Re: Water probe II

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Bob (R.A.) Jones" <a1accounting@xxxxxxxxxxxxx>

Hi,

I thought I posted a message to you about high Z problems when you first
suggested using pure water.  But apparently it only made it to a draft
sorry.

Is your simulation software full 3D simulating distributed R and C?  If so
that a nice tool.

If you can design a high bandwidth high impedance divider it would be very
useful to the list.

Note your make shift hand on the top and hand on the side that improves the
response is effectively a parallel C between the input and output that
compensates for the self C of the probe as is done on regular scope probes.
Apparently some high voltage probes use an adjustable cylinder to achieve
the that.

If your going to experiment with the inner outer thing here are some of my
thoughts.  Add an air gap between the inner and outer to reduce the C
between them and therefore more tolerant of differences in voltage.  In the
limit this is say 1/4in inner tube with a 2in air gap. If one outer improves
things perhaps two outers are even better and allow high R for all of them.
You will still need high Z amp to avoid loading on the inner.

Bob


----- Original Message -----
From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Monday, December 27, 2004 9:42 AM
Subject: Water probe II


> 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
>
>