Yes but water can be "superheated" too if there are no nucleation points for
bubbles to form and like super-cooling this can amount to a couple of deg C
in a very clean container. However the vapour cannot be superheated without
increasing the pressure .....as in a steam engine, or autoclave. I may not
be as accurate as "temperature nuts" would like but unless you are very sure
of your conditions it is probably more reliable. I notice the wiki on
"temperature scales" doesn't include boiling water (or steam) these days,
but if does say is 17mK low of an exact 100 :-))
http://en.wikipedia.org/wiki/Temperature_scales#International_temperature_scale_of_1990
I note it doesnt actually say where you place the sensor :-))
Alan
G3NYK
----- Original Message -----
From: "Brooke Clarke" brooke@pacific.net
To: "Discussion of precise time and frequency measurement"
time-nuts@febo.com
Sent: Monday, July 21, 2014 9:11 PM
Subject: Re: [time-nuts] temperature sensor

Steam superheats only if the pressure is raised above standard pressure,
otherwise, steam at standard pressure will be exactly 212F, or 100C.

-Chuck Harris

Brooke Clarke wrote:

Hi

If you look at the people running very precise thermometers (sub 0.001C) they are doing better with thermistors than with PRT’s. Both the PRT’s and the thermistors come with notes on them requiring on location re-certification below he 0.01C level. A triple point of water cell is typically what’s used for both.

Bob

On Jul 21, 2014, at 10:12 AM, Attila Kinali attila@kinali.ch wrote:

Tom wrote:

There is no such thing as "still air," unless there is no temperature
gradient.  If there is any temperature gradient (typically due to
power dissipation), there will be convection currents.  In a closed
space (for example, internal to a TBolt or in a sealed box that
encloses a TBolt), these convection currents will set up a flow
pattern that may be benign or malicious with respect to keeping a
particular part of the device at a constant temperature.  If the part
you are particularly interested in is a creator of thermal gradients
(as the OCXO in a TBolt is), analyzing this gets very complicated very fast.

Fans (speaking here of fans that circulate air internal to a closed
volume, not fans that exchange air between the inside and outside of
a volume) tend to mix up the air and reduce thermal gradients.  Then,
the question becomes whether the circulation due to the fan has a
patterned or a random thermal flow.  Typically, a random (diffused)
pattern is best -- but it is relatively hard to achieve.  With
careful design, active circulation is usually better than passive
convection.  However, "careful design" is not easy.  Also, fans raise
a concern about vibration, which is a real worry with any precision oscillator.

One other possibility is to use passive techniques to randomize (more
or less) the passive convection.  This can be achieved (to a degree)
by filling the internal volume with low-density, very porous
insulation.  On a larger scale, a sealed box of, say, 2 cubic feet
can be filled with common packing peanuts and the isolated object
placed in the middle.  Air will still circulate by convection, but in
a more random manner.  (There will also be less bulk circulation, so
the thermal gradient will be somewhat larger than before.)  Applied
to a TBolt, one might fill the inside of the TBolt itself with
smaller pieces of styrofoam (irregular shapes perhaps 6 or 7mm in
size).  [Spheres (styrofoam beads) may pack a bit too tightly for
this, impeding airflow more than desired.]  The same can be done for
a sealed box that encloses a TBolt or other oscillator.  I have
achieved very good results with this method, when properly applied.

I have done a fair amount of experimenting with and without fans (but
one must recognize that there are so many variables, even a lot of
experimenting really only scratches the surface), and have always
found that passive circulation (within sealed volumes) works very
well when the object ultimately being controlled is an ovenized
oscillator.  For tight control, which is needed for precision voltage
references, DAQ circuits, and other precision process-control
applications, I do use a thermostatically operated fan to exchange
air between the outermost sealed volume and ambient -- but even this
I usually find unnecessary if the ultimate object is minimizing the
frequency drift of an ovenized oscillator.

Finally, re.: fan control.  For a brushless DC fan to run slowly, you
need to feed it full voltage with pulse-width modulation
("PWM").  Even then, they will not run all that slowly.  The
Microchip TC642B fan controller (8 pin IC, about $1.20) is a very
handy part when you need a wide range of fan speeds.  It uses
commutation noise to sense fan rotation, and has a "stall routine"
that gives the fan a kick if it stalls (NB: this is a feature of the
642B, absent on the 642).  So, not only will it run the fan at its
lowest possible self-sustaining speed, you can also run the fan much
slower than its self-sustaining speed by letting it stall and be
restarted periodically.  The fan looks like a windmill with three
sheets to the wind below its self-sustaining speed, but it works
extremely well and this operation does not damage the fan or the controller.

Best regards,

Charles

the broiling temperature is dependent of the atmospheric pressure! the
water broils at 100C° only a see level!

