Fresh water versus salt water
@efattah: I think Your statement in a
DB-Article is very accurate:
"Unfortunately the whole issue about setting a record (at least for constant ballast) seems more about knowing how to get good conditions, rather than knowing how to dive well."
After reading about Your record attempt in May 2001 in the treacherous waters near Miami (currents!) it seems to be wiser to prefer lakes for record attempts. The "hypothermic diving system" seems to work for me also, according to my experience.
Still I would like to mention also the "other side" of lakes:
1. Are lakes calmer?
The surface of a lake may also become quite choppy, making breath-ups really difficult (at least there are no bottom currents to be expected!).
2. Equalization is harder in cold water
By and large fresh water is colder than sea water at depth (Vancouver excluded
).
Conclusion: generally, equalization will be harder in fresh water. The frenzel fattah mouthfill technique has helped me a lot (thanks!) and enabled me to increase my personal best substantially inspite of a very limited training schedule. However any equalization technique, including the mouthfill technique is getting harder in water temperatures around 4°C.
3. Equalization depends on the relative pressure increase between surface and target depth.
Let's suppose we are freediving in a lake in Tibet at an altitude of 5500mt: At the surface we start with 50% of atmospheric pressure. At a depth of -50mt this pressure will have increased by approximately 5 extra atmospheres (we don't split hairs now by differentiating between bar, atmsophere, kg/cm², ... they may be used interchangeably). The ambient pressure will have increased from 0.5 atm to 5.5 atm, relatively spoken: by 11 times.
Conclusion: equalization will be just as difficult as diving from sealevel to -100mt (where the absolute pressure will have also increased by 11 times). Less density of fresh water will (almost) never compensate the disadvantage of starting at a higher altitude.
Herbert's last world-record dive on Sep 04 to -95mt CW at Millstatt Lake analyzed:
First I denote:
P(A) = Pressure at the surface of a lake at altitude
P(DA) = (Total) pressure at a depth of a lake at altitude
P(SL) = Pressure at sea level (assumed average of 1013mb, practically: 1)
P(DSL) = Total pressure at an equivalent depth at counted from sea level. (what we want to resolve, [X])
The altitude of Millstatt Lake = 588 Meters.
To obtain the air pressure P at the surface of Millstatt Lake I use the barometric formula:
P(A) = P(SL) × exp (-0,122×Altitude in km's)
P(A) = 1013 mbar × exp(-0,122×0,588) = 931 mbar (rounded to mb)
95 mt of fresh water cause a pressure of 9,5×980 mb = 9310 mb.
Hence the total absolute pressure at -95mt (air + water) will be:
P(DA) = 931 + 9310 = 10241 mbar.
Now we use:
P(A) / P(DA) = P(SL) / P(DSL)
931/10241 = 1/x
10241 = 931 X
X = 11
Finally we calculate the Virtual depth for an equivalent dive at sea level:
VD= (X-1)×10 = 10 × 10 = 100 Metres.
Conclusion: Regarding equalization Herbert is able to dive to -100mt CW in the ocean.
Actually his attempt to -100mt in CW on the next day was partially successful: He was able to manage equalization but had LMC at the surface, because this was done only shortly after his successful FI-World Record to -100mt under stressful conditions. Applying again the same formulas Herbert should be able to equalize for a CW dive to -105mt in the ocean (in an inverted position).
You are welcomed to look at my complete treatise about
Freediving at altitude.
Being someone that represents divers going to "intermediate depths" [=most of us
], I found out:
It's easier to make personal bests in clear and warm sea water (as opposed to cold and dark fresh water) and I am not surprised to struggle with my last equalization much sooner at some altitude in 4°C of fresh water as I would in a warm, crystal clear and light flooded ocean, starting with maximum air pressure (~1013 mb).
I wish You all safe, joyful and successful dives,
Gerald