Salt Water Capacitor Discussion

[Aggie]

Hello all,

I am researching Tesla ideas, which led me to salt water capacitors (SWC). I have built two out of a plastic drink bottle and two salts (KCl and NaCL), but I am not measuring any capacitance.

How do you measure the capacitance of a SWC with a multimeter, which has capacitance capability? The range on the MM is nano and micro Farads. Currently, I connect the two leads of the MM to each lead of the SWC, but I am not reading any capacitance, except when I walk up closely wearing my fleece jacket. Guess I'm the one generating the static electricity.

Also, I would love for the thread to become a list of past experience and current work with SWC's as I am not finding much in details.

My goal is to determine a model of capacitance based on salt type and maybe pH.

Thanks,

Aggie

 

[(*steve*)]

If your meter can't measure any capacitance then there is likely not very much to measure.

Do you understand how capacitors work?

Perhaps you can describe and/or show us some pictures of what you've done.

 

[Aggie]

If your meter can't measure any capacitance then there is likely not very much to measure.

Do you understand how capacitors work?

Perhaps you can describe and/or show us some pictures of what you've done.

[/IMG]

My meter does measure capacitance: Ranges: 2nf, 200nf, and 20uf.

I basically following this type of instructable: http://www.instructables.com/id/How-to-build-a-Tesla-Coil/step5/Build-the-Capacitors/

I've tried a copper and steel center electrode, different types of salt (K and NA) and nothing on the meter.

I thought I understood capacitors in that you just connect a positive and negative terminal to the meters inputs. I've also tried swapping the leads from the MM as well.

Edit: Here is the actual instructable i am following and a pic: http://www.instructables.com/id/Make-A-Water-Bottle-Capacitor/step10/Completed/

As you can see, my image of measuring capacitance and the instructable one are almost identical in design.

Thanks

 

[jackorocko]

Maybe the plastic bottle you used is to thick for the voltage you tried?

 

[Resqueline]

The capacitance is independent of the conductivity of the electrolyte, that will only affect the ESR of the capacitor.
The capacitance is dependent of the insulator (glass/plastic), its thickness, and the area. So a big thin-walled PET bottle will give a high capacitance.
It's important to wrap the foil tightly around the bottle. If you use spray glue that may give the best results.
What kind of bottle are you using? Are you sure the red wire has good ohmic contact with the aluminum foil?
Does your meter respond correctly to a proper capacitor?

 

[jackorocko]

http://www.sparkbangbuzz.com/els/stat-gen-el.htm Don't you have to charge these first or is that not part of the capacitance test?

 

[Resqueline]

The meter would not take kindly to the capacitor being charged, but not using a meter you could of course try the charge & spark test.
The salt water capacitor is just a variety of the Leyden jar btw..

 

[Aggie]

The capacitance is independent of the conductivity of the electrolyte, that will only affect the ESR of the capacitor.
The capacitance is dependent of the insulator (glass/plastic), its thickness, and the area. So a big thin-walled PET bottle will give a high capacitance.
It's important to wrap the foil tightly around the bottle. If you use spray glue that may give the best results.
What kind of bottle are you using? Are you sure the red wire has good ohmic contact with the aluminum foil?
Does your meter respond correctly to a proper capacitor?

The foil and wire electrode are making enough of a connection for the meter to find connectivity. I might have some spray glue nearby.

The bottle I'm using is a sports drink plastic bottle. I can transfer this whole apparatus to a glass flask if you think it would work better. Unfortunately, I do not have a capacitor on hand (at work now).

Thanks for the help on this. just want to understand SWC's better.

 

[Aggie]

http://www.sparkbangbuzz.com/els/stat-gen-el.htm Don't you have to charge these first or is that not part of the capacitance test?

From what I can tell, people don't charge the capacitor first. I think the multimeter is doing the charging.

The mass of the SWC might be more than can be charged by my MM. Hadn't considered that.

 

[jackorocko]

From what I can tell, people don't charge the capacitor first. I think the multimeter is doing the charging.

