Grasping the basics- dive computers PART 1
What is underneath?
Schematics of a Dive Computer
Sensors/Clock → Storage Unit ↔ [Control Unit / Arithmetic and Logical Unit]CPU → Output [Display]
A dive computer is a device that operated upon collected data, it is an electronic board which does arithmetic and logical operations accordingly with the programmed decompression algorithm and provides with the actual (real time) ascent profile information.
This device supposed to guide the diver from depth back to the surface with some degree of confidence that the user will get out of the water without symptoms of DCS and if they are present, should be easily managed.1
The computer contains sensors that reads depth/altitude (barometric pressure), temperature and a clock. With the collected data performs real time-depth calculations base on the programmed algorithm, stored detailed dive statistics and profiles.
Some computers include a high pressure transducer assembly that records and track gas supply and it is able to predict the remmaning dive time in conjunction with the “No Decompression Limits (NDL/ NoD/ NDC). Also this computers takes biological data (i.e., breathing frequency, breathing volume and heart rate) to show the level of exertion of the diver which is used as parameters in the calculations of the ascent profile.
Remember that algorithm used by the computer is a model predictor
The computer interface is how the diver introduce data and reads the display. This feature represent how the user engaged with the computer itself.
Aslo it is important that the computer can be interface with a PC2 for dive planning function, print backups tables, review past profiles, and make parametric computer cloning.
List of regular futures and displays
- Easy-to-read display (sometimes in color) that provides the following information:
- No stop limits,
- Depth,
- Time,
- No stop time remaining,
- Ascent rate,
- Decompression status,
- Previous dive information/ Log Book,
- Low battery warning,
- EAN, HELIOX, TRIMIX compatible,
- Automatic adjustment for altitude diving,
- Replaceable or rechargeable batteries,
- CCR (Closed-Circuit Rebreather) mode,
- Interface with your laptop/regular computer so you can download your dive data,
- Electronic compass,
- built-in thermometer.
The Major benefits of a dive computer is the flexibility that allows the diver to do multilevel profiles without the limitations of the maximum depth for the entire time as it is prescribe by the tables. Most tables use only one compartment to calculate repetitive dive allowances; dive computers have accurate depth reading (± 0.5 msw) and provide the diver with information continuously through the dive.
Within the scope of a multilevel recreational profile within the Haldane’s model, the 5 min. compartment control the bottom time at for example 30 meters maximum depth profile; when you ascend above 20 meters, this compartment no longer restrict the NoD since it can no longer reach the allowed loading limit. Other lower compartments control each depth as you ascend and because these slower compartments haven’t reached their loading limits, additional dive time is permitted without enter decompression status.
How do I choose?
There’s still some discussion about the validation procedure of a dive computer, up until now no other computer has been validated except for Cochran Dive Computer, which is what the NAVY use. Outside the military environment, the EMC-20H has been used and evaluated by Hamilton on the Archeological research of the Monitor, which also was used to elaborated and validated his DCAP program, especially for mixes like TMX 18/50.
Regarding the validation, Dr. Doolette defined it as a process that measures whether your product meets its “requirements”, and “verification” evaluates whether it works or functions. In other words,
Validation confirms that a decompression algorithm performs to the level you
want in terms of risk.
Verification determines that the dive computer does the proper calculations to perform that validated algorithm.
Another concept to have in consideration is the function of the computer, Dr. Hamilton recommended in its own word “never use the word ‘safe’ in relation to decompression, it just does not apply and is misunderstood by people. Use ‘acceptable risk’, which gives a different perspective although it really is the same thing.“
Although considered that the
primary function of the computer is to get the diver to the surface without any residual or long-term effects of DCS.
One more point before you read the next section of this lesson is what Dr. W. Gerth alleged about the process of acceptable risks on decompression models: “In the U.S. Navy in the ‘noise’ of our risk of DCS estimates we include factors such as what the diver ate for breakfast, body temperature, etc. Then we assert that the model that we use prescribes schedules that are within an acceptable risk of DCS and we say that anybody can dive that profile – that will be your mean risk of DCS with such and such an error. We are not going to live long enough to get enough data to parameterize a model to incorporate all of the different factors that have been posited as controlling DCS risk.”
So before you choose a computer, you should ask yourself:
1-With which decompression model am I comfortable accepting the risks of DCS and,
2-With which computer works as it was intended for or function as it was designed for.
In the next section (title) you will read about the procedure took by the NAVY on validating and verifying their dive computer (NAVY), having a glimpse on it will give you an understanding on the effort implied on developing such endeavor. This is the main reason why commercially over the counter dive computer doesn’t publish their algorithm implementation nor validation or verification of their computers. Just change the circuit model board of a dive computer will require a new verification. I invited you to read along to understand such
complexity.
Conclusion
One interesting
conclusion on the workshop were made by S. Lesley Blogg from SLB Consulting and Andreas Møllerløkken the Norwegian University of Science and Technology, in which they described the need of test each computer separately in relation on how many bubbles can be detected using VGE measurements. Such method could easily be employ without taking to much effort in man-power (man-tested) nor money investment.
Once the target population has identified their need for a dive computer, then ideally their most commonly used dive profiles could be used to test different models against one another to find the optimal DC for the populations’ use. It is necessary to test individual DC models, because each is driven by a specific, but usually unidentifiable, algorithm. Although this might not be ideal, it is a cost-effective approach and with objective endpoints, an eminently testable approach to take. This method obviously could not be employed if using DCS as an endpoint. Using VGE measurement, the algorithms in each DC for each specific profile can be rated for decompression stress, then paired comparisons can be made and the optimal DC (producing the lowest amount of VGE across the test population over selected profiles)
chosen for use in that specific population.
completed article
here
