Re: Calibrated Thermometers

"Pieter Litchfield" <pvcl@*nospam*plitch.com> wrote:
All - many thanks for replies.

I apparently provoked a bit of a discussion about the theoretical
temperature of ice water with my question. I reasoned (based on my 8th
grade physics skills) that ice turning to water would result in the loss of
a large number of calories at 32 F before the temperature of the surrounding
water increased from 32F to 32F+ (latent heat of fusion?), so an insulated
mug of ice water would stay at exactly 32F while there was significant ice
in the water. I guess I will go looking for a physics book.
Here's a link to a site about "latent heat":
http://www.physchem.co.za/Heat/Latent.htm#fusion

That may or may not be the best discussion I've ever seen of the subject,
but it certainly *does* detail the significant points.

Note that what I was thinking about is point "b" on the heat curve at the
bottom of the page.

Exactly.

Also note there are some other substances listed with their melting points.
It appears that most are out of range and of no use in checking calibration
near 20C except for Ethanoic acid at 17C.

I'll stick with H2O... ;-)

I like the suggestion of checking against a good digital thermometer but of
course that begs the question - how do I know the digital thermometer is
accurate? I need a couple of reference temperature points near the useful
operating range of the thermometer to calibrate it, and your comments have
got me thinking more about it.

Unless you can get the digital thermometer calibrated, you are
simply going to trust it!

Digital thermometers may or may not be either more accurate or
more precise than a typical dial thermometer though, so the
trick is to find a "good digital thermometer". Of course, good
enough for darkroom work is not really very good. I'd think the
point would be to simply have one thermometer that is used as
the standard, and is typically stored away where it will not be
damaged. That could also be a good mercury or alcohol
thermometer too.

I've set up arrays of a dozen or so digital temperature sensors
and charted the differences between them over a period of days.
It was very interesting, and lead me to the conclusion that I
would need to use three sensors to get sub 1F degree accuracy.
I averaged the three, tossed out the value farthest from the
average of the three, and used the average of the other two
values! It turned out that these sensors would drift back and
forth within just about the limit of their specification (plus
or minus 0.5C), and over a week's time they *all* seemed to
eventually wander near the limits. These were relatively
inexpensive ($2-3 or some such each, Dallas Semiconductor type
DS18B20), and may or may not be indicative of the accuracy that
might be available from various digital thermometers.


"Floyd L. Davidson" <floyd@xxxxxxxxxx> wrote in message
news:87iroe7loo.fld@xxxxxxxxxxxxx
"Pieter Litchfield" <pvcl@*nospam*plitch.com> 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" <see@xxxxxxx> 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 <http://www.apaflo.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) floyd@xxxxxxxxxx

--
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) floyd@xxxxxxxxxx
.

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