• Welcome to ScubaBoard


  1. Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.

    Benefits of registering include

    • Ability to post and comment on topics and discussions.
    • A Free photo gallery to share your dive photos with the world.
    • You can make this box go away

    Joining is quick and easy. Login or Register now by clicking on the button

Deep stop question

Discussion in 'Advanced Scuba Discussions' started by ross9, Jun 17, 2020.

  1. KWS

    KWS ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 500 - 999
    Location: SE TEXAS
    4,925
    1,264
    113
    I have a relative very low understanding of deco but It strikes me that the deep stop rule of half ATM is just another global rule of thumb. The words unneccessary and increase deco overall is very dependant on IMO what you have done so far. By that I mean doing a 100 ft dive to the ndl limit vs doing a 100 ft dive as a bounce to get a dropped tool etc. Time has to have a lot to do with just how much continued on gassing you would be capable doing let alone anything else. Yes I think a 60 ft dive for 20 min and a "deep" stop at 1/2 ATA is a waste. but a dive to 130 for 5 minutes and stopping at 50 ft is a whole different thing. IMO especially when you look at the aspect of potential harmful continued on gassing. Again from a pure novice perspective. The idea of any rec stop deeper than 15 ft is not a good idea is questionable. That too is riddled with IMO problems as rules like,,,,, do a 3 minute stop at 20 is a global one based on the worst condition dive. If you take the argument of rec diving is to 130 ft on dive tables and you start to ascend 10 seconds prior to NDL I dont think 40 or 20 should be a no no or a waste. but if you use the RECOMMENDED max depth for an OW of 60 ft on a computer it is again another story. Personally I have always believed that deep stops are a TECH THING and use it for a deep stop and combine other activities inthat period. That many rec divers follow suit for the 100 ft plus rec dives for when they screw up their ascent rates or run at min consrervitism. I can also see that opinion of physics if you are a skilled master at ascents and not having to do some stops. but the real world is not like that. You can beet feet to 60 and slow your ascent to 30 depending on what era you certed in . the 60 fpm or the 30 fpm or the mix. Thankfully the GF displays on my shearwater take the mistery out of what to do when that question of stop or dont stop arrises. stop depth for safety or other ,,, gf99 tells me that min depth limt.,,,,,,,,,,, how long to stay???. gf surface tells me that min time.
     
  2. Storker

    Storker ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 100 - 199
    Location: close to a Hell which occasionally freezes over
    15,769
    12,218
    113
    *bing, bing, bing*

    All available literature indicates that deep stops are either based on anecdote, or a theoretical construct with no basis in proper data. The jury is still out, but the evidence is rather compelling.

    If doing a deep stop makes you feel good, by all means. As @boulderjohn mentioned, people are different and physiologies are different. But unless you have evidence to the contrary, I sincerely believe that your best bet is to go with established science, valid for the masses. And current established science indicates that stopping deep isn't a very good idea for most of us.
     
  3. ChuckP

    ChuckP Loggerhead Turtle

    # of Dives: 200 - 499
    Location: Cozumel
    1,259
    1,175
    113
    You are a recreational diver, you are staying within the NDL of whatever your computer is set to, comments below are based on the recreational NDL dive only.

    The PADI planner says on air you can dive to 90' for 21 minutes and still ascend directly to the surface. Your computer is evaluating different "compartments" looking at saturation levels. If you ascend directly to the surface (or safety stop), your body is continuely off gassing during that ascent, which they have determined is safe.

    When you stop at half max depth, in this instance 45', you will continue to on gas in any compartments that have not yet been saturated - the root of your question is this good or bad for you, is there any benefit to doing it.

    Consider two different dives, a square profile wreck dive verses a reef dive here in Cozumel (max depth 90' and consistantly shallowing up as the dive progresses). The reef dive will spend 15 minutes at 90' and gradually shallowing up and maybe spending the last 15 minutes at 40' - total dive time being 60 minutes - any way you slice it, the Cozumel dive is a deep stop dive done 100's of times a day everyday.

    For me it comes down to this, why allow yourself to on gas if you aren't looking at something? There's no benefit from doing it but yet there's no penalty either other than repetitive diving where lessening the on gassing increases your NDL for second and third dives.

    The question of deep stops really doesn't come into play until you start diving deeper where you encounter higher pressure gradients and more tissue loading in the mid to slow range compartments.
     
  4. Dr Simon Mitchell

    Dr Simon Mitchell ScubaBoard Supporter Staff Member ScubaBoard Supporter

    614
    2,091
    93
    Hello,

    As boulderjohn said, this is a fairly good summary of the situation. I would just add that the trend away from over-emphasizing deep stops is an evidence-based phenomenon rather than a community fashion. I linked to it in another thread, but I recently gave a lecture on the relevant theory and evidence in a covid lockdown presentation on-line organised by a Polish group. It is a little long and slightly laboured because I was asked to speak slowly to allow simultaneous translation into Polish, but if you are interested in this subject you will probably find that acceptable and it will answer most of your questions. It obviously is in English once I start talking. You can see it here.

