Dive Computer Evolution - 2004
I bought this SOS Decometer in 1987, it
had been on the market for 5 years or so and was firmly established as the
cutting edge in diver convenience. Over the years I have used literally dozens
of dive computers and the latest one is the VR3. The VR3 makes the same miracle
device claims as the Decometer did some 22 years earlier. I quickly stopped using
the Decometer in favour of an early Aladdin with a digital display and never
completed a decompression dive using it until recently.
I thought I would compare the Decometer performance with the VR3 on a
fairly deep dive where both computers would be tested against a trusted software
plan I had used many times. The dive was to 78m for 20mins to explore a wreck
here in Thailand. Setting the gas mixtures into the Decometer could not have
been easier, only using air as a choice made using quite simple! The VR3 accepts
10 different gas mixtures (including Trimix) and these can be pre set or even
modified underwater. Setting the VR3 took the same amount of time as filling my
decompression tanks! Hopefully, it will get a little faster with practice.
Dropping down to the wreck near 80m, meant changing some settings on the VR3
while descending, while the Decometer simply looked like it wasn’t
working. The mechanism in the Decometer is a series of small plastic bags which
are crushed by increasing pressure. This action acts works through some kind of
filter system, which at a prescribed rate starts to register that decompression
stops are necessary. The VR3 has a small computer that makes calculations based
on breathing gases and time against changes of depth.
After about 10 minutes at depth the Decometer had started to look like it was
working and the deco stop slider had begun to move saying that stops where
necessary around 6m. The VR3 was suggesting stops were needed at 27m, but these
were deep stops mainly to control micro bubbles (a theory not imagined in 1981,
when the Decometer was introduced.) The conditions at the bottom were fairly light and the
Decometer display was easy to read. The display on the VR3 was a grey dot matrix
and its contrast suffered in the dim glow of depth. On pressing the right hand
slider of the VR3 the back light came on and this made everything easy to read. After
20 minutes at depth the VR3 was indicating deep stops near 55m for 2 minutes.
I ascended to this stop depth and noticed
the time to surface was now 73 minutes. The Decometer does not display the
actual time to surface, but rather the decompression stop depth only, in this
case 12m. On completing the VR3 stop at 55m, I ascended to the next predicted
deep stop, this time 43m for 2 minutes. These deep stops are predicted in a
different manner to my software schedule, the deep stop depths generally not be “pre
viewed” prior to the dive, this can cause a group of ascending divers to become
separated if some are using a software schedule.
I followed the ascent plan from the VR3 and this could not have been more
different to the Decometer. My software plan which was worn on a wrist slate was
a similar total time to the VR3 (while still at depth), but by doing the 2
minute stops each time, gave a much
longer time to the surface than originally predicted.
By the time I reached 15m the Decometer computer was saying a 6m stop was
necessary, I switched gases on the VR3 to a 60% deco gas. By 81 minutes
the VR3 display indicated that it was safe to surface, the Decometer was
still saying that I should stay at 3m for an undisclosed time. The Decometer
still actually says that I should be at 3m even 2 weeks after the dive. (I
opened it up and it looked like the bag mechanism had flooded) I had lost the
instruction booklet almost 17 years ago, I don’t think it mentioned a maximum
depth!
The two computers gave completely different ascent solutions for this
dive, both were also very different from my software decompression plan. The
only lesson to be learned from this exercise, is that divers in a buddy team,
should use the same schedule for the ascent, either both using a software
solution or both a similar computer with a table back up. This will help avoid being forced
apart by doing decompression stops in different depths.
Dive computers and decompression methods have certainly changed dramatically
over the years. I think that the deeper stops advised in modern methods are a
benefit to diver physiology. New, deeper stopping decompression methods are an
emerging science that need careful
consideration, a whole other article.
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