[time-nuts] temperature sensor
EWKehren at aol.com
EWKehren at aol.com
Mon Jul 21 23:50:07 UTC 2014
Thank you Charles
. As a first step I will fill one of my Tbolt boxes with small foam
particles. Sounds like a good idea. I have one unit where I have given particular
attention to low power from the power supply.
Bert Kehren
In a message dated 7/21/2014 7:43:49 P.M. Eastern Daylight Time,
csteinmetz at yandex.com writes:
Tom wrote:
>There have been several discussions over the years about variable
>fan speed based temperature control. I can't explain it, but I've
>always been suspicious of this technique. It seems to me still air
>is inherently better than moving air. Passive (no fan) is better
>than active (fan). And constant velocity is better than turbulence
>is better than variable velocity. But I don't know for sure.
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
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