ABOUT DIVE COMPUTERS
Our Advice: Buy a Shearwater Perdix or equivalent to allow yourself room to grow into the full scope of diving possibilities.
Dive computers help the diver track depth and bottom time, calculate necessary decompression stops, and minimise the risk of decompression sickness (DCS).
There are three main decompression models that are used by modern dive computers; Buhlmann ZHL-16C, VPM, and RGBM. Each of these models will not match the other in terms of required stop times and depths for an identical dive profile.
Better dive computers using the Buhlmann (ZHL-16C) tissue model make further adjustment for gradient factors, allowing the diver to adjust for their own physiology and conservatism preferences.
Console or wrist mount?
Wrist mounted vs console dive computer: console dive computers are vulnerable to damage, difficult to check, add unnecessary bulk to the SPG, and require you to dissemble the console for battery changes. Wrist mounted dive computers are recommended for being far easier to check frequently and to watch for while maintaining stops.
Replaceable batteries
If your dive computer’s battery runs out at the dive site or on an overseas trip, it is essential that you be able to easily change the battery yourself from common battery types available at any corner store. For this purpose AA or AAA batteries are best. Poorly designed dive computers require a service centre to change the battery, or require special (difficult to find) battery types.
Multi-gas capable
Nitrox is increasingly the standard gas mix from dive centres around the world. For recreational purposes, at a minimum your dive computer must be programmable with air or variable nitrox mixes.
We recommend that all divers, including new recreational divers, should consider a technical capable dive computer which will give them room to grow in the sport, rather than buying a cheap recreational computer that will soon need to be replaced.
Shearwater dive computers offer a simplified recreational mode for new divers, and will not need to be replaced even as the diver moves into advanced trimix and CCR.
A technical dive computer must be capable of programing both open circuit and CCR for multiple gas mixes of air, nitrox, and trimix that can be switched during the dive.
Upgradeability and support
Diving decompression and decompression modelling are not an exact science and are constantly improving. Better dive computers will offer ongoing support and updates so that your dive computer will always be running the latest software.
ABOUT GRADIENT FACTORS
Buhlmann’s tissue model has 16 hypothetical tissue compartments (with different gas absorption rates and saturation values) that are tracked during a dive in order to determine each tissue compartment’s inert gas pressure.
As a diver ascends, the tissue compartments start to release pressure (off- gas). Buhlmann gave each tissue compartment a maximum pressure value which, if exceeded, will result in getting bent (M-value).
Gradient Factor = (Tissue Compartment Pressure - Ambient Pressure) / (M Value - Ambient Pressure)
The gradient factor formula tells us that (a) at or above GF=1, you are at Buhlmann’s M-value and should be bent, and (b) when tissue compartment pressure reaches ambient pressure, then the GF=0.
Baker’s ascent strategy is to ascend to your first stop which gives a Low GF such as LowGF=0.30 and to exit the water at the surface with an acceptable High GF such as HighGF=0.80. At each stop depth the acceptable GF is higher but may not exceed the MaxGF determined by the line between HighGF and LowGF:
MaxGF = HighGF - (HighGF - LowGF)/LowGF Depth * Current Depth
Therefore at each stop diver must off-gas enough tissue compartment pressure so that Max GF is not exceeded at the next stop.
E.g. a gradient factor of 20/80 means the first stop is 20% of the M-value, and the diver exits the water at 80% of the M-value (or 80% of the way to being bent).
