Inner Life of a Dive Computer
The basic components are:
As a matter of course the housing is waterproof. But normally these are not gas-proof! That is, during an excursion in a recompression chamber, put your computer in a water-filled bucket: otherwise the air is diffusing into the housing and upon ascent the box will crack due to the expanding air ...
The basic different housing designs are:
This means, either the housing has to be sturdy and robust or the chips themselves ...
On the circuit board all the components are soldered according to the following principle:
once upon a time, the interior of a good old DECO BRAIN looked like that:
The small piezo crystal transforms the ambient pressure into a measurable voltage (Piezo Effect): with a small operation amplifier this voltage is measured and then the depth is calculated. It could be as well a capacitive resistance. All these things are temperature-dependant, so the whole set-up is temperature drift compensated ...
Now you see immediately, that the computer has to be salt- or fresh water calibrated in order to calculate
the correct depth. This is no safety concern:
the depth is just for your dive planning or logbook entry or so: the decompression calculation does not depend on the displayed depth but on the real, the measured pressure.
The power supplies are nowadays small Lithium-Ion batteries with a minor self-discharge coefficient so these things last for a couple of years. This was possible only with the modern chip design, which resulted in full custom ASIC chips with very low power consumption. The chips in the old Deco Brain used up power, oh boy, I can't tell (as well the Orca EDGE ..)
Normally, by switching off, you switch off just the display, the computer carries on calculating all the desaturation as well it checks the ambient pressure all the time. So it sees, if you are on the way to your favorite mountain-lake.
Quite a new model looks like that:
Basic Working Principle of a Dive Computer
The data from the quartzclock (= dive time) and from the piezosensor (pressure -> depth) are feed directly into the inertgasloading calculations for the various compartments (the notion "compartments" pls. cf. below). This is quite comparable to what you do with your dive table: with depth and dive-time you look up a certain repetitive/pressure group. The only difference is, that you do it once for a rectangular dive profile and the computer does this job normally every 2 seconds or so and calculates a "Multi Level Profile". This calculation is repeated for every compartment (normally a resonable dive computer has 9 or more compartments, 6 beeing the absolute minimum) and thus checks the remaining no-decompression time, or, in case of a deco-dive, the resulting deco-times on the various deco-depths. During the surface intervall the computer takes the desaturation into account and you could check this via a rolling NDL display, showing the adjusted NDLs for your repetitive dives from 9 to 42 m or so. Why are the adjusted NDLs shortened in comparison to your first dive? Well, from your first dive you have a certain amount of inertgasloading.
What are thus the practical consequences?
1. through the basic difference between a rectangular and a multilevel profile are the multilevel NDLs longer: normally you do not stay the complete bottom time on your planned table-depth, instead you dive increasingly shallower. With the desaturation through the ascent your dive computer will show you increasing NDLs by going up.
2. different dive computers differ by:
- number of compartments, and their:
- respective halftimes
- respective allowed critical or super-saturation
- the principal model, that is the basic algorithm for saturation/desaturation calculations
this implies regularly:
- the NDL for the first dive of the day differs, not by much, say + / - 10 to 25 %. For e.g. the NDLs for 40 m are around 7 to 11 min.
- for repetitive dives and, in particular for dives with deco stops. Then the difference can be as great as 500 %: now the different models reveal their hidden agendas ... Another example you will find there. Why this? Have a look at the next but one chapter.back to: Contents
What kind of computers are there?
We could differentiate between:
Which one do you want? Well, have a look in your moneybag! And as well at your certifications ...
That is: for what kind of diving do you want this unit?
Damn sure you should visit a reasonably good dive shop with a profound guidance:
AND: you should have the opportunity to rent the box in question for a couple of days,
OR: to jump into the next pool, like in our diving tower to check the box on-line!
For those wanting to know a little bit more: this is the PADI / SSI Specialty you should attend.
Which table models are used?
