# rec.photo.darkroom

## Subject: Re: Calibrated Thermometers

"Pieter Litchfield" wrote:
>Dumb question time:
>
>I have dial thermometers with a dial that can be rotated for
>"calibration" purposes. 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)?

Use an insulated container. A foam cup snuggly fitted into a
thick coffee cup would do fairly well, for example.

The idea is to *isolate* the water from heat loss or gain to the
surrounding (warm) air. What heat is picked up necessarily goes to
melting ice rather than raising the temperature of the water,
and hence the temperature will be constant at the melting point
of the ice.

It will be very close to 32F for most ice cubes... :-)

>Since the scales don't go to boiling, I can't use boiling
>water. Are there any other useful points?

Nope. Or, at least not using water.

>I would think that if there is a way of setting the calibration near 68
>degrees F to 75 degreesF this would be better than calibrating at 32 degrees
>F, so are there any natural processes that happen in this range that could
>be the confident basis of a calibration?

The thermometers should be linear, so they will almost certainly
be accurate at 75 degrees even if they are calibrated at 32
degrees. However, that should probably be verified for a new
thermometer.

It isn't easy to check them accurately, but if you have more
than one (and the more the better) it can be close enough. Put
them all into a relatively good size (half gallon or more)
container of water at room temperature. Ideally this would be a
room where the temperature doesn't change, but it won't really
make much difference because the thermometers should all react
faster than the water can change temperature. When the water is
at room temperature, all of the thermometers should read close
to the same. Simple as that.

In this case you want the water in the container to lose or gain
heat from the air around it, so don't use an insulating
container. An aluminum container would be best perhaps, but
another metal or glass is okay too. Just don't use a foam
insulated jar! The ideal container would probably be a double
boiler from the kitchen... with the outer container basically
buffering the inner one, which might take several hours to
stabilize too.

And, to top all of that off... buy any half decent digital
thermometer to use as a reference to calibrate all the others
against.

>"Nicholas O. Lindan" wrote:
>>
>> You can always stick it up your a---. Should
>> be 98.6 +-.

Not necessarily true. That is just the average, but
individuals vary greatly.

>> Fever thermometers are accurate to +- .2 degrees
>> and make good calibration standards: put both
>> thermomters in hot water in a styrofoam cup -
>> keep the water gently stirred.

Do that with a modern digital thermometer though, not
one of the traditional glass types that has to be
shaken down.

Regardless, it won't work well because it is too hard to
maintain a cup of water at 98F in a room that is 68F.

>> Only color photography is sensitive to
>> temperature, and that gets processed at 100
>> same as body temperature: they are both organic
>> chemistry.
>>
>> B&W is a +-2 F game, and you only need
>> to have a repeatable temperature and the ability
>> to read the deviation to +-10%. A grossly
>> inaccurate thermometer will do as long as
>> it is linear.

That is true, and is the *most* significant thing to keep in
mind.

For example, lacking calibration... always use the *same*
thermometer for film developer. All that counts is whether it
has precision, not how accurate it is. It can be 5 degrees off,
but if it is always within 1/10th of a degree at the same
temperature, all is well. (Until a different thermometer is
used!)

>> Temperature calibration is tough to do with
>> great accuracy. A triple-point ice cell is
>> the only common temperature reference.
>>
>> http://www.its-90.com/wtpguide.html
>> http://www.omega.com/ppt/pptsc.asp?ref=TRCIII&Nav=temk03
>>
>> If you would rather have someone else calibrate your
>> thermometer:
>>
>> http://www.omega.com/toc_asp/frameset.html?book=Temperature&file=INTRO_GLASS_THERM
>>
>> Or buy a 'Kodak Color Thermometer' or 'Kodak Process Thermometer' on
>> ebay and be as Alfred E. Neumann: "What, me worry?"

--
Floyd L. Davidson