a dive computer. Dive tables assume you stay at your maximum depth for the entire duration of your dive, while computers will adjust for your dive profile. Dive tables don't know how long you stayed at 100 ft, and then ascended to 72 feet, and then ascended to 20 ft. They need to assume worst case scenario and that you stayed at 100 ft for the entire time and your body didn't have the chance to off-gas. So exceeding times by a few minutes on a dive table in a normal dive profile where you start off deep and gradually shallow over the course of the dive until you decide to surface, would obviously not result in DCS as you've off-gassed, on-gassed less during the dive than the tables would assume.

[@mikesbigadventures194]

I love your videos Bryan. Learning so much. What’s your opinion of Ratio Deco different agencies use?

[@LakeHickoryScuba]

Hello Mike's Big Adventures. I've talked about this very subject in the past. When I think about Decompression Theory, I focus on the last word, "Theory." Decompression theory is always going to be theoretical at best. We can see this in its simpliest form, by looking at different agencies Decompression Tables and the maximum allowed time each table allows at given depths. A quick example would be PADI says you have a total bottom time of 55 minutes at 60ft, where SSI says you have 50 mintues at 60 ft. According to the theory, a diver would go into decompression by 5 minutes if he used PADI tables, but he was an SSI diver. As for me personally, it depends on the purpose of the dive and other varibles on how I plan and execute dives. Most of the time, my computer is my go to (tables being used for training purposes only), and it is usually set on the most conservative setting. Oh, and it should also be mentioned that I usually leave my computer in Air Mode, even while diving Nitrox. I truly believe in conservatism while diving. Hope this answers your question. Great question by the way, one that most divers don't even think about.

[@mikelewellen1114]

Great video.

[@LakeHickoryScuba]

Thanks mike lewellen, glad you liked the video.

[@coreybaldwin989]

Another great video! Always very informative!!

[@LakeHickoryScuba]

Thank You.

[@tomhyde9293]

I don't believe the 99 minutes on the dive computer is a no deco time. It is simply the fact that the computer can display 2 digits. My computer does the same, until my no deco time drops below 99 minutes. Try this. Start descending slowly. Watch your computer. At some point, the no deco time will decrease below 99 minutes. Prior to that, the no deco time calculated in the computer was greater than what the computer could display in its 2 digit display. That's why that once you went beyond 100 minutes, the computer did not show a required deco. The computer's calculated deco was still greater than 99. Thanks for early voting, I'm going today.

[@LakeHickoryScuba]

Hello Tom, you would be correct as others have mentioned as well. Most computers can only display 2 numbers thus making the given No -Deco Time max out at 99 minnutes. What we were wanting to see was, if the computer would go into DECO Mode after exceeding what was displayed, and based off what the dive plan featured showed. Thank You for voting as well. Happy Diving.

[@divingmadd6219]

Great video Bryan! Very interesting theories. You can upload your critique of my twinset shutdown drill if you want. And show others how buoyancy of lungs affect us underwater and muscle memory etc etc.

[@LakeHickoryScuba]

I will try to get one made for you, and will add your KZbin channel in the video as well.

[@divingmadd6219]

LakeHickoryScuba only if you have the time and want to.

[@LakeHickoryScuba]

I don't mind making a critique video at all. It just may take me a little while, as we are extremely busy right now starting our winter trips. I will make sure to link your channel in the video as well. Stay tuned.

[@mikesonntag7698]

Not sure if naui updated the tables since this video but the NAUI table does have an ndl at 6m or 20 ft. It is 461 minutes

[@LakeHickoryScuba]

I will have to look into that one Mike Sonntag, thanks for the information.

[@ryba7846]

Hello. I got bent doing 2 dives. The first dive to 72 feet(22meters) and second dive to 130 feet (40 meters) I know about you should do deepest dive first but i was always thinking its just about the longer bottom time. My computer did not enter deco. The doctor said that it was much safer back in the days when diving with physical tables and that the computer is recalculating all the time and keeping you on the limit. You say dive computers have safe margin so my question is, do they really?

