[time-nuts] temperature sensor

[EWKehren at aol.com]

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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