Effects of water density...

[trux]

The viscosity of fresh water is around 33% less than the viscosity of ocean water, although this ratio changes with temperature. But certainly there is a HUGE difference in viscosity.

Yes, there is a difference, but the 33% sound a bit exagerated to me. Perhaps in a highly concentrated brine, but from what I could find, the difference between fresh water and salt water is just around 5%.

For example at 20°C, the viscosity of fresh water is 1.00374 [10^6 m²/s], while at salt water of 3.5% salinity (average sea water salinity), it is 1.05372 - ~5% difference (source: http://ittc.sname.org/2002_recomm_proc/7.5-02-01-03.pdf)

 

[efattah]

Another good example of the difference in properties is shown by a simple experiment. Try to swallow large awkward vitamin pills using a glass of cold, fresh water -- easy. Now try to swallow the same pills using a glass of warm or hot water -- incredibly hard. The warm/hot water just doesn't seem 'thick' enough to swallow the pills with.

 

[trux]

Interesting, I didn't notice that, but it is real that the difference of viscosity between cold and hot is surprisingly higher than the difference between fresh water and saltwater. For example it makes 30% between 10°C and 20°C. So finally, the water temperature would have bigger impact than fresh and salt water.

 

[cdavis]

The jelly experiment is a scream, recommended reading.

Can't speak to the difference due to water temp, I can mostly stay away from really cold water, but salt/fresh viscosity/density issues seem significant in the free fall stage. Even if you weight to be neutral at the same depth, seems to me the lower density of fresh water,would make for less buoyancy change with depth, slower freefall and easier return because you would be less negative. Viscosity, if significant, would make for faster free fall and not much difference, if any, in return. The latter is what I experience. Fresh water seems much "slicker" and freefall feels faster, even though I am often not going straight down in fresh water. No doubt both are operating, but in the depths I inhabit(0-30m) viscosity seems to me to be more important.

Connor

 

[uw_ikarus]

Density is certainly an important factor. What counts for bouyancy is the density difference between the human body and the water. The human body has a specific weight similar to water, so even small changes in the density of water (lets say 2-3% between fresh and salty) count a lot.

 

[trux]

The feeling may be misleading. Much more interesting would be seeing and analyzing dive profiles in different conditions. But if you look at the viscosity table, you can see that the difference between fresh and salt water is quite small (5%) and practically constant over wide range of temperatures. In contrary, already difference of 1-2°C makes for the same difference in viscosity. So from this point of view, if you feel a difference in the glide, it is much more likely that it is due to the temperature difference than due to the salinity.

You would have identical viscosity at sea water of 22.15°C and at freshwater of 20.00°C. Now, the surface temperature is little important, it is more the temperature/depth profile which may significantly change, but here I'd tell that more typical is that the temperature changes faster in lakes. But that's quite individual and often it may be the opposite case.

So yes, if you dive in a warm freshwater lake with a deep thermocline, the viscosity will be lesser (and the sink phase faster) than in cold ocean. But in typical case of a lake with quickly dropping water temperature with depth versus a warm ocean, the viscosity will be actually much lesser (and hence the sink phase faster) in the ocean.

But again, there are so many other factors, that it is hard making any prediction without carefully measuring and evaluating all the conditions: not only salt vs. fresh water density and viscosity, the temperature/depth profiles, and different buoyancy changes, but also vertical and horizontal currents, different turbulence, variable salinity with depth, variable impurities in fresh water having influence on the viscosity too, and perhaps more.

Otherwise, I agree with Eric, that the high viscosity would be rather an advantage on the ascent (an likely in the first meters after immersion too).

 

[trux]

Density is certainly an important factor. What counts for bouyancy is the density difference between the human body and the water. The human body has a specific weight similar to water, so even small changes in the density of water (lets say 2-3% between fresh and salty) count a lot.

Ikarus, we do not speak about density here, but about viscosity. Although there is some relation, these are two entirely different aspects with very different effects. The effect of density on buoyancy is probably quite clear to every diver and freediver, but it is real that the effect of salinity and temperature on the viscosity and due to it on the speed or energy consumption of the freediver is little discussed or studied,

 

[uw_ikarus]

Ah, maybe I was mislead by the title of thread...sorry

 

[trux]

Ah, yes, sorry, you are right - I forgot about that. It is real it turned towards the viscosity in the discussion, since the effect of density on buoyancy is rather evident and does not really need much discussion.