    Simon M
     
    Dark Wolf, scubadada, DavidFL and 6 others like this.
  5. KWS

    KWS ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 500 - 999
    Location: SE TEXAS
    4,925
    1,264
    113
    all good points. does that direct surface assume a steady 30 fpm? IF so Ok what about those that do a higher ascent rate untill they get to 50 or 60 to reduce gass use would that deep stop for a minute maybe 2 be beneficial on over all gass usage? Im thinking no if you stop more than a minute. The topic comes up a lot with people and many justify it as time to stablize buoyancy and get a little heavy before finning up. I hear a number reasons to do it but I have to admit that most sounds like the ""benefits of a deep stop" is a secondary concern. I am wondering that so long as you use reasonable ascent rates it probably has no benefits. for those sports car divers I think they have different opinions. Most the time I hear that some do it if the total dive is a deep one other wise they skip the deep stop. I have never had any real opinion from casual divers on the subject because the NDL is a direct surface dive. Of course that brings in the safety stop discussions and the 3 minute and the 5 minute issues which are always presented as no longer optional, MANDATORY SAFETY STOP. I have done semi deep stops after multiple dives that were deep. One can also reason that a deep stop is no different than a multilevel dive If you ignore that the primary goal is to ascend to the surface. doing a 5 minute dive at 100 ft and then going shallow to 40 ft until gas says ascent to the surface would be the same deep stop type of issue. That is the main argument that seems to be the most valid when I hear it.

    Ill ask the question. is the on gassing one would do on a deep stop be a serious problem or just a math issue saying you could avoid most all addition on gassing if you do a direct surface. If you did a dive to 100 for the entire NDL would there be a price to pay for doing a deep stop vs doing a 100 ft dive for half the allowed NDL. would you not be off gassing more than you would on gassing on say a 2 tank dive day would it make any difference at the end of the day. Would it also be reasonable to say that the more dives per day the less impact the deep stop would have in the later dives? And then is there any residual N2 consequence after the surface interval???
     
  6. KenGordon

    KenGordon Rebreather Pilot

    3,360
    2,141
    113
    OP, have a look at http://archive.rubicon-foundation.org/1520 which ought to be the DAN 2004 study of brief safety stops as deep as 15m on NDL dives. My internet isn’t happy going there this second so I cannot be sure. The conclusion is that for some not outrageously deep dives (25m if I remember correctly) stopping deeper helps a bit, at least with ultrasound detected bubbles.

    What a deep stop means depends on what deep means. As per some earlier posts that can mean a stop deeper than 99% of people ever dive, or it can mean a stop shallower that 90% of dives.

    Much of the argument is about the depth of deco stops on fairly length, quite deep dives. The NEDU stop redistribution study was a 170 ft dive with substantial stops at depths like 70ft or 50ft. That demonstrated that the additional on gassing to slow tissues was disadvantageous, however the earlier DAN study shows that a short deeper (than 6m) stop helps on NDL dives.

    Also, all this obsessing over the “pure” plan ignores that in practice few people are diving to the algorithm. They are usually much slower ascending than tables or diver computers allow for. It may be the case that a 4 to 6m/minute ascent on an NDL dive gains nothing by spending a minute or two at 10 or 15m.
     
  7. tbone1004

    tbone1004 Technical Instructor ScubaBoard Supporter

    # of Dives: I'm a Fish!
    Location: Greenville, South Carolina, United States
    17,536
    9,635
    113
    @KWS trying to follow that post a bit and maybe it's not enough coffee on my side.

    This is all a math game. The original discussion of deep stops was a redistribution of decompression time from shallow to deep. I.e. I deduct 5 minutes from my 20ft stop and I put that 5 minutes at 60ft, or whatever the numbers actually are. Same total TTS. Argument there was from the same arguments as bubble models in that it minimized bubbling in the fast tissues since they were oversaturated. The price for doing that is additional ongasing in the slow tissues while you're waiting for the fast tissues to blow off.
    If you treat this as a multi-level dive, then you are not redistributing deco time, you are just doing a multi-level dive. If the last level is shallow, then yes that is mimicking the original idea of the deep stops by allowing your fast tissues to stabilize under more pressure, but some of your slow tissues will still be ongasing. In the context of sport diving depths I don't think it actually matters because you aren't down for particularly long and 4-5ata's isn't a whole lot. From memory because Rubicon is down for maintenance right now basically says that a pause at half pressure for a minute or two can help prevent the fast tissues from seeing a big over-saturation event when you go fairly quickly from 4-5ata's up to 1.5. This could be mimicked by going say 50-60fpm up to half pressure, then say 15-20fpm up to your safety stop depth. That half pressure though is what is important as on "big" dives there are almost always stops around that depth. They aren't necessarily very long.
    In my first post I showed a dive to 10ata, where there were 6 minutes of stops from 6ata's up to 5ata's. It was less than 4% of the total decompression time in the example, but if you extrapolate that to NDL diving and give a brief pause at half pressure, you can help the fast tissues as described in the study. For NDL diving does it actually matter? I don't know, but if anything a planned pause in the ascent at half pressure helps most divers control their ascent rates which are usually far too fast anyway.
     
