# Single probe capacitance meter

Discussion in 'Electronic Design' started by Germán Schuager, Oct 25, 2003.

1. ### Germán SchuagerGuest

Hi, I'm studying electronics and I've to make a final project to finish my
studies.
I've saw an artifact that is used to measure the grass density (in Kg/hect)
and I'm very interested about it.
It's composed by a stick of aprox. 1.5 meters with an aluminum sphere in one
side and a small computer on the other (used to show, log and average the
measures).
Apparently, what this device does is to measure capacitance, and the grass
in the field is used as the dielectric (the same principle of a humidity
meter I think).
My question is against what it measures capacitance (against ground?) and
how it does it.
I would appreciate a lot some information or guidance on the subject (I've
been searching the net and found almost none information)

Germán Schuager

2. ### John WoodgateGuest

I read in sci.electronics.design that Germán Schuager <gschuager.n.o.s.p
> wrote (in <bned7r\$107i0e\$-
berlin.de>) about 'Single probe capacitance meter', on Sat, 25 Oct 2003:
There appears to be nothing else possible.
How about doing some *experiments*? You can make a sphere from a child's
ball covered with Al foil. Then with a battery-operated capacitance
meter, you can do some *field* trials.

You MAY need to connect one terminal of the capacitance meter to a metal
plate on your shoe, but connecting it to your hand may be sufficient.
Depending on what sort of shoes you wear, the capacitance of your body
to earth is about 100 to 200 pF. The capacitance of the sphere depends
on its diameter and the distance above the grass (of course!). Expect
only a few pF, so your 100 pF capacitance in series won't have much
effect.

3. ### Germán SchuagerGuest

Somebody told me it could be that the measure principle has something to do
with the internal geometry of the device.
Like if the device may has something like an antena facing the floor inside
the stick, the sphere irradiates electrical signals and the measures are
done trough the electrical field.

Have you ear something like that before?

I don't know if the user gives a path to ground (but it seems more logical
to me).