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Subject: Re: Calibrated Thermometers




"Nicholas O. Lindan" wrote in message
news:IHm8g.2420$u4.1962@newsread1.news.pas.earthlink.net...
> "nailer" wrote
>
>> you are right, during phase transition period ,the
>> temperature will
>> stay constant (in an isolated setup).
>
> But with Kitchen Kounter Kalibration it isn't.
>
> I STRR that a reasonable setup can be had with distilled
> water slush,
> such that the ice and water are at the same temperature.
>
> Wait for a winter day when the temperature is 32F scoop up
> some clean
> snow in the proverbial styrofoam cup, shake a bit to
> generate water,
> put a cap on it and keep it in the shade.
>
> The ice, the water and the air/water vapor have to be at
> the same temperature:
> the ice is neither melting nor freezing.
>
> A home made slush of crushed distilled water ice cubes
> should work.
>
> The NIST/National Bureau of Standards has a publication on
> 'Practical
> Temperature Standards'. Much easier to call Omega and get
> a triple
> point cell.
>
> If you use a glass fever thermometer as the standard be
> aware they
> hold the highest temperature. Look at the thermometer to
> be tested
> and note it's highest temperature. Then look at the
> reading on the
> fever thermometer. The quantity of water needed has to be
> sufficient
> that it's cooling time constant is much greater than
> heating time
> constant of the thermometer. And keep a lid on it. A use
> for
> fast-food litter: the little hole for the straw is the
> right size for
> a thermometer.
>
Just to stir things up further, note that glass column
thermometers are calibrated for a certain depth of
immersion. Usually this is marked on the column. Some
thermometers are made to total immersion, mainly for
measuring air temperature. For using boiling anything as the
high temperature standard one must know the barometric
pressure. In addition, freezing and boiling points of water
are given for chemically pure water. Its been too long since
I did this to remember the size of the errors. Barometric
pressure can result in significant error if it isn't taken
into account.
As far as knowing if a thermometer is reliably
calibrated, if this is really important, one must have a
NIST tracable calibration. That is hand work so the cost of
a certified calibration is high. Even relativelly low cost
digital thermometers appear to be accurate enough for color
work, i.e., 1/2 degree F. in the relevant temperature range.
For B&W I use a large Weston dial thermometer which has
been adjusted to match a certified mercury thermometer. I
also have a digital thermometer made by CheckTemp. I prefer
the dial thermometer for working use because its
indestructable and will stand immersion which the digital
thermometer will not.


--
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@ix.netcom.com




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