[time-nuts] OXCO insulation

Bill Hawkins bill at iaxs.net
Tue Feb 24 23:44:30 EST 2015


I've lost track of who said what in this thread, but someone cautioned
against
over-insulating the oven. The quest for the best insulation is not
necessary or
desired.

If you use aerogel and your oven controller has the least bit of
overshoot, it
will take a long time for the overshoot to dissipate, during which time
your
controller has no control over the slowly decreasing temperature.
Indeed, the PID
algorithm could drive the output to the heater all the way to zero while
the
temperature comes back to and slightly below the setpoint. This is
guaranteed
to produce another overshoot as the temperature continues to drop while
the
controller raises the output from zero.

You need to have enough heat leakage to keep the heater running at a
reasonable
value. If you want to do something different, try using coarse and fine
heaters.
The coarse heater and control provide fast warmup to a small deadband
where the
fine heater takes over. Commercial ovens don't add the expense of a
coarse heater
because they assume that warmups will be infrequent. The user can wait
for some
warmup time before using the device. OTOH, fast warmup may be bad for
the crystal.

One other thing to consider when designing an oven: put the temperature
sensor
as close as possible to the crystal. Distance delays the sensing of
crystal temp,
called dead time. Dead time will cause the controller to oscillate at
lower gain,
so the gain has to be reduced. This reduces the ability of the
controller to
respond to external temperature changes, called load disturbances.

Hope this was of some use.

Bill Hawkins



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