Cold water diving

[KOMPRESSOR]

One interesting chestnut (I think) is how education is pre-dominantly in Europe all about theory and very-little-to-no practical application. *cough* *cough*

Lots of posers livin' in the EU circle....

If you're talking about me I'm absolutely more into practical diving. Oh, and I live in Norway. It's not within the EU. Didn't you know that?

 

[String]

4c water is at its densest. Any colder than that its less dense so will rise above 9ice floats remember), any higher than 4c the same.
So a large enough body of cold water you're likely to find 4c water at the bottom with the other temperatures sitting on top (obvious example are seas and oceans with a typical 4c bottom temperature everywhere). Exceptions for things such as currents, feeding streams and so on but for a static body of water the above is true.

Very common to have icey water/cold surface temperatures start to rise up to eventual 4 degrees when descending down the water column for that reason.

Also worth noting the great lakes aren't a body of pure fresh water. They do have a salt content and although not as much as the ocean can be high enough to make a slight difference to the freezing point (however the 4c rule still stands).

Oh and Norway isn't in the EU - very lucky for Norway! Also seems a lot of people are guilty of "just doing" without thinking first. In some cases, its a national trait.

 

[KOMPRESSOR]

4c water is at its densest. Any colder than that its less dense so will rise above 9ice floats remember), any higher than 4c the same.
So a large enough body of cold water you're likely to find 4c water at the bottom with the other temperatures sitting on top (obvious example are seas and oceans with a typical 4c bottom temperature everywhere). Exceptions for things such as currents, feeding streams and so on but for a static body of water the above is true.

Very common to have icey water/cold surface temperatures start to rise up to eventual 4 degrees when descending down the water column for that reason.

Also worth noting the great lakes aren't a body of pure fresh water. They do have a salt content and although not as much as the ocean can be high enough to make a slight difference to the freezing point (however the 4c rule still stands).

Oh and Norway isn't in the EU - very lucky for Norway! Also seems a lot of people are guilty of "just doing" without thinking first. In some cases, its a national trait.

They won't listen to us. We're foreigners you know

Even though there is a higher level of salt and minerals in the Great Lakes I doubt it is enough to affect the point og freezing of the water. It takes quite a lot. Some places in the ocean it can keep kind og fluid, or slushy, down to as low as minus 2 to minus 3 degrees Celsius. I have measured -1.5 in the ocean, not with a diving computer though.

 

[off the grid]

Some additional reading on the net, and Kompressor's and String's points are well taken. Even with slight changes in freezing points due to chemistry and pressure differences, the temps at depth should not be much different from the 39 degrees stated. I'm surprised the computers could be that far off. "not spot on" is an understatement. But I will have to do some measurements of my own just to prove the point. Thanks for all the imput and photos. Just one more question for the cross Atlantic team. What is the lowest temps your computers have recorded?

 

[KOMPRESSOR]

Some additional reading on the net, and Kompressor's and String's points are well taken. Even with slight changes in freezing points due to chemistry and pressure differences, the temps at depth should not be much different from the 39 degrees stated. I'm surprised the computers could be that far off. "not spot on" is an understatement. But I will have to do some measurements of my own just to prove the point. Thanks for all the imput and photos. Just one more question for the cross Atlantic team. What is the lowest temps your computers have recorded?

Well, when diving in the winter, with low air temps, and VERY slow updating on some computers, one can read temps way off real temps even a bit after going under water. During ice diving it is more common than not to read 0C/32F in the surface, but it doesn't mean it is correct. As soon as divers stir up the water with bubbles, warmer water from beneath comes up with the bubbles, and it will even melt the ice! This can be a bit dangerous if one uses the same hole in the ice over a period.

I dove in the sea while ice was forming. -As salt water freeze up at lower temps than fresh, I'm willing to say that my reading of -1C could have been about correct.

 

[DOkie]

Then I have a serious question for you. What are temps at depth in the summer?

 

[KOMPRESSOR]

Then I have a serious question for you. What are temps at depth in the summer?

Since water is at its heaviest at 4C/39F, you will likely find this temperature at the bottom of a deep lake even in the summer. That is unless you are in a very warm all year round climate or the lake isn't all that deep.

