What happens when you take multiple computers and dive tables on the same dive?

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About 5 years ago, I went for a dive with some friends; all of them Instructors on a 40m recreational dive.

The dive ended with all of us separated, some still doing safety stops and some divers already on the surface, whilst one was doing a mandatory decompression stop. Of course, decompression stops are not within the scope of a recreational dive, however, due to the fact that other divers had to make deeper stops, and divers were trying to stay together, this particular dive ended in one big mess.

Aside of the chaos and confusion that occurred underwater, this is obviously not the safest and most fun way to dive. What had happened? Why had it happened?

The divers had been pushing the dive, trying to stay away from deco by following their computers, not realising that the five of us were using different computers, with different settings and algorithms. On that day, I decided to make an example of this and make it a topic that I would highlight on my courses.

A lot of my time on courses, recreational or technical, is spent on talking and discussing decompression. In the many years that I have been teaching, I discovered that people that were already technically trained had to compromise amongst themselves in order to plan a dive together. The softwares or computers did not agree on what they were about to plan. Between us, with myself as the Instructor, we had to find a compromise.

Some students would show up with plastic dive tables, others with the latest software, and others would still have their old recreational computer with them. Many students, even now, are completely certain that their program or computer is the only and the right way.

During these discussions I normally bring out an old dive US Navy table that I used to carry with me as a backup plan in the early days. Suddenly they realise that things weren’t always like they thought they had been and this was my moment, as an educator, to start a healthy discussion on the ever-changing decompression theory.

I like to compare a 19 year old US Navy diver with a 60 year old Tech diver that smokes. Obviously the diver that is in better shape will be able to off gas more efficiently, not even talking about cold and warm water and demanding environments.

A few years ago, I decided to take several computers on a dive and as I suspected, the differences were astonishing.

Last month, I decided to do the same thing but a bit more seriously, using dive tables, a few of the most common technical computers, and a few recreational computers.My intention was to take them on a 45m, 15 minute dive.

The objective of this experiment, as an educator, is to create awareness of the fact that decompression theory is something that is not written in stone. Computers and tables might produce very different numbers, which given together with the physical differences in between each individual (weight, gender and age), and environment we are diving in, makes it virtually impossible to create a perfect decompression plan.

Before the experiment, I made sure there was no surface interval time on the computers by removing batteries. Then I put all of them on a PVC pipe and took them for a dive.

The dive was a 45m, 15 minute bottom time on air, dive with the exact same depth and time for all computers since all these devices were mounted on a PVC pipe.

I expect you will want to know which computer did what but I think it is irrelevant since no one really can tell what the best result is or was. The objective was of the dive was only to create awareness of the fact that all these computers we can buy create different profiles; therefore we need to have a good foundation in order to choose for a certain profile and here, brands are irrelevant.

The experiment was NOT a scientific experiment so I do not want to get into exact numbers, brands or theoretical discussions. The objective was simply to show the audience that different computers give different numbers, very different numbers, and that tables give us different numbers again.

The table I used was a table based on the US Navy tables and still in use by instructors in Northern Europe. Believe it or not, with this table we can plan an air dive to 91m and do a safety stop.

Not one of the computers I used comes close to the profile produced by this table. This table can take us from 91m to 6m whilst any of the computers would have made us stop at least 5-10 times as deep.

I decided not to make any personal conclusions writing this article, however, I think I can safely say that this table is extremely aggressive and should not be used by Sport or Technical divers.

Summary of my findings on the 45m, 15 minute air dive: 

  • The table produced a 3 minute stop at 3.5m whilst the most conservative computers gave around 10 times that amount of decompression time starting stops 3 times deeper.
  • I used two different computers from different brands using the same Buhlmann algorithm and settings using the same Gradient Factors (30/70) and the results were not consistent; not only the total run time was different, also the first stops were at different depths.
    • The first stop from brand A was at 9m and brand B wanted to make me stop at 12m.
    • Brand A gave me 15 minutes time to surface whilst brand B gave me 23 minutes when we arrived at 18m.
  • I also used the same brand computers using different settings: the default setting and another common setting (30/70 and 25/85). The difference on this relatively shallow dive, at 18m was astonishing. At 18m settings 30/70 gave me 23 minutes and 28/85 gave me 11 minutes, implying that a diver needs a very good reason to choose a different setting than the default settings or vice versa.
  • The computers that were using RGBM were recreational computers. These gave me shallower stops than the technical computers with less decompression time. An interesting event was a 15m stop from one of them, afterwards directly pushing us to 4.5m.
  • Comparing all computers: The first stop of one of the computers was at 15m, whilst the less conservative computer took us to 3.5m in order to finish the deco.
  • The most conservative TTS (total time to surface) between all computers was more than double the time of the most aggressive computer and the table really didn’t give us any decompression time.

I don’t want to make any conclusions. I will leave you to draw your own. The objective was to demonstrate that tables and computers give different numbers, numbers that are completely different!

Mike is the founder of www.thelonghose.com and a Technical Instructor Trainer for various agencies and the current Training Director of I.S.E, Innerspace Explorers.

Photos: Alexia Dunand

Mike Van Splunteren

Mike Van Splunteren

Mike took his first breath underwater more then 25 years ago after discovering diving during a two year period in the Army. Teaching is the thing he likes most about scuba diving, learning new things and transmitting them to his students. He still loves recreational diving, just being in the water for the sake of being underwater, but over the years Technical Diving has 'infected him with its virus'. Not only depth and overheads attracted him, but the quest for optimal configurations, swimming techniques and getting really safe and comfortable became much more interesting! Mike is a fulltime Technical Instructor Trainer for RAID and ISE and the current Training Director for ISE for more than a decade. You can find out more about Mike at www.thelonghose.com.

5 Replies to “What happens when you take multiple computers and dive tables on the same dive?”

  1. Robert Helling says:

    I am not really surprised by the findings. Besides depth/pressure sensor jitter and different levels of conservatism fudge factors there is no such thing as “the Buhlmann model with gradient factors” as I explained on my blog: https://thetheoreticaldiver.org/wordpress/index.php/2017/11/02/why-is-buhlmann-not-like-buhlmann/

    And to be fair, you should not really compare relative differences (“A wants twice as much deco as B”) but rather compare absolute runtimes since a few minutes are are always in the uncertainties (like rounding errors) of the models. Six minutes of deco is twice 3 minutes but both are “just a few minutes” and most divers will probably surface just fine when skipping either. I would be much more worried about differences like 60 vs 80 minutes (even though that is only 25%).

    For similar reasons, the first stop is (given that it tends to be very short) almost meaningless: What one computer may call 1 or two minutes of stop is likely very very similar to what somebody else calls just a slow but continuous ascent.

    Have you tried to run the profile through a software like Subsurface. There you could try to determine to what differences in model parameters (like gradient factors) the plans by the different computers amount to.

    1. Mike van Splunteren says:

      Hi Robert, The idea was to keep it simple and didactical. I am not surprised that an experienced technical diver is not surprised. What you are explaining and writing is for many divers complicated and irrelevant, they are clueless and don’t know what Buhlmann is or bubble mechanics or even the word run time…. I think there is much too much emphasis on theoretical information in technical (and not enough in recreational) that might cause confusion and doesn’t bring the diver, especially the normal diver anything. What I tried to do is to make it easy to digest and in the face….This is simple : Tables / computers give very different results, go and sort yourself out !! What you are doing is exactly the opposite…you are trying to look for what the best model or table or way is and what stop is important and we get into this “I know more than you” discussion etc.. An AOWD with 61 dives that just finished his nitrox has no idea where you are on about 😉 BTW look at the huge amount of shares and comments from divers that had no idea…and thought it was all the same…Not kidding you 😉 Regards
      Mike

  2. Dan Marelli says:

    An additional piece of information would be the most recent US Navy air decompression tables (revision 6) on the dive profile in question would require 1 decompression stop at 20 fsw (6.1 m) for 8 minutes. All Navy tables are empirical in that they are tested with multiple dives and divers and limits are determined from the data collected using a probabilistic approach. Obtaining multiple results from a variety of computers is disturbing, are they all just best guesses based on assumptions and what difference does it make whether the computer is a “technical” or “recreational” model if they all profile the dive in question?

    1. Mike van Splunteren says:

      Hi dan, yeah, I am the guy that wrote the article and for sure there will be a second part or test.
      This experiment was “dedicated” to the normal recreational diver….

      1. Dan Marelli says:

        Thanks. I actually mis-typed Rev. 6 when I meant Rev. 7, which was released about a year ago by the Navy.

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