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