73
Alex

On 7/21/2014 11:56 AM, Alan Melia wrote:

Steam is always 100C in open air at sea level BUT there are real
problems if you try to use it

Steem quickly condenses back to water.  If you think you can see steam
you are mistaken.  What you see is water aerosol that is condensed
when stem hits the colder air. Water in vapor form is invisible in
air.  For the same reason clouds are water, not vapor.  If you place
the sensor in steam it is hard to really know what you have.  Is it a
mixture of vapor, re-condenced vapor and air.  And then what about the
thermal conductivity?  You really can't know.  But with water it is
pretty easy to see that it is nearly 100% water.  Experiment with tap
water v. RO water and I doubt you will find much difference as "hard"
water has only maybe 12ppm dissolved minerals.

Same with dissolved gasses O2 and N2 at room temperature are present
at the small fraction of a gram per liter but at 100C there is not
much gas in the water.

Remember the raise in boiling point is (from memory) about .5C per
mole per liter and how many moles "stuff" is in a liter of 100C tap
water?  You can calculate the effect.  But I'm thinking it's way below
the 0.001C level.

Air pressure or altitude above sea level will make a real difference.
I once tried to cook rice at 12,000 feet.  It didn't work.

On Mon, Jul 21, 2014 at 1:11 PM, Brooke Clarke brooke@pacific.net wrote:

--

Chris Albertson
Redondo Beach, California

On Mon, 21 Jul 2014 18:00:59 -0400
Chuck Harris cfharris@erols.com wrote:

Uhm.. you are the second one claiming this. Could you please explain
what physics limits the temperature of vapor?

The ideal gas equation says that p*V/T = const, ie that the temperature
can rise at a constant pressure, as long as the gas is allowed to expand.

			Attila Kinali

--
I pity people who can't find laughter or at least some bit of amusement in
the little doings of the day. I believe I could find something ridiculous
even in the saddest moment, if necessary. It has nothing to do with being
superficial. It's a matter of joy in life.
-- Sophie Scholl

Ok... a couple hours of reading later.... ;-)

My excursion into temperature measurement has brought some results:

1. PT sensors can be secondary standards for temperature calibration.
   But standard industrial sensors do not have the stability or linearity
   of the standard grade sensors. But at least they do not break when you
   glare at them. Those in ceramic housing are supperior to those in glass
   or metal housing. Thin film are inferior to wire wound. (in terms of
   stability and accuracy, thermal coupling is a different matter)
   (the price for a commercial standard grade PT sensor seems to be in
   the order of 3kusd)

2. The uncertainty of the calibration of the standards grade PTR seems
   to be in the order of 100uK to 10uK.

3. Making a triple point of water cell for calibration with an accuracy
   better than 10mK seems to be quite simple and doable at home, most
   likely something around 1mK is achievable. Also judging the quality
   of the cell is quite simple: make multiple of them, the one with the
   highest temperature is the most accurate one.

4. A well done ice bath gets you into the ballpark of 10mK accuracy.
   Most of the error is due to impurities and gas in the water.
   The air pressure effect is much smaller (and thus inconsequential)
   unless living on a high mountain. Also an ice bath is easier to
   do than using an triple point cell.

5. There are people on ebay who sell very pure Gallium and Indium that
   could be used for (not so accurate) melting/freezing cells for
   ~29.7°C and 156°C.
   (if anyone knows what the non-nut would use those for, please tell me)

6. The book "Traceable Temperatures" by Nicholas and White is a very good
   reading on temperature measurement and calibration. It explains the
   procedures with what can go wrong and what accuracies are achievable.
   It also contains a list of references for further reading. I did not
   have a look at those yet, but from the titles they look very reasonable.

    		Attila Kinali
   

--
I pity people who can't find laughter or at least some bit of amusement in
the little doings of the day. I believe I could find something ridiculous
even in the saddest moment, if necessary. It has nothing to do with being
superficial. It's a matter of joy in life.
-- Sophie Scholl

Chuck Harris cfharris@erols.com wrote:

Saturated steam at standard pressure will be exactly 212F, or 100C.

Superheated steam is steam raised above the saturation temperature, and so
is dry, and could be any temperature above the boiling point at that
pressure.

Saturated steam under (positive or negative) pressure can boil at other
temperatures, from memory (50 years ago!!) 30psi is 131 degC and 15psi is
115 degC.

Steam in steam engines is generally dry, and is thus to some extent
superheated.

--
Brian Duffell  Yarm    England

Hi

At least where I have lived around the country (and actually measured the tap water) the “stuff” in the hard water has come out in the > 100 ppm range. Each time I’ve asked about it, the answer comes back: “we add (trade name of stuff) to the water to make it hard so the pipes last longer”.  Anything under 30 ppm is pretty soft water.

Now, does that have a massive impact on the boiling point - nope. Checking your barometer is a better thing to do than checking your TDS meter if you are calibrating the boiling point. Now for the triple point …. you need good clean water.

Bob

On Jul 21, 2014, at 7:05 PM, Chris Albertson albertson.chris@gmail.com wrote: