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Subject: Re: Calibrated Thermometers




"Pieter Litchfield" wrote in message
news:gRn8g.7191$Gg.3865@twister.nyroc.rr.com...
> Well - the experiment continues. I remembered that I have a cheap digital
> thermometer (used for measuring snow temps). This is the self-contained
> pen style that go for about $20. It has more than sufficient range for
> both the freezing point and boiling point of water. Here's a link:
> http://www.professionalequipment.com/xq/ASP/ProductID.3915/id.22/subID.174/qx/default.htm
> The stated accuracy is +/- 1.8 degrees F over the range of 32 - 212 F,
> which would not be sufficient, in my opoinion to insure phot chemicals
> were at the right temperature. So I decided to at least test the end
> points to see just how accurate those were.
>
> I got out some distilled water ice cubes (used to mix working developer on
> ultra warm days) and used the blender to make a slush of ice and steam
> distilled water. After stirring for a few minutes, the thermometer
> stabilized at 32.1 degrees F. I found that if I didn't stir the temp was
> about 32.3 - 32.5, and I theorize that the shaft of the probe may have
> been conducting enough room heat south to create a "microclimate" around
> the probe. Stirring would keep the colder water around it all the time in
> such a case.
>
> I next dipped it in a pan of slowly boiling water (but not touching the
> bottom). The thermometer stabilized at 212. Since it does not read
> decimals when displaying higher F temps, I tried changing to the C scale
> and found it read 100.1.
>
> So my cheezy digital thermometer isn't too far off at those two points.
> If I can assume good liniarity in the range between them, I should be able
> to use it to calibrate my dial darkroom thermometers and my water temp
> control valve. The amount of error at the two "end points" of the
> calbration are well within my limits - I'm just developing B&W, so a 1/10
> of a degree plus or minus isn't the end of the world. Heck, it's almost
> impossible for me (even with a controlled water temp valve) to hold any
> chemical to a 1/10 degree tolerance when in use over time anyway.
>
> As suggested elsewhere, perhaps I'll try for a "mid range" value to
> determine if there the cheezy digital thermometer has a linear scale. In
> the meantime, thanks for all the useful discussion. Hope nobody got a
> bloody nose!

The best "inexpensive" thermometers I've found are those for the tropical
fish enthusiast. A couple of points...unlike my experience with photography
thermometers in this price range, the aquarium thermometers when compared
one with the other at the store reads pretty much the same. That bespeaks a
measure of consistent quality control. Then too, the life of the fish
literally depends on accurate temperatures (and the maintenance thereof).
Finally, such aquarium thermometers compare favorably in accuracy with my
standard - the Kodak "Color" thermometer acquired many moons ago.

A small added benefit is that some aquarium thermometers are designed with
what looks like BB shot at the bottom so that they float upright in a
solution.



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