ehsjr wrote:
Joerg wrote:
ehsjr wrote:

Joerg wrote:

ehsjr wrote:

You call that a coil? That's not a coil.
*This* is a coil!


No, it's a petunia. Or whatever. (I am not a botanical expert)

So, what's that super-stretch edition of coil for?


For a permeability tuned oscillator. It's an experiment to
measure water level, which changes only 3 feet nominal at
the site.

The finished coil is 42" of wire on 46" of PVC. A float
makes the coil ride up and down on a galvanized rod as
the water level changes.

It is accurate to within an inch, with +/- 3" being acceptable,
so that part is good. There is a big (ie 3" change in reading
with a 30F change in ambient) temperature problem to be solved,
but I ran out of warm weather to work on it. Looking for +/- 3"
accuracy between 50 and 100 F.


Can't you run some additional DC through the coil and regulate that so
you get a constant DC resistance through the coil?


Hadn't thought of that. As I recall, there's no DC in the
coil. My notes, circuit, schematic and coil are all buried
in the garage waiting for warm weather. But I'm fairly sure
it's a standard Colpitts with the coil connected to the base,
with no DC path.


Well, you would need a mux. Don't know your frequency but it's not a big
deal.


I'm measuring frequency, which is what changes with temperature
change - I don't remember the direction. As I recall, it makes
an excellent thermometer, implying linear change. Anyway, my
assumption has been that either or both the capacitance and the
inductance are changing with delta T, and I expect the biggest
change is in C, not L. It seems highly probable that the caps
are the biggest source of the delta in Hz. Those assumptions are
what I need to examine next.

BTW - that's just one of the wacko coils I made. :-)
It started out only as a proof of concept thing, but
since I had already Rube Goldberged the "coil winder",
I did a few more of various configurations.


Yep, once you have built a toy it has to be used :-)

--
Regards, Joerg

http://www.analogconsultants.com/