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Dielectric Constant Meter

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oucivileng

Fri Feb 19 2010, 04:18PM

Registered Member #1772 Joined: Tue Oct 21 2008, 05:23AM
Location: Athens, OH
Posts: 71

I'm trying to build a device to measure the dielectric constant of fluid for some research I'm working on and am struggling with a few things. To determine the constant, I need to measure capacitance between two plates with the fluid as a dielectric. I was originally going to measure the capacitance with a multimeter but someone damaged our nice, expensive Fluke somehow and it no longer works reliably. I've tried using some hand-held multimeters lying around but the reading jumps around too much for me to make an accurate measurement. So instead of fooling around with the malfunctioning Fluke or waiting for the university to replace it, I've decided to put together a capacitance bridge that I connect to my signal generator. To do this, I need some calibration-grade capacitors for my capacitor bridge. Can anyone suggest some capacitors with extremely low tolerances? I've been looking at mouser and newark but it seems like if I find a capacitor with a low tolerance in their online catalogs it's usually a typo. I would prefer to have something with leads that I can work with as I am a civil engineer with limited electrical experience. Also, I am cursed with shaky hands. Please help.

Thanks!

MinorityCarrier

Fri Feb 19 2010, 04:34PM

Registered Member #2123 Joined: Sat May 16 2009, 03:10AM
Location: Bend, Oregon
Posts: 312

What values do you want/need?

I have a pretty good selection of Component Research Corporation 1% film capacitors plus a variety of lower capacitance 1% silver mica capacitors from various mfg's, and am willing to provide some.

radiotech

Fri Feb 19 2010, 05:38PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Is the fluid an insulator or a conductor ?. For insulating fluids, a relatively inexpensive , easy to use meter is the Sencore LC103 Z meter. about $3000, US It also will measure leakage currents up to 1000 volts, with a range from farads to picofarads. It can be interfaced to a computer. Also the LC103 can be calibrated.

If your fluid is an electrolyte, this meter can measure capacity of the interface layer at the plate if it exhibits any polarity.

Get the optional carry case it you need to lug it around. I used the LC102 for 10 years at work and own one myself.

If you do get one for for work, you can use it at home for TC work.

link

oucivileng

Fri Feb 19 2010, 05:58PM

Registered Member #1772 Joined: Tue Oct 21 2008, 05:23AM
Location: Athens, OH
Posts: 71

I was hoping to go for less than 1% but maybe that's not realistic. If I decide to go with 1% I can just order some caps from newark. It'd be easier to get the university to pay for it. Thanks for the offer though.

I'm running into another problem though. I could either use some method of varying one of the resistances to find the null point using an oscilloscope, or I could calculate the unknown capacitance with a voltmeter. I know there are unbalanced wheatstone bridge equations that are used for strain gages. Does anyone know where I can find equations for calculating the unknown capacitance by measuring AC voltage across the bridge?

oucivileng

Fri Feb 19 2010, 06:03PM

Registered Member #1772 Joined: Tue Oct 21 2008, 05:23AM
Location: Athens, OH
Posts: 71

radiotech:
I don't think I can talk my department into spending that kind of money on just a capacitance meter, as great as it may be. I only need to run a few tests. The material is an insulator. It would be good to have one for TC work though. My mmc caught fire a few months ago and needs replaced

I forgot to mention in my previous post that the capacitors will probably have to be 200-400pf.

radiotech

Fri Feb 19 2010, 06:39PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Broadhurst, , Martin G and Bur, Anthony J Two terminal Dielectric Measurements up to 6 x 10^8 Hz . J. Research NBS Engineering and Instrumentation Vol. 69C No.3 July-Sept 1965.

For AC they aren't called Wheatstone, That paper has a lot of references , theory of measurements and also measurements at audio frequencies. The Instrument Society of America stuff is great if it has been made available online by the NBS.

As an illuddirate, paper is still great !

Proud Mary

Fri Feb 19 2010, 06:44PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

A convenient method of determining the static relative permittivity of a non-conductive liquid is to measure the capacitance of a parallel plate cell in air, then again in a pure standard reference liquid like benzene, and then finally the capacitance of the cell when filled with the fluid whose permittivity you want to measure at a given temperature:

Ex = 1 + [(Co â€“ Cx) / (Co â€“ Cr)] x (Er â€“ 1),

in which -

Ex is the unknown static relative permittivity
Co is the capacitance of the cell measured in air
Cr is the cell capacitance using the reference liquid
Cx is the cell capacitance when filled with the unknown liquid.

oucivileng

Fri Feb 19 2010, 07:33PM

Registered Member #1772 Joined: Tue Oct 21 2008, 05:23AM
Location: Athens, OH
Posts: 71

Radiotech:

Yeah, I know they aren't wheatstone for AC, I just couldn't remember the name. I think it might be Schering Bridge or something like that. Thanks for the reference though. I'll look it up.

Proud Mary:

I was planning on just measuring the capacitance of the plates in a vacuum and then just referencing that value:

Ex = Cx/Co

Co being the capacitance of the parallel plate cell in a vacuum.

We have a little vacuum chamber that we use for various soil and asphalt tests that I'm going to try first. If that doesn't work out well, I'll use the method you suggested. Thanks!

radiotech

Fri Feb 19 2010, 08:54PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

That will give you a bogey for that physical arrangement and you can compare it the the permitivity of space which is 8.85 E-12 F/m.

How will you get the capacitance meter into the vacuum chamber. I dont think the electrolytics in it will do well.

The C0 mentioned by ProudMary should be easily calculated by just using the parallel plate radio formulas which will be fairly accurate for the tests you are doing. Then you could borrow a Q meter from your universities EE dept (mentioned in thar ref. previous.

Also reading your posts seems to tell that you are interested in the force side of capacitor work. The AC testing method depends on the admittance which has a reactive component and perhaps hidden ferroelecrtric effects. There are lots of old electrostatic methods which use force balances to determine capacity.

klugesmith

Fri Feb 19 2010, 11:58PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

oucivileng wrote ...
I was planning on just measuring the capacitance of the plates in a vacuum and then just referencing that value:

Ex = Cx/Co

Co being the capacitance of the parallel plate cell in a vacuum.

We have a little vacuum chamber that we use for various soil and asphalt tests that I'm going to try first. If that doesn't work out well, I'll use the method you suggested. Thanks!

Unless you're aiming for ridiculous accuracy, you can skip the vacuum chamber part & measure the test plates in air.
Ca/Co is about 1.0006 at all frequencies you are likely to be using.

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