Effects of water density...

[Krakenlims]

ok, so water density increases as temperature drops, phytoplankton sink slower in colder water,,, does this mean a diver in very cold water exerts more effort to get down or gets down slightly slower than a diver in the warmest of waters, or is the difference negligible for a large human diver?

 

[trux]

Density and viscosity have very little effect on the speed of a propulsing body - I remember about a test, when scientist measured the speed of swimmers in pool with normal water, and a pool filled with jelly. At most test subjects there was no or very little difference of speed. Although the drag is bigger, the propulsion is better.

The density might have some influence in the sinking phase, but first the difference of density between fresh and salt water is rather negligible, and second in salt water you have also greater change of buoyancy, compensating again the slowing effect in the sink phase.

All in all, the difference of the relative change in buoyancy in salt or fresh water will have much bigger impact than the effect of the density (or viscosity) on the motion.

 

[wet]

Ivo, do you have a ref for that study? I thought it measured viscosity only. Jelly and water may have the same density or different?

 

[trux]

Yes, the reference should be somewhere here on DB - I posted about it a few years ago. And yes, it was about viscosity, but aside of the influence of density on buoyancy (which is not what the question was about), its effect on the speed will be through the viscosity anyway, so I think it is pretty relevant.

 

[uw_ikarus]

I am left with some doubts...moving with fins in a jelly pool should be difficuly anyway ??

 

[Kars]

The terminal velocity falling through gelly could be much, much lower.

But I think the difference in Velocity through cold versus warm water is significant or meaningful in a freediving competition setting.

Warm water does help equalising, relaxing, having less suit etc.

 

[trux]

Regardless of what you think about jelly (look up the study if you want to know the details), the answer to the initial question is as follows:

ok, so water density increases as temperature drops, phytoplankton sink slower in colder water,,, does this mean a diver in very cold water exerts more effort to get down or gets down slightly slower than a diver in the warmest of waters, or is the difference negligible for a large human diver?

I do not know whether phytoplankton has some capacity of buoyancy control, but I'd tell it sinks slower in denser water because, unlike a freediver, it cannot add more weight on their weight belts, not because the water with higher density slows down the motion. And as explained above, the viscosity would slow down passive motion (or terminal velocity as Kars tells), but not really active propulsion (though the energy demand may be higher). However, all this is completely negligible in comparison to the other effects of cold water (also mentioned by Kars). Additionally, even assuming the same wetsuit, in water with higher density, the change of buoyancy will be greater (due to the compression of the wetsuit and lungs), hence after reaching the neutral buoyancy, the sinking may be slightly faster (but the ascent will require more effort).

 

[Kars]

Hi Ivo, I expect athletes to become tired quicker in denser water.
This because of the more effort that is needed to displace the water. Did the experiment compared energy expenditure to that of the same training covered in normal water?

With higher density material you'll reach "hull speed" and terminal velocity quicker, going faster than that speed requires a logarithmic amounts of energy.

My expectation is that I think swimmers were able to swim similar times, but needed more energy to accomplish it.

 

[trux]

Yes, that's what I wrote - that the energy demand may be higher. It is not so important at a sprinter (in the jelly test, it was demonstrated that the loss due to the high drag is compensated by a much more efficient propulsion), but certainly at a slow pace freediver who relies on long glides. However, the impact of the water density due to the temperature change will be absolutely negligible in that sense, simply not detectable at all.

 

[Bill]

'ok, so water density increases as temperature drops'
First things first. That is not a true statement. It depends on what the starting temperature is, because it reverses at 4-5C, if my memory is any good. At some point below zero the density decreases dramatically. Picky, picky you think but at the temps that divers face, the curve is so flat that it is insignificant.
By the way, the unique and magical properties of water and their affect on living things, is the most powerful argument for the divine creation theory. IMHO.

 

[Mullins]

After all the terrible screw-ups he/she/it made, it's good to know he/she/it got something right!

Agree about the density difference being negligible in the ranges we're talking about. Check out the density/temperature table: [ame=http://en.wikipedia.org/wiki/Properties_of_water]Properties of water - Wikipedia, the free encyclopedia[/ame]

 

[Krakenlims]

Bill: my statement was not specific enough for you to say it is not true..
"if my memory is any good" - haha, good old wikipedia..
trux: some phytoplankton do have their own buoyancy control, but control is a bit of a strong word, if they had any real control they wouldn't be plankton

 

[Simos]

Although the simple answer is that it's irrelevant for all the reasons mentioned (not huge density change, ability to add/remove weights) air density presumably changes more dramatically with temperature so does that mean that the buoyancy of my 5mm suit will be noticeably less in cold water? ie will I need less weight if I use the same suit?
Ps I am talking in practical terms as the theoretical answer is probably yes
Posted via Mobile Device

