Brooke Clarke wrote:
Hi:
For some time I've considered surrounding a free running 32678 Hz
oscillator (like a Dallas 32khz, or one of the newer Maxim units) with
thermal mass and insulation in order to get the time constant into the
range of some days. To get a feel for it a simple experiment shows
that a half inch diameter brass rod 3.75" long (102 grams) has a
thermal time constant of about 6 min 35 seconds when wrapped lightly
in a towel.
Is there a way to calculate the amount of aluminum and Styrofoam
needed to get a time constant of say 100 hours?
This came up in relation to WWVB clocks that free run for most of the
time. When you compare WWVB clocks it's not uncommon to see tens of
seconds difference between them. http://www.prc68.com/I/Loop.shtml#TC
Start with the maximum thermal resistance the application can withstand
(determined by internal dissipation and acceptable temperature rise
above ambient).
If for example the dissipation is 10mW and acceptable temperature rise
10C then thermal resistance will be about 1000C/W.
The thermal capacity required can then be calculated from the time constant:
C= 3.6E5/1E3 = 360 J/C requires about 7.2 kg of aluminium.
The required thickness of styrofoam can then be calculated from the
surface area of the aluminium block.
Achieving a thermal resistance of 1000C/W may be a little difficult
without using radiation shields.
Bruce
_______________________________________________
time-nuts mailing list -- [email protected]
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
