rec.photo.darkroom

Subject: Re: Calibrated Thermometers

"Nicholas O. Lindan" wrote:
>"Pieter Litchfield" wrote
>
>> If I put the thermometer in a glass of water full of ice cubes, should it
>> not read 32 degees F (or 0 degrees C)?
>
>No.
>Try it.

Yes. And you really should try it.

>Read:
>http://www.its-90.com/wtpguide.html
>
>The short form is:
>
> o The ice is melting in the water so the water must be
> above freezing.
>
>Ergo, the water is > 32F

Or exactly equal to 32F.

> o The ice that has not melted is below freezing. If
> cold enough it will freeze the water. For water to freeze
> it must be supercooled.
>
>Ergo, the water is < 32F

Or exactly equal to 32F.

>At some time in the process the water may pass through 32F.

Yep, as soon as all of the ice is melted.

>When demonstrated by the 7th grade science teacher cold ice
>(<32F) is added to the mix as the teacher stirs and peers at
>the thermometer. When enough ice has been added for the
>thermometer to pass through 32F the teacher promptly announces
>success and tosses the mixture in the sink. If the teacher
>had kept going and the the water was pure enough and well
>polished with no air in it and the ice cold enough the water
>can get as cold as -40F/C.

Have you ever seen water at -40 that was not ice?

Or better yet, have you ever even so much as seen -40?

>> I can't use boiling water. Are there any other useful points? I would
>> think that if there is a way of setting the calibration near 68 degrees F
>> to 75 degreesF
>
>To do this you need a known good thermometer to know that the
>water bath is at 68F so the questionable thermometer can be read
>to determine it's error.

Glad you got something right!

--
Floyd L. Davidson
Ukpeagvik (Barrow, Alaska) floyd@apaflo.com

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