[time-nuts] E1938 oven design

Richard (Rick) Karlquist richard at karlquist.com
Sun Jun 11 13:43:07 EDT 2017

On 6/11/2017 8:59 AM, Attila Kinali wrote:

>> The exact insulation is relatively unimportant.
>> We even tried still air using a knife edge
>> cradle.  Didn't make much difference.

> What is a knife edge cradle?

We wanted to test still air as insulation.
We couldn't just replace the insulation with
"nothing".  There had to be some kind of mechanical
support to keep the oven mass suspended inside the
outer case.  Our ME built a skeleton framework
made of plastic to support it.  In order to minimize
conduction thru the plastic, he designed in knife
edges where the plastic came into contact with the
oven mass.

> One big question remains: How did you set the ratio
> between face and rim heater? Was it determined at design
> time and then just set the same for all units or was
> it part of the production test?

During proof of concept phase in R&D, I peaked up the
thermal gain on each unit by trial and error.  I could
usually get a run with positive thermal gain, then
increment the ratio, and get a run with negative
thermal gain.  I could then interpolate to get the
ratio that should give "infinite" gain.  Maybe one
or two more runs after that would get me to where
the gain passed through infinity at some ambient
temperature, and was well into the millions over the
whole range.  At extremely high thermal gain, the
gain is not constant over ambient temperature.

I collected data on a number of units and then used
the average ratio as the production setting.  I
checked production units from time to time and they
typically ran well in the 100's of thousands for gain.

This compromise was workable because we individually
programmed the oven set point to the exact turnover
temperature.  This is a lot easier because it doesn't
require environmental chamber runs.  The E1983A software that
I "leaked" to the time-nuts community I believe has a
command that can be used to search for the turnover.

Rick N6RK

> 			Attila Kinali

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