The lake near my home is 1400ft deep. It doesn't freeze up every winter, and summer temps may reach 20C/68F in the surface. But at the deep end it is a CONSTANT 4C/39F.

 

[DeepSeaExplorer]

There was a documentary on TV last month about global warming where they mentioned super cooled water underneath glaciers that remained liquid at 29 degrees due to the pressure. Not at all sure this could happen in a lake at SCUBA diving depths though...

 

[dave4868]

Some additional reading on the net, and Kompressor's and String's points are well taken. Even with slight changes in freezing points due to chemistry and pressure differences, the temps at depth should not be much different from the 39 degrees stated. I'm surprised the computers could be that far off. "not spot on" is an understatement. But I will have to do some measurements of my own just to prove the point. Thanks for all the imput and photos. Just one more question for the cross Atlantic team. What is the lowest temps your computers have recorded?

Yes, it appears they've got it about right.

I was hoping the net would provide some specifics about the freezing point of typical fresh water in lakes, but I couldn't find anything.

I agree with the point that naturally occurring freshwater in many lakes will have a freezing point practically the same as pure water.

I'll just have to suggest that freshwater temp readings below 32F should be questioned. If it were me, the first thing I'd do is give more time for my temp gauges to equilibrate, then certainly check my gauges for accuracy later.

Freezing point aside, the inaccuracy of scuba temp gauges is a well-known issue, as we've all seen fellow divers report differences of 1 to 3F from the same dive, same profile.

As was mentioned, it will take quite a while for gauges to equilibrate to the water temp. Pre-chilling by surface conditions might cause an inaccurate low reading for quite some time.

A few years ago, I checked the accuracy of the analog thermometers in my Sherwood spg's. These are the typical bi-metallic dial-type temp gauges. Through years of buying and selling used scuba gear, I've retained the spg's with temp gauges that seemed most accurate and sold the others, which were off by up to 3F.

Just for fun, I rechecked my temp gauges last night in an ice bath. Both finally read 32F, but it took quite a while for them to equilibrate with the surrounding icewater. And that was in a cooler filled mostly with ice and only enough water to immerse the gauges some hose.

For a long time they only went down to 34F. When I refreshed the ice and reduced the amount of water, they went down to 32F in a couple of minutes. (Thanks, DA Aquamaster!)

In the past, I've also checked them at 40F and 60F, using several other reliable outdoor thermometers for comparison. My spg thermometers appeared to be about a half degree low at 40F, but right on the money at 60F. Of course, they could be off a bit at other temps....they're only bi-metallic coils after all.

Anyway, I'm pretty confident in their readings below 40F, so I went back through my dive log to see what I've recorded that might demonstrate dense 4C/39F water settling to the bottom. Looked for dives in still conditions, like an ice-covered lake where mixing and currents would be minimal. Also looked for dives into the "deep holes".

My ice dives were of no help, since they were in shallow water on the edge of sloped bottoms. Water temps were in the mid-30's.

However, I did find quite a few early season freshwater dives where I got into the deeper parts of these bowl-shaped 100-foot-deep lakes and 39F is indeed what I often recorded. Does that mean anything in itself? Probably not, since I didn't try to measure colder water above it.

However, one of those early season dives was just before complete ice-out. Much of the mile-long, 110' deep lake was still covered with 1/2" thick spongy ice. Wind and wave action hadn't gotten much of a grip on the small area of open water, so the water was pretty static.

That day, I again measured 39F at 85', but most interesting was the appearance of a very distinct cloudy layer at that depth which dramatically reduced the visibility from 10' to less than 2'. It was so distinct as to appear like a layer of thick smoke or fog.

Now it makes sense that if it was denser water it would suspend particles which had gradually sunk through the less dense water above.

Next time, I'll have to try to get good temp readings above that layer to see if there is some colder water above it!

Having an interesting goal will spice up these otherwise commonplace dives. Thanks for that!

Dave C

 

[KOMPRESSOR]

Well, I'm just out of the water. 39F from 15ft and down. 37F from 15ft and up to and in the surface. Those darn neoprene gloves sure are compressed at 100ft. After 35 minutes I felt like I had clubs instead of hands! 30F air temp wasn't to bad, but 2" of clear invisible ice on the docks sure was... Ouch!