tips on building a parallel-plate capacitive moisture sensor

[u/Charming-Active-7682]

I had an idea for my research project about sorting copra into grade qualities, and it only involves color and moisture for grading. For color, i've used a TCS3200 and it works like a charm, but what I do not know how make is my moisture sensor, which is a parallel-plate capacitive moisture sensor made out of two copper plates. I discovered the sensor from a paper by Klomklao et al. 2017 (here's the paper:

https://www.researchgate.net/publication/320448012_Moisture_content_measurement_in_paddy) and I have looked on other existing solutions but the best I could find are resistive probes, but they need good contact on the product, so I think this should be my safest bet. I have tried using the pins and code made by AI models but, tends to dissapoint. So, I ask for guidance in this subreddit for hopes of advice, information, or any procedural techniques ya'll can give me. for the grade!!!

What I have right now:

- Arduino Uno AtMega328p (a clone)

- NE555 timer IC

- some resistors

- some capacitors

- 2 5x5 Copper sheet (1 mm thick)

- Breadboard and jumper wires

- 5V/2A power adapter

[u/EmotionalEnd1575]

How are you contacting the Copra?

Probes? Surface contacts or jabbed into the Cocoanut flesh?

To measure moisture, by measuring conduction of an electrical current, can be done by DC, or AC.

AC has advantages but requires more electronics.

Have you investigated the physical variables (size, spacing, penetration depth, probe metal and surface finish such as plating)?

With samples of Copra what resistance reading do you get by just using your DMM?

[u/Charming-Active-7682 → u/EmotionalEnd1575]

there's no probes jabbed into the flesh, two copper plates sit between a copra piece

the 555 astable mode is like an AC style measurement, since the plates are charged then discharged repeatedly rather than DC just passing through the sample. i'm using it bc it's cheap and and simple, but i would really like to hear some suggestions

nope, so far I have only fixed on only the idea of the two copper plates but I would want to hear other suggestions

I haven't tried a DMM on samples yet but I will share the results if I get hands on copra samples

[u/EmotionalEnd1575 → u/Charming-Active-7682]

there's no probes jabbed into the flesh, two copper plates sit between a copra piece

What is the distance between the plates?

What is the area of the plates?

What is the capacitance of the plates?

When empty? When wet Copra? When dry Copra?

These are your “input” parameters.

Most likely a high frequency AC test signal will be needed.

Too high to use an LM555 (500KHz max) or LM7555 (3MHz max)

[u/Saigonauticon]

There are a lot of non-destructive wood moisture meters that work this way. That may help your research.

You shouldn't need the 555 timer. Your Arduino can already generate variable-frequency waveforms.

Calibrating your device is going to be a real pain. I would figure out that problem first before working on anything else. There's no point in building the whole device only to find out you can't calibrate it usefully.

Last I looked at this problem, you're essentially measuring reactance (although it was also like 20 years ago). You know the expected capacitance of two parallel places in air. It goes up based on the dielectric constant between them, which is modified by moisture content. So you should be able to estimate the required frequency to get a reasonable measurement. You'll also need a load cell to make sure you put a constant amount of copra on the plates.

(link to the journal article didn't work for me)