A couple of quick definitions of notions, which "surface" all along these discussions:
In a simple scheme it looks like that:
The mother of all dive tables is the model of Haldane. This clever guy, John Scott Haldane, lived from 1860 to 1936 in Scotland. He developped, together with 2 other good guys, Arthur E. Boycott and Guybon C. Damant, approx. 1907 for the British Royal Navy, the first dive table of the world with staged decompression! It was first published in 1908 officially with the goal, to make diving as safe as possible and decompression as short as needed by avoiding the, then-called "CAISSON" disease, vulgo Decompressionsickness, DCS.
The direct siblings are the U.S. Navy Tables from 1933 and 1957. And these in turn do have a direct child: the PADI / DSAT table, aka RDP (Recreational Dive Planner). This took place approx. 1988. The RDP No-Decompression-Limits (NDLs) are shorter and the complete nitrogen-clearance has been set to 6 h (instead of 12 h as with the USN table).
Quite a bunch of other models are based on the ideas of "Bühlmann-Hahn". Basically these are extensions of the old haldanean ideas to reduced ambient pressure (mountain-lake), the basic Schreiner equation ist still used in this "ZH-L 16" Algorithm. An algorithm is just a rule to do certain calculations. ZH-L 16 means: ZH like Zürich (Prof. Albert A. Bühlmann was Professor for internal medicine at the clinical center in Zürich, Switzerland and my late friend Dr. Max Hahn; he was physicist and CMAS Instructor), L like Linear, 16 is the amount of compartments. This model has been developped and tested very thoroughly in cooperation with Hannes Keller approx. 1960. Even with viewy self-experiments with HELIOX in depths up to 300 m! Later on the oil industries took interest in these things and supported these guys financially.
The grass roots have been the works of Robert D. Workman (1965: Nitrox and Heliox
Decompression, M Values, 9 Compartiments) and
Heinz R.Schreiner with already 15 compartments (1968) and the pioneering works of 2 german professors,
Siegfried Ruff and Karl Gerhard Mueller (1966). They racked their brains concerning bubble nucleation and calculations
and as well with saturation diving and Helium-Decompression. Also they wanted to make the decompression of mine workers more safely.
Now since approx. 1994 we find in the ALADIN computers the model ZH-L 8 ADT, ADT like adaptive for cold and air-consumption / exertion, with 8 compartments.
Since 2002 we find similar models, like the ZH-L 8 MB. The MB stands for micro bubbles. The box allows for micro bubble reduction through user-programmable deep stops, which are called there "level stops".
The least common denominator of all these models is:
the inertgas saturation / uptake (and: de-saturation / release) is an exponential process.
This saturation process during diving is in parallel within all the considered compartments, during ascent and the SI the desaturation is as well a parallel process. Thus these models are called "parallel" models.
Some other models differ as well in the desaturation calculation:
There are some other models, just to name a few:
The biggest part of all dive computers worldwide are useing, in one way or another, the ZH-L models due to the following facts:
A more or less schematic overview for the compartments, their halftimes and their respective a- and b coefficients:
definitive more infos and (copyright licenced) copies of approx. 20 dive tables you will
find in this PADI / SSI specialty
Still more infos needed? No problem:
see you in this workshop!
back to: Contents
Dangers and Limitations!
Why so? Very easy:
your dive computer does not have a clue about your hydration status, nor does it know if you have a bad hang over!
As well it does not know if you took any medication, or if you stupidly smoked before the dive!
As well the box is ignorant if you felt cold during the safety stops or if you battled against a current ...
All of the above may increase your susceptibility to DCS!
Everything pumping more inert gases into your body (cold, exertion) and everything interfering with desaturation should thus be avoided:
Cold changes not only the perfusion of your skin and muscles, as well it increases the solubility of inertgases (Henry's Law).
Skip breathing may increase your CO2 level and thus change the acid-base balance in your blood.
Lots of so called "undeserved DCS" i.e.: DCS despite correct dive table or dive computer handling and despite correct ascent procedures has been explained through insufficient fluid intake and / or dehydration through sweating, vomiting, squirts, dehumidified compressor air or alcohol abuse the day before.
So what is the practical consequence for the dive computer handling?
Otherwise you dive according to the motto (Dr. Tom Neumann, in , p. 122):
"It doesn't matter, which tables you don't use!"
"It doesn't matter, which tables you don't use!"
a brief description of a couple of historical dive computers in: