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Old 12-27-2011, 11:58 AM   #1
Mink Islander
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Default Black Cat Water Temp question

I thought that water as it cools becomes more dense until it reaches 39.4 degrees and then as it cools further, it becomes less dense -- explains why ice can form on top of water that is above freezing in temperature (usually 34/35 degrees). So why is then that the Black cat water gauge at 10 feet says 36.2 degrees while at 2 feet it is 38.8 degrees. What am I missing? Maybe it's just a calibration issue....
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Old 12-29-2011, 11:04 PM   #2
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These probes have taught me a lot of things about the lake water since I installed them. In a cup of ice water they read 32 F.

The key to the water behavior you describe above is this: The water must lie still. Stillness is when the thermal layers take their "rightful" places.

Both air and water move and behave as fluids, with very little exception. When the thermal layers get disturbed by movement, you get wave action, stirring and mixing. When the air is dry and calm, the water surface chills by evaporation into the dry air. The air, when dry and calm at night, is losing all its heat to space. The lake heat goes with it, until enough evaporation has taken place that the air above the lake is 'full' -- dense fog of the "steam" type, on the water.

Warmth is buoyant and solids sink in a sea (or atmosphere) of cold, but turbulence (wind, waves) messes that up. You'll see sand (or rocks!) in sea waves. In calm water those objects just sink by gravity, and the thermal layers assume their "textbook" order -- also by gravity.

At this location we get a lot of wind. That leads to lots of wave action in the air and water. The water often reaches "freezeable" temp long before conditions become right for ice to form.

When the lake is ready to freeze, the temp at both probes is usually 35 degrees or so. Then it needs calm, and a force to cool down the surface. That force can either be a clear, dry night (for black ice = strong) or snowfall (for flake ice = weak.)

Typical temps during iced-over lake are 34 F at 10 feet, and 33 F at 2 feet. The coldest weather brings the 2-foot probe to 32 F. Don't forget, it's near south-facing shore, where the ice is never amazingly thick, thanks to sun-absorbing rocks & land near.

2-foot temp readings vary everywhere on the lake. In places where the ice gets more than 2 feet thick, those readings would be 32 or less all winter, until the ice thinned to less than 2 ft. The 10-foot probe is the indicator of what the 'underlying' water temp of the open lake is... and that doesn't vary as much by location.
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Old 01-11-2012, 08:31 AM   #3
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Default It a calibration problem

Just as it has for over a month, the 10 ft probe continues to show a temp several degrees below the 2 ft probe. Consistently. Every day. Regardless of weather conditions. Right around 3 degrees difference. And now it's suggesting that we have ice (31.9 degrees) at 10 feet on top of water nearly 3 degrees warmer. And of course the water at the surface is below 32 degrees -- it's ice now. Both probes should be reading somewhere between 33 and 35 degrees at this point with the colder temps more likely at the 2 foot probe since 33 degree water is less dense than 35 degree water. At this time of the year the lake has its most consistent temperature from top to bottem -- around 35 degrees throughout -- except very near the surface where the air temps can further cool it to make ice or warm it for that matter. While wind and other currents can swirl things around (and keep that warmer 35 degree water at the surface so ice won't form), I would not expect to see this type of temperature inversion at 10 feet and these temps.

Sorry to be such a nudge, but I think the 10 ft probe is reading about 3 degrees colder than it should. As you can tell, I really enjoy your site and check it often along with a number of other lake cams to get at least a partial view of conditions on the lake.

Thanks!
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Old 01-20-2012, 06:39 PM   #4
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Uhhh, yup. (Sigh)

I've been watching it ever since R2B brought it to my attention in a PM a few weeks ago. I've got a few months to think about it, potential causes, etc. That's because I won't be able to swim down there and visually inspect it until that season is back. Will be interested to see if it straightens out during the winter, or in the start of swim season. Last time I doubted its reading enough to check it, the season was summer and I swam down there. My nerve endings confirmed the reading and the temperature difference that the two probes were showing at that time.

If anyone has any guesses about the cause (other than calibration, the obvious) feel free to speak!
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Old 01-20-2012, 06:54 PM   #5
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CLA,

I believe the difference is related to "unintended couples" in the leads. Before I retired, almost seven years ago now, I was involved in engineering management for a large company. We used thermocouples in many applications. When we had questions about accuracy and unusual readings, it almost always came down to the leads.

Here is some technical information that might help:

http://www.picotech.com/applications/thermocouple.html

Thanks for all you do!

R2B
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Old 01-23-2012, 12:17 AM   #6
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Quote:
Originally Posted by Resident 2B View Post
CLA,

I believe the difference is related to "unintended couples" in the leads. Before I retired, almost seven years ago now, I was involved in engineering management for a large company. We used thermocouples in many applications. When we had questions about accuracy and unusual readings, it almost always came down to the leads.

Here is some technical information that might help:

http://www.picotech.com/applications/thermocouple.html

Thanks for all you do!

R2B

You're welcome. And thank you for the info. I just realized something: The other probe, at 2 feet, began reading 5 degrees too high, last year. I put the 10-foot probe in about a year after the 2-foot one. So, when each probe reached a certain age (the same for both) it began misreading.

I tested the 2-foot probe and found it to be 5 degrees too high. The computer corrects it with a script that subtracts 5, and I can change that number if I need to.

If I assume the 10-foot probe is reading low by the same amount, that brings the actual temperature to 35 degrees instead of its current reading of 30.

35 is what it's almost always shown during lake ice season. The lowest I ever remember it going was 34.

Your thoughts?
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