    ChuckP likes this.
  8. boulderjohn

    boulderjohn Technical Instructor ScubaBoard Supporter

    # of Dives: 1,000 - 2,499
    Location: Boulder, CO
    26,603
    19,154
    113
    In my research on deep stops in NDL diving, I learned that this study is no longer highly regarded. Here is the 2004 article as it was published by DAN. Note that the article starts with a warning that it does not reflect current thinking.

    That warning suggests another DAN article, also fairly old, which is a panel discussion of experts. In that panel discussion, Peter Bennett gives something of a lukewarm endorsement, probably because he was part of another study that liked them. (That study is the basis of the NAUI endorsement of deep stops.) All the other experts in the panel disagreed. In my research, people endorsing deep stops on NDL dives gave me links to this article as if it were a ringing endorsement of deep stops, when it is actually quite the opposite.

    DAN Europe endorses deep stops, much to my amazement, since there is precious little research to support that point of view.
     
  9. Storker

    Storker ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 100 - 199
    Location: close to a Hell which occasionally freezes over
    15,769
    12,218
    113
    When I took my 3*, we did a simulated mandatory decompression dive. Using tables. Now, our tables are calculated for an ascent rate of 10 m/min, so we were supposed to ascend by 10 m/min until we reached... well, to be honest, I can't quite remember, but I seem to believe that it was either 9m and then slowing down, or the deco stop at 3m. We were taught that if you go slow from depth to the deco stop, you would be ongassing more than the tables were calculated for on your way up.

    Except for when I was a n00b and corked from the SS, I can't remember to have ascended so fast. And IIRC, when I took my OWD, PADI's maximum ascent rate was 18 m/min. I have some problems reconciling my experiences with your blanket statement that most divers' ascent rates are usually far too fast.
     
  10. boulderjohn

    boulderjohn Technical Instructor ScubaBoard Supporter

    # of Dives: 1,000 - 2,499
    Location: Boulder, CO
    26,603
    19,154
    113
    I used to be of this mind, and a thoroughly unnecessary search of my long past posts will find ones in which I cautioned against ascending too slowly while using tables, arguing that the numbers on the tables were invalid if you were not ascending at the rate at which the tables were created. In my research, I talked with someone who was well versed in the research done on the PADI tables, and he told me that wasn't true. He said they saw no problems associated with ascending slower than the 60 FPM used for the tables. The PADI language for the RDP says divers should not ascend faster than 60 FPM; it does not mention ascending slower than 60 FPM. If you look at the DAN articles on ascent rates, they all advocate ascending slowly, and they have no caution against ascending too slowly.

    I agree. I have done NDL dives all over the world with countless divers I had never met before. I have rarely seen anyone ascending at what I thought was seriously too fast.

    I have supervised a number of dives for divemaster training in which the DMCs are supposed to be leading dives for certified divers. Whenever they started the ascent, I noted the exact time and calculated their ascent rates. I almost never had any DMC lead an ascent faster than 10 FPM, and I don't think any of them ever went as fast as 30 FPM.

    The ascent rate issue is more pronounced in tech diving, where ascending too slowly can be problematic for people ascending using tables, because if they ascend much slower than planned, they will reach their first planned stop having added unplanned bottom time during the ascent. When using computers, the computer will adjust. Here is a story that illustrates this.

    I was on a dive boat doing technical dives, and the DM went around the boat to each group asking for their expected run times and expected times at 40 feet. (They wanted to know when to send someone down to until the ascent line.) Pretty much everybody was in a reasonably close range. Sitting next to my team was a team of rebreather divers who had exactly the same dive plan as my team. As one would expect, when my team reached the ascent line (on a wreck), we could see other teams had already starting up to the surface, and we saw that the rebreather team, the only one behind us, was ready to go. We watched our 30 FPM ascent rate carefully. When we got to our first stop, I looked down and could not see the rebreather team below us, despite reasonably good visibility. I was concerned. After the first couple of stops, I could finally see them. I assumed they must have had a problem that had been solved.

    We were back on the boat long before them, and people were concerned. When they finally got on board, the DM asked them what had happened, and they said they had no idea. They said all through their ascent, their computers kept adding time, and they did not know why. I pointed out that they had ascended very slowly, and they responded that you are supposed to ascend very slowly.​
     
    ChuckP likes this.

Share This Page