The mass of the SWC might be more than can be charged by my MM. Hadn't considered that.

my first thought was the MM charged the cap as well when it was connected. But I don't know enough about them to actually say.

 

[Aggie]

Failed attempt #2:

I don't know why I didn't think of it earlier, but I have tons of used centrifuge tubes for this test.

But as you can tell, I'm not getting a reading @2nF resolution. The only time the meter responds is when I put my hand near the meter.
I did used spray adhesive to attach the foil to the tube. Maybe the walls are too thick as well.

I will try a thinner walled bottle next and maybe a glass flask. grrrr

 

[jackorocko]

Just out of curiosity, have you tested a standard capacitor to see that the meter is functional? Just thought I would mention it.

 

[Aggie]

Just out of curiosity, have you tested a standard capacitor to see that the meter is functional? Just thought I would mention it.

Not yet. I don't have any laying around the lab. I will have to bring one from home tomorrow and report on that. As it is, the meter does respond to me approaching the SWC. For what it's worth.

 

[Resqueline]

You're using the meter incorrectly..
The picture is still not clear, but clear enough that I see that the capacitor is not supposed to be attached to the leads but put into the socket near the cap' range.
The meter test the capacitors at 2.5Hz as stated in the manual. The mass of the cap' is irrelevant.

A glass bottle has a whole lot thicker walls so it'll give a whole lot less capacitance but may be able to withstand a whole lot higher voltage.
Physical tables will list the electrical properties of different insulating materials.

 

[JimW]

You are trying to build and test a Leydon Jar (300 or so years prior to Tesla). Google that for more info. People use these for Tesla coils sometimes. The capacitance is strictly a function of surface area of your outside aluminum foil and the thickness of the bottle. I build these all the time for a variety of classes. From what you have shown, you should see around .1nF of capacitance. Make sure the nail makes contact with the salt water in the jar. Disconnect one of the multimeter leads and use the "Relative" button to zero out any stray capacitance, then connect it back to the capacitor lead.

-Jim

 

[Aggie]

You're using the meter incorrectly..
The picture is still not clear, but clear enough that I see that the capacitor is not supposed to be attached to the leads but put into the socket near the cap' range.
The meter test the capacitors at 2.5Hz as stated in the manual. The mass of the cap' is irrelevant.

A glass bottle has a whole lot thicker walls so it'll give a whole lot less capacitance but may be able to withstand a whole lot higher voltage.
Physical tables will list the electrical properties of different insulating materials.

And we have a WINNER!!

That was the issue. I thought (incorrectly of course) that you could use either the leads or the insert port.

Centrifuge tube is measuring 0.081nF!

Thanks again!

 

[Aggie]

You are trying to build and test a Leydon Jar (300 or so years prior to Tesla). Google that for more info. People use these for Tesla coils sometimes. The capacitance is strictly a function of surface area of your outside aluminum foil and the thickness of the bottle. I build these all the time for a variety of classes. From what you have shown, you should see around .1nF of capacitance. Make sure the nail makes contact with the salt water in the jar. Disconnect one of the multimeter leads and use the "Relative" button to zero out any stray capacitance, then connect it back to the capacitor lead.

-Jim

It's the high-voltage fun research that brought me to the SWC idea. I have been reading about Leyden jars as well. Thanks for the reassurance i was doing something right with this.

 

[JimW]

We build a few historic Tesla coils for fun at a museum I work at. We generally use glass plates with aluminum foil sandwiched in between. It is a lot easier to get to the capacitance level desired. Heavy though...

-Jim

 

[crond]

We build a few historic Tesla coils for fun at a museum I work at. We generally use glass plates with aluminum foil sandwiched in between. It is a lot easier to get to the capacitance level desired. Heavy though...

-Jim

I have buit a few SWC and i am charging them with a flyback and they hold quite a charge. I am not sure of the capacitance but i am getiing 1.5 - 2 inch arcs. Also wiring two of these SWC in parallel and attaching them to the flyback I am getting very loud bright arcs. I plan on building a tesla coil soon and I am going to try to use SWC's for the tank cap. Any comments or suggestions are welcome.
And as always be careful playing with high voltage! : )