A question about capacitor

Two plates is the minimum, often exceeded.

Yes, although what would be called the charge is fully contained whether the dielectric conducts or not.

If you know your chemistry, you know that water becomes something like an insulator when you get the free ion concentration low enough. Are you using leakage to guage that concentration? The capacitance will stay just about the same unless you have very high impurity levels.

So doesnt capacitance depend on conductivity of the dielectric ? I mean wont a more conductive sample of water give a different capacitance than less conductive water ?

No. The effects act in parallel. When you apply an electric field to a slightly conductive dielectric, some bound charges move a limited distance then stop, and some unbound charges drift continously. We call the first displacement (or capacitive) current. We call the second leakage. The movements of those different charges are largely independent.

I don't think you will be able to measure a capacitance change as you alter the ion concentration until you have such a large concentration that the dielectric property of the solute itself is a factor. By that time, measuring the capacitance would be quite a trick.

In C= KA/d if K does not depend on conductivity, what makes a dielectric have different dielectric constant K. What are the parameters on which K depends on?

Usually, 'K' refers to the relative dielectric "constant" and would be multiplied by the permittivity of free space in the above formula.

It is affected by how easily bound charges can be displaced and the density of those charges. For example, in water, the bound charges are at sort of opposite ends of the H2O molecule and displacement occurs as they become polarized in an E field.

Your question covers a lot of territory.

So doesnt this imply that if there are free ions in water there will be a different K from a nuetral sample of water with no free ions ? I am actually adding chemicals(ionic) to water which nuetralises harmful ions in water to form a nuetral particle floc. So I need to detect zero crossing. As i am adding ions into water i need to detect a zero crossing in going from postive charge to negetive.

I don't think so. The free ions will drift under the influence of an E field, contributing in-phase current, not lagged current such as displacement produces. The only way free ions could change the K would be if there were enough of them to act as a dielectric and they had a K different than the H2O they displace.

Sorry, but I cannot make sense of that. Maybe it would help to show your circuit or block diagram.

"Larry Brasfield" wrote in message news:piyae.45$ snipped-for-privacy@news.uswest.net...

Woops, that should be: not leading current such as displacement produces.

I am adding salt to water. I read something in one of the forums below which confuses me. It also says "adding virtually any contaminant to water will alter its conductivity. That also ruins the diaelectric constant . Only pure water will have a measurable diaelectric constant."

It says that pure water is insulator and adding salt changes conductivity. I am adding salts to contaminated water to which makes it an insulator. But if more salts are added water crosses the point at which it is an insulator(pure) and again starts conducting. So I need to detect that zero crossing.

But according to the forum in the link above, conductors cant be dielectric. But what if I use conducting water as dielectric. What happens. I know dielectric is supposed to support electric field but oppose current. But if there is leakage current the capacitor can't hold charge or what ?

Also what is meant by "Only pure water will have a measurable diaelectric constant."

I don't know what a ruined dielectric constant is. That is gibberish as far as I am concerned.

I don't understand the zero crossing. Pure water conducts. There are always H+ and OH- ions present, and they can carry current. What you will see, if you can reduce free ions by adding salts to react with already present ions so as to produce a non-ionic product (a precipitate, I presume), is a decrease in conductivity which will approach the conductivity of pure water, possibly followed by an increase if you add too much. There will be no zero crossing.

To me, it looks like you should almost forget about capacitance and measure conductance. If you are using AC for that measurement, it may be a good idea to measure only in-phase current so as to ignore the capacitance.

For good conductors, the dielectric properties do not matter and are difficult or impossible to measure. A counter-example to that statement can be found in any electrolytic capacitor. They all leak current, so, because the material between their plates is a conductor (albeit a poor one), and taking the above as true, they cannot also be capacitors. Countless devices say otherwise.

The question should not be "Can it hold charge?" but "How long can it hold charge?" Leakage limits the time that a capacitor can hold charge.

That overstates the case. If we are to believe that, then adding one ion molecule to a vat of pure water will turn it from being a dielectric to being a poor conductor.

A more accurate statement might have been that adding things to water that increase its conductivity interferes with many dielectric measurement methods.

Imagine measuring the value of a capacitor with a capacitance bridge. The null is nice and sharp and determining the value with high precision is easy. Now, connect various resistances in parallel with that capacitor. The bridge null gets shallower and wider as more and more of the instrument's bridge current detours through the resistance and less and less of it passes through the capacitance. But paralleling those resistors did not alter the dielectric constant of the insulating material in the capacitor.

The key word, here, is "measurable". Using a higher AC excitation frequency lowers the capacitive impedance while the Resistive component is essentially unchanged. So measuring conductive samples is more practical with a higher excitation frequency. The caution is that the actual value of high dielectric materials, especially liquids can vary dramatically as the excitation frequency passes through motional resonances of the polar molecules. So you will need calibration standards that you can use at the frequency you choose.

There will be some ions, but not many. Only about 1 in 500 million water molecules in pure water break up into H+ and OH- ions.

Pure water is actually a good insulator (conductivity < 0.06 uS/cm)

What you're actually looking for is a pH meter.

This isn't really related to dielectric constant at all, unless, as Larry said, the solute has a dramatically different K than water, and if it were in enough concentration to affect a capacitive readout, you wouldn't have a capacitor but an electroplating tank.

Good Luck! Rich