[@LakeHickoryScuba]

Hello ryba, computers will always be safer when doing multi level diving. Even in a square profile, computers algorithms tend to be more conservative than tables. Sorry to hear that you got bent. We encourage all divers to stay well with in their no decompression limits, and never push their limits.

[@jimmymifsud1]

My Shearwater Teric, at 12m or in yanks terms.. 40ft it allows 121m on 21% O2

[@LakeHickoryScuba]

Hello James Mifsud, a lot of divers here in our area love the Shearwater Teric.

[@jimmymifsud1]

LakeHickoryScuba it’s a great watch, an even better dive computer 😉

[@Warhamster55]

Jules Verne Lodge Florida There is an underwater hotel at Key Largo, Florida. The moon pool in the hotel is at about 20 feet. Guests of the hotel stay overnight. You could check with them about decompression sickness at 20 feet. :-)

[@LakeHickoryScuba]

Hello Warhamster55, I am very familiar with the Jules Verne Lodge in Key Largo. I have visited it many times over the years. It’s a really neat thing for new divers to experience as well.

[@forgot_my_name_again]

Isn't the no deco limit on your computer limited to two digits because there isn't any more space on the screen? I think your computer takes something like 6 hours or more of no deco time in consideration at those depths.

[@LakeHickoryScuba]

Hello Carlo, that is the general consensus, that most computers simply do not have enough real estate to have more than two digits on the screen.

[@rotflol6666]

What is the safety stop depth in feet? 20? (metric is 6m) That would explain why you are not on-gassing, the models don' t consider on-gassing at that depth

[@LakeHickoryScuba]

Depending on whether you use a computer or dive tables, it ranges between 10 feet and 20 feet. Thus, averaging around 15 feet (as taught by the majority of diver training agencies). Most computers during the ascent phase, will turn on the safety stop feature around 20 feet, and will stop the counter if you descend deeper than that, or if you ascend shallower than 10 feet, with in a 3 minute period. In regards to on gassing Nitrogen, our bodies still on gas anytime we are at more than 1.0 atmospheres, as can be seen on a dive computer with an AC mode. If a diver fails to turn on his computer prior to a dive, they usually will automatically start upon reaching a depth of 4 feet, thus showing a diver is on gassing nitrogen. But it is believed to be so minimal that keeping track of the nitrogen on gassing is not needed. It was shocking to learn from the Diver's Alert Network, there has never been a recorded case of Decompression Sickness on any diver, from a depth of 40 feet and less. This may be another reason that most tables start their depths at this range.

[@rotflol6666]

the ppN2 at 20ft is/might be too low to get you bent

[@LakeHickoryScuba]

I would agree for the most part, as long as we left the variable of a rapid ascent out of the equation. Any and all Nitrogen in a diver's tissue still could expand to quickly versus leaving the tissue and re-entering the lungs during a rapid ascent. Thus, Decompression Sickness and Arterial Gas Embolism is still a big risk. However, just staying at that given depth (20 feet), like you, I don't believe we would ever retain enough Nitrogen for it to matter.

[@rotflol6666]

I meant in the computer models, mine all go to a safety stop at 6m, so "off-gassing"

[@LakeHickoryScuba]

In that case, you computer is basically saying we don't start on gassing any Nitrogen until we reach a depth deeper than 20 feet. This is another reason we decided to make this video, to show divers that dive theory and reality are not the same thing. Obviously we have to be safe while diving and stay with in our limits, but in short, there is a lot of conservatism built in to Gear Manufactures Algorithms and Training Agencies Tables.

[@bloodymarvelous4790]

You could've known that after 1 minute. If your computer says your NDL is 99 minutes, that's because the counter doesn't go higher. If after 1 minute under water it still says your NDL is 99 minutes, that means you can still stay at that depth for another 99 minutes. If after 99 minutes your NDL is still 99 minutes, that means you can still stay at that depth for another 99 minutes.

[@LakeHickoryScuba]

Hello @bloodymarvelous4790, thank you for sharing your thoughts with us.