Hi, am facing a strange issue. In brief, the objective is to measure di-electric constant of a varnish film (approx 30micron) applied to a copper plate. To achieve this, i have made a capacitance meter (using a CC Bridge method) where the electrodes are two brass plates of 11cm diameter, between these i keep a copper plate with varnish applied to one side.

1. Electrodes (11cm diameter)


2. Copper plate with one side varnished


3. Electrodes with plate in between





Based on theory – i should get a capacitance of around 3500 pf, however, what i get is only approx 600pf. I have tested the setup using paper as the dielectic (80 micron thickness) – again same problem, capacitance is much lower than expected.

I have tested the capacitance meter using various commerical capacitors in 470- 10000 pf range, the meter is accurate.

What am i doing wrong in either the electode setup or the measurement method itself? please advise...

 

Just a guess ...but surely the surface finish of the metal is all important ...I can see scratches on the surface... parallel mountain ranges many microns in height...

I would think you need a mirror finish then you have close to two flat surfaces facing one another upon which the charge resides .

 

I would think you need a mirror finish then you have close to two flat surfaces facing one another upon which the charge resides .

Thanks - i will try after buffing the electodes and copper plate

 

Electrically-conductive grease on the electrodes might help.

 

Bridges (at least any I have seen until now) are designed to have a rough result in rather large tan(dellta) value. For higher frequencies and small loss factor the thingy need for exact measurement is the VNA. In order to cope with a tiny budget, there are two options - 137-2700 MHz region the one port N1201SA (130 USD) or its competitor 50 kHz-900 MHz (but 2-ports) Nano-VNA for about 50 USD.

 

By the way - paper is damn bad isolator by loss factor. Please look into neighbours theme where yesterday I gave an extensive set for different dielectrics, polar and non-polar, loss factor.
2) Do Your trodes must be movable, or cap will be stationary. If last, there are not bad option to use a pcb (about 2 pF per cm2). Contrary, if the frequency is sub kHz but voltage is the alter priority, very good results may be obtained by using a monotytic PVC carpet rolls. Just apply the metal film be rolled in-between and 40kV-80kV per 3 mm layer is warranted. However not suitable for large frequency. Not suitable for air-filled porous PVC carpet.
3) The cap formula is designed for rather large sized caps. If the platelet length become to concurrent with the distance (ie becomes larger than some 1/100 or least 1/10 of length, then fringent field emissions out of cap "ends" become prevailing over the Area effect, thus the cap formula gives a tenfold or even larger mistake. Thus, if the expected moving distance is large, just dont be surprised.

 

Do Your trodes must be movable, or cap will be stationary. If last, there are not bad option to use a pcb (about 2 pF per cm2).

Yes - the electrodes do need to be movable, since differnet samples of varnish need to be tested. For this fresh copper plates are used with varnish applied to one side, which is then inserted in between the two brass plates.

The cap formula is designed for rather large sized caps. If the platelet length become to concurrent with the distance (ie becomes larger than some 1/100 or least 1/10 of length, then fringent field emissions out of cap "ends" become prevalate over the Area effect, thus the cap formula gives a tenfold or even larger mistake. Thus, if the expected moving distance is large, just dont be surprized.

The plate length (or diameter) is large ; dia 11 cm, area 95cm2. the thinkness of the dielectic (varnish) is only around 30 micron. therefore length is much much greater than distance.

 

Electrically-conductive grease on the electrodes might help.

i assume this wont count as another layer of dielectric since its electrically conductive ?

 

For higher frequencies and small loss factor the thingy need for exact measurement is the VNA. In order to cope with a tiny budget, there are two options - 137-2700 MHz region the one port N1201SA (130 USD) or its competitor 50 kHz-900 MHz (but 2-ports) Nano-VNA for about 50 USD.

I only need to measure around 1KHz, is a Vector Network Analyser still needed ? or are there other methods to determine dielectic constant more accurately?

 

Electrically-conductive grease on the electrodes might help.

Will EEG gel be suitable ?

 

The 1 kHz is sure to tiny for any VNA. Then the AC bridge is a thing be used. Grease - for what? Be sure the screaming sound will happen only if both plates will contact for short-circuit.
Aah, the reason is to make a measurements f the paint quality? Yes, then the most wise way would be push between paint layer and upper electrode the sub-mm thick plastic film with as much as possible high epsilon. Simply it will warranty the micro-pores will not cause the short-circuit but itself will not influence the paint layer thickness readings.
By the way, as I bought at Biltema some lacker thickness measures for so cheap as few Eur, isnt it worth better to buy the professional device??

 

The scratches, vaults, hills etc on the varnish will create the three-layer pie model. Pie consists on 1st layer 30 micron, 2nd layer the air but thickness is ca few microns and thin mechanically soft high capacitance layer for better contact.
Thus three caps are in series first is dominant second is so large that not much disturbs a first reading, and third must be much larger than first as well. However, I doubt is on the ral car any pach being glade so larg as 10x10 cm? Probably better is to diminish the size of apparatus to tenfold smaller. Mine has about 3x3 mm trodes.

 

PPS - what heck is organic varnish. Organic chemistry is the science about the hydrocarbons. Any imaginable resin is hydrocarbon and most of the pigments as well. Thus from inorganic (pure mineral) paints I may label only guash. Chalk and carbon anyway needs a sort of organic resin.

 

Then the AC bridge is a thing be used

can you please suggest some ideas / circuits which could perhaps be integrated with a small MCU like an arduino?

 

Probably better is to diminish the size of apparatus to tenfold smaller. Mine has about 3x3 mm trodes.

Could you perhaps share a model / measurment technique used ?

 

Thus, do Your aim is to measure the thickness?? (means fraud de-masking and later paint repair places detection?? Then ideal instrument is something like this. Only the difference that mine have an another color itself https://www.ebay.com/itm/LCD-Digita...258773&hash=item3b2a8f7b51:g:cPgAAOSweEFcX5c1

 

Techniques - just push electrodes tight to the surface and read the thickness. If somewhere is the pach with bigger thicknss - means here was the impact beat, later worked by putty and paint. Car with beats has price 10x cheaper as non-beated.

 

Your aim is to measure the thickness??

no, my aim is to eventually measure the dielectic constant, by first measuring the capcitance value!

 

Will EEG gel be suitable ?

I don't know. Try it (as a very thin smear) and see if you get cap values closer to the calculated ones.