Taking Water Temperature Underwater

[Paraaronoid]

Curious to know, can a low-end mercury thermometer be used for taking accurate water temperature readings at depths up to about 30-35 ft? Does the pressure down at that depth adversely impact the mercury reading in anyway?

I'm looking to take temperature readings this year when I am out in various freshwater lakes at various latitudes. I want to be able to log data from year to year regarding lake temperatures at various depths. I am interested in doing this because I want to know how close/far I am to lake turn-over (4 degrees C in spring, 10 degrees in fall), as my spearing success in these lakes hinges greatly on visibility.

I'm not wanting to spend the money to purchase a dive watch/computer or anything like that. I'm willing to just do basic readings with a hand-line if that is possible. Any opinions on whether I can achieve this using 'grade-school' data gathering methods?

 

[trux]

The capillary is closed, so the pressure will not influence the reading, but the thickness of the capillary wall and especially of the reservoir (which is usually thinner) is not designed for such pressures, so the thermometer can easily implode. Besides it, you'll have problems reading it in low light conditions, and the reaction time is quite long, so I would rather suggest using a cheap electronic or mechanical (i.e. bimetallic) thermometer.

 

[sross]

Heh, heh,

In my day I have had to measure water temperatures around clam beds, among other parameters, and I have to confess to having tried just that- and it didn't work so well. I also agree with Trux that the risk is too high with a mercury thermometer. The amount of mercury in a thermometer is enough to cause local environmental damage, and if it broke I don't know how you would ever find the little mercury blobs and safely recover them. I wouldn't want to eat clams, or much of anything else, from some place where a mercury thermometer had been broken.

Anyhow, in my time I unthinkingly tried a mercury thermo in a lake, and found that when you get down the mercury becomes harder to read as light levels go down, at least against the white backing mine had.

OTOH, a red alcohol lab thermometer I tried, and still have, works great. As you go down the red alcohol starts to look darker against the white background. I also had one with a yellow background, but that one seemed to me to be harder to read.

Eventually I got a really cheap Casio watch with a large temperature display that hasn't leaked at all the depths I've taken it to, even though it's only got a 100m/300ft water resistance rating. I used the lab thermometer to calibrate it, and it is as accurate as I need (+/- 0.2 degrees C) reads fast in the water, and is much more convenient.

Here is the kind I got, you can get 'em ridiculously cheap on eBay nowadays:

[ame=http://cgi.ebay.com/Casio-Moon-and-Tide-Graph-Thermometer-Watch_W0QQitemZ220568089243QQcmdZViewItemQQptZUK_Jewelery_Watches_Watches_MensWatches_GL?hash=item335ae1f29b]Casio Moon and Tide Graph Thermometer Watch - eBay (item 220568089243 end time Mar-21-10 14:14:04 PDT)[/ame]

One note though- the Casio watch reads really fast in the water, but very slowly in the air. If you're doing air temp measurements an alcohol thermometer is the way to go.

 

[Paraaronoid]

Thanks guys for the responses. I had a feeling that a cheap mercury thermometer just wouldn't do the trick...I was looking at the Casio Sea Pathfinder watches, maybe I will have to take a look at them more closely. I do like that they can display depth as well. But, that involves more coin too...

Or maybe just an alcohol thermometer...

You've given me a lot of food for thought...Thanks!