 

[Krakenlims]

Although the simple answer is that it's irrelevant for all the reasons mentioned (not huge density change, ability to add/remove weights) air density presumably changes more dramatically with temperature so does that mean that the buoyancy of my 5mm suit will be noticeably less in cold water? ie will I need less weight if I use the same suit?
Ps I am talking in practical terms as the theoretical answer is probably yes
Posted via Mobile Device

- practically no, coz no huge density change, it's irrelevant for all the reasons mentioned above

 

[Bill]

I stand corrected then.
Never thought about the air in the suit though. Say about 8 kilo and a maximum of 20 degrees change on the surface, that could be 6-7% if air density is directly proportional to absolute temperature. Only be meaningful in the pool.
Enjoyed the comment Dave but I'll wait to see if there is a 'judgement day' before I pull Mother Nature's chain. I'm just too conservative for this century.

 

[sross]

'ok, so water density increases as temperature drops'
First things first. That is not a true statement. It depends on what the starting temperature is, because it reverses at 4-5C, if my memory is any good. At some point below zero the density decreases dramatically.

Hi Bill,

4.1°C is the temperature at which pure water reaches its density maximum.

At 0°C water freezes, and yes, ice is much less dense than water, but its viscosity is pretty high. In fact, diver velocity falls to zero when he or she hits water in its frozen state :head

The difference in density between water at 0°C and ice at 0°C is due to the entropy change, and it is much greater than the density change due to temperature differences in the liquid state, which is why ice floats on the surface, no matter what the temperature of the water underneath.

The amount of density change caused by temperature is pretty negligible for freediving considerations. For example fresh water at 25°C is approximately 0.3% less dense than fresh water at 4°C. This sort of a difference will be below noise levels in terms of water resistance experienced by a freediver.

Note that the above is true only for fresh water. Addition of salt changes everything. Sea water at a normal salinity of, say, 3.5%, will freeze at approximately -1.9°C.

So in the ocean it is actually possible to dive at a temperature below 0°C. I've done this myself with a bunch of undergrad students in Nova Scotia, Canada. They were doing their checkout dive for a certification program in a cove where the water temperature was measured at -1.2°C. They were wearing a motley collection of wetsuits, some borrowed, and you've never seen such a stiff, red-faced collection of bodies as that crew when they came out of the water. We had kept a bunch of vehicles running with the heaters on so we could stuff 'em in and warm 'em up after the dive.

Also, in normal sea water there is no density maximum, unlike fresh water, it just keeps getting denser as you go down to its freezing point. As with fresh water, though, the changes in density with temperature are so small that they have no practical effect on the swimming resistance of a diver.

The difference between diving in fresh and salt water due to salinity could be significant, though. For example, at 4°C the difference in density between fresh water and 3.5% salinity sea water is 2.8 %. This means that in sea water the work required to displace water as the diver's body moves through it will be 2.8% greater, and during the freefall phase of a deep dive, this could make a significant difference to the depth achieved.

As Trux has pointed out, though, the effect of increased water density on active swimming is complicated, and it's hard to say whether a 2.8% difference might just get lost in the noise of factors such as wetsuit fit, weighbelt profile, the diver's swimming style, etc..

Anyhow, divers doing dives with a long sink phase might find themselves doing a little bit better in fresh water than sea water.

 

[Bill]

Thanks for taking the time. I didn't realize that the four degrees only applied to fresh water. Long ago I read something about submarines 'floating' on a layer of water. Interesting stuff.

 

[trux]

Long ago I read something about submarines 'floating' on a layer of water.

Yes, that's true, but that's so called halocline - a distinct layer where salinity increases sharply. See more details at Wiki :

Halocline - Wikipedia, the free encyclopedia

 

[Simos]

...and there I was, thinking that freediving was just about holding one's breath and swimming under water )))
Posted via Mobile Device

 

[efattah]

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.

This is shown by the drastically faster freefall you get in fresh water, even if ballast is adjusted to the same 'negativity'.

My experience has shown that a constant weight dive in fresh water is much easier than the same dive in the ocean, if ballast is adjusted to be equalized. For no-fins diving, however, I have reason to believe that the thicker viscosity of salt water may be preferable.

Of interest is the fact that all dynamic apnea distances attempted in the ocean have been far shorter than distances in the pool -- even though no turns are required. Of course there are other confounding factors there, but I think the viscosity may be one of them. 'Dynamic in ocean' used to be a semi official event with world records as well.