[sdiy] Being accurate

[Paul Perry]

----- Original Message ----- 
From: "Paul Schreiber"
> About 10 years ago, I had to design a capacitance meter that could resolve
10fF
> (10 femtofarads) on a scale of 0 to 100pf. For $25. Working outside
at -40C to
> +100C.
> Any guesses how it was done?
>
In 1965, I had a lowly assistant tech job at a weather research place that
(among other things) involved climbing a mast & connecting a humidity
sensor,
consisting of an open air capacitor made of invar (dimensionally stable
over temperature extremes) that formed part of an oscillator that was (I
think)
hetrodyned and then frequency detected..
It was to look at fine structure in humidity variation, if you
had three of them some distance apart & correlated the signals, you could
work out
wind speed as well (because the fine structure decayed only slowly).
I'm sure your meter would have made things easier (most of my time was
spent converting the data from 10 metre rolls of photographic paper to punch
tape).
We had good resolution but the baseline was pretty screwed (used a sling
psychrometer for that).

paul perry melbourne australia