Water probe II

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Denicolai, Marco" <Marco.Denicolai@xxxxxxxxxxx>

Hello all,

Thanks for your help.

--- B2: ---

I am aware that professional probes for this purpose are in the range of
kohms. I just think that it would load too much the TC. I am pretty sure
it would be a lot easier to built a low resistance version of my probe.

>C = 2.7 meters (your probe length) x 111pF/meter = 300 pF

By the way, I am suspicious about your formula for the capacitance to
gnd. I have got another one (at home) and it looks really different.
Let's see. I have a measured rise time of 20 us, that makes C=6T/R =
6*20 us / 20E6 = 6 pF. This is in line with my calculation and my
simulations (dated 2-3 days ago).

>I would try the toroid without the resistor.  Run a copper wire or tube
through a sliding feed-through such as a SwageLok.
But that would probably be sensible also to displacement currents,
should be calibrated every time, etc.

>The output of a liquid resistor probe should be around 100 volts to
overcome distortion from electrolytic effects associated with the metal.

I guess you meant "at least 100V". Good to know.

--- Mike: ---
No, that 20 Mohm is an upper limit. I am confident the real value is
lower. I have to measure again (and for good, this time).

--- Bob: ---
>The only other way I can imagine you can do it is by putting a high
input impedance unity gain amp at the
>output but you will still have the self C charging problem.
I agree.

>Perhaps you can shield the whole probe in an outer tube filled with
water to shield it from external C effects and it
>will reduce the effects of its self C because the outer and inner are
at the same voltage at each point but the outer
>will still need to be sufficiently low impedance that its voltage is
primary determined by its resistance not its self
>C or C to the top load.
This is a great idea! I am going to simulate that. I am even in the
position to try that if the simulation looks good.

--- NEW STUFF ---
Before Christmas I remembered that I did have a prototype of the probe
at home. It was only 70 cm long, using the same idea then the big one.
That too exhibited a 20 us rise time. I realized it would have been a
lot easier to measure and get to work the small one, and then to use the
same medication for the bigger one.

So I made a simulation model for the smaller one and I setup a benchmark
for it at home.

In the simulation model (Bela) I set the tap plate to 1V, the top plate,
bottom plate and ground plane to zero volts. Then I read the charge
accumulated on each plate. As C=Q/V the charge gives me the capacitance
to the tap plate.
For the small probe I get (the tap plate is near the bottom here):
Ctap-top = 2.8 pF
Ctap-bottom = 311 pF
Ctap-gnd = 0.87 pF
Charge on tap = 318 C (this is the total capacitance in pF)

This means that the divider isotropic capacitance must be 318 - 311 -
2.8 - 0.87 = 3.33 pF. That is where the missing charge goes.

In my lab, I setup a power opamp, driven by a function generator and
driving the small probe. I use a continuous 1 kHz square wave and I can
see in real time the rise time of the tap voltage. It is easy to see how
it get worse by getting close to the water column with a hand. As well
it is easy to get how it gets better by touching the upper electrode
with one hand and the water column with the other hand. That is,
increasing the water column capacitance to the upper electrode
counterbalances the isotropic capacitance and results in a better
response.

More simulations, more measurements and I'll be back with the results.

Best Regards