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Dielectric Constant Meter

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LAN_403

Sulaiman

Sat Feb 20 2010, 08:11AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I don't know if it helps, but amongst amateur radio experimenters this capacitance (and inductance) meter

is highly regarded.

oucivileng

Mon Feb 22 2010, 04:48PM

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

Radiotech:

The would not be going into the chamber. I would just put the parallel plates in the chamber and run wires to the outside. If I were to get too much of a leak by just running the wires over the rubber seal, I'd just connect it to two bolts in the acrylic lid.

I've been searching for that article you recommended but cannot find it anywhere. I can find it cited in about a dozen other articles (none of which are relevant to what I'm doing unfortunately) but the actual text eludes me.

Klugesmith:

Yeah, I don't know why I didn't think about Ca being so close to a vacuum. I need this to be accurate only to about a tenth. Thanks for bringing that to my attention.

Proud Mary

Mon Feb 22 2010, 05:19PM

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

Why not just cover two microscope slides with adhesive metal foil, and put them in two adjacent slots in an ordinary slide-staining trough. Measure C with the dry trough, and then pour in your liquid goop, and see if this alters the permittivity in a significant way. If initial results look promising, you could work up the experiment to improve accuracy and repeatability.

radiotech

Mon Feb 22 2010, 05:44PM

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

Stuff is hard to find. I realized that the files may be preprints so here is image of first part.
Face identifies J Research NBS .

oucivileng

Mon Feb 22 2010, 08:47PM

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

That's some great stuff. I think it will be a tremendous help. I really appreciate it!

oucivileng

Tue Mar 09 2010, 06:54PM

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

I'm having some problems. I'm testing out my bridge circuit using a known capacitance and am unable to balance the bridge.

I'm using some very precise potentiometers (10 turn!) to vary the resistance and an oscilloscope to find the null point.

My AC source is an old EICO 324 signal generator that I bought on ebay and repaired. The signal is clear and the frequencies are surprisingly very accurate. When I hook it up to my bridge circuit however, the signal shown on the scope is fairly noisy. Could this be the reason that I can't find null? Would a higher voltage AC source solve my problem? Could I just plug a variac into the wall and use that as my source?

radiotech

Tue Mar 09 2010, 07:31PM

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

The Eico 324 is RF signal generator with very low voltage output
and an oscillator made to produce overtones for extending the dial ranges. What F are you using? Also either the null detector of a
bridge or the generator would have to be balanced input/output. The capacitance bridges use "transformer bridges with specially wound secondaries. The 324 has an internal AF modulation oscillator
again, the waveform might be a problem.

Back to the 324, what was you signal level at the null detector?

if you go to 60 Hz, you can get more level but the reactance would be very high in the uufd range.

radiotech

Wed Mar 10 2010, 12:14AM

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

Your post seemed to indicate you are using the RF output; There is the modulating signal available too about 5 volts/10Kohms, ~~400 Hz

hope edit works!

teravolt

Wed Mar 10 2010, 03:27AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

if your fluke is a newwer model can you get it repaired? genraly they are prety good about it. I found this for 134$

it has a 11000 count resilution.
if you want to contiue with your bridge the caps that you use should be NPO so they don't drift with temprature. Is it pausible to just look up the dialectric constant of your fluid some one must have checked it?

oucivileng

Wed Mar 10 2010, 04:49AM

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

Radiotech:

Thank you for that last post! I switched to AF out (431Hz) and my bridge worked. I don't know why I didn't try it. Live and learn i guess. I was able to change the resistance until I found a spot where the scope more or less changed to a flat line. I measured a 470pF cap (1% tol) at 481pF. I know I could have been much more accurate if there wasn't so much noise on the scope. Could I just put in a low pass RC filter or something to clear that up?

Teravolt:

Our fluke 8846 is fairly new but not new enough to be covered under the warranty. They want something like $600 to repair it. They're about $1000 new so we decided we might as well buy a new one. I should mention that it's the CE department at my university buying it, not me.

I'm dealing mostly with mixtures of chemicals so I can't just look up the dielectric constant.

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