A change in the dielectric constant changes the current or voltage?

[sabishaw]

hi everyone!
i am not into physics, but need it now,i am a programmer workin on an IOT project...so kinda need help.
i want to use a capacitor to see what passed between two plates...knowing, it is the change in dielectric after certain intervals...how do i measure the change at the other end...should i see the change in current or change in voltage, while not disconnecting the source power...
thanks

 

[Baluncore]

Welcome to PF.

The capacitor may be maintained at a fixed voltage, while current flows to maintain the voltage.
Fundamental equations; C = Q / V, where Q is charge; Q = I * t.

Will the dielectric be a moving continuous insulated belt, or be a non-conductive liquid?
If charge is removed with the moving dielectric, I = Q / t.

We need a better idea of what you are intending to do.
How many capacitors?

 

[sabishaw]

thanks Baluncore for your warm welcome and reply
actually i am planning to make long capacitive soil moisture sensor...
where i will be collecting data by calculating change in dielectric, for i recently learned that change in wetness leads to change in dielectric constant...am i right??

 

[Baluncore]

Yes, you are on the right track. It has been considered before. Have you searched the literature?

Liquid water has a dielectric constant of about Er = 80 times free space, very easy to detect. If I remember correctly, as ice it is much lower, say Er = 3.5, similar to dry soil. When water is bound in a tissue, or in clay, the constant will be closer to maybe Er = 30.

One problem will be arranging the capacitor plates to have soil between them. You will also need to consider how you will measure changes with depth. Will you use a long thin capacitor to approximate an average, or will you lower two plates of a capacitor, one plate down each of two parallel test holes?

You might start by experimenting with a simple oscillator that has period proportional to capacitance. You will need to insulate the plates from the soil so salt conductivity and corrosion do not cause problems.

You must also consider why you want to make the measurement. Water available to plants will be held in fine mineral grain and organic matter. Depending on soil pH, plants may have trouble extracting water from clays. There is no such thing as a standard soil, it is different everywhere.

Precision systems use a neutron probe to measure total water content, that includes water unavailable to plants. https://en.wikipedia.org/wiki/Neutron_probe

 

[sabishaw]

thanks Baluncore for your reply...i will be adding other sensors along with this sensors...like pH,temp...that will make it work, i guess...neutron probe will make it an expensive product... and a few points you made made me to think about a few things...anyways...should i calculate change in current or voltage...

 

[Baluncore]

Neither current nor voltage.
You should make an oscillator and count the frequency.
What type of electronic controller, recorder or display will you use ?

 

[sabishaw]

in my experiments i will be using a multi meter...
for my project node MCU
...could you please tell me what and how and why make osc ... and frequency,...
i didnt quite understand it

 

[Nik_2213]

If you do not insulate the capacitor plates from the medium, you'll have all sorts of problems with conductivity, electrolysis, corrosion, lime-scale, bio-fouling etc.

So, given two thinly but sufficiently insulated metal plates with a medium between, the problem becomes reliably measuring the capacitance.

IMHO, unless you have a Wheatstone Bridge handy, the easiest way is to incorporate your capacitor in an electronic oscillator and measure the frequency. This may display analogue, digital or both. Former gives you 'at a glance' like car speedometer, latter is 'logging data'. Calibration to your taste, due care for temperature effects...

Although you could go the 'Theremin' route, with a beat-frequency between 'base' and 'sensor' RF oscillators, a 555 / 7555 chip is probably easier, a two-transistor 'flip-flop' 'multivibrator' minimal. Doing it with an OpAmp may be over-kill at this stage. YMMV...

OT: Long, long ago and far away, I built an elegant 'Santa Claus' detector for my young brother, using a simple RF oscillator on one of my solder-less S-Dec 'breadboards', but fitting a germanium audio transistor. Wired between bedroom's metal door handle and some kitchen foil under the mat, there was insufficient capacitance for the unfortunate transistor to oscillate as-is. Some-one touching the door handle, either side, sufficed. Even touching it wearing thick, perhaps sleigh-driving gloves sufficed...

 

[Baluncore]

in my experiments i will be using a multi meter...
for my project node MCU
...could you please tell me what and how and why make osc ... and frequency,...
i didnt quite understand it

google 'capacitive soil moisture circuit' to find existing technology.
You will find capacitive probes and oscillators of various price and quality.

An RC oscillator, such as an LM555, has a period proportional to capacitance. Use a multimeter that includes a frequency measurement range. Frequency will be inversely proportional to capacitance.
Alternatively, you can build a significantly more complex circuit that generates a voltage proportional to oscillator period.

https://www.naturalresources.sa.gov.au/samurraydarlingbasin/publications/measuring-water-in-soil-capacitance

 

[sabishaw]

Thanks everybody...
it looks i am at an awesome place to find help...
as for insulating my device...i am thinking of varnishing or painting(waterproofing) the plates of my capacitor plates...
for buying capacitive moisture sensors...i already have those...but now i want to make a long one...which is not even available in the market...
so got to make my own...also i think, if can make it locally i can put a huge bar on controlling my device's cost...
now, i was surfing the net and learning about LM555 ...it got me real confused for i am not really that good with circuits...
googling the circuit of soil moisture sensor...didn't yield anything relevant...
https://www.google.com/search?hl=en...1j3j1j0j1...0...1..gws-wiz...0i71.BjP4wqjf2VU
i wasnt able to find how would i hook up LM555 to my requirements...can someone guide me to hookin up LM555 or any oscillator to my sensor...so that i would finally get an output pin whose change in voltage, i would calculate using my controller...
thanks again for these prompting replies

 

[sabishaw]

please see if this is what i need...thanks

https://www.flickr.com/photos/146752278@N07/shares/w255ux

 

[Baluncore]

...so that i would finally get an output pin whose change in voltage, i would calculate using my controller...

I fail to see why you are pursuing this project, judging by your insistence that you have a voltage output to a multimeter. An automatic system would work better, over a wider range with a frequency output rather than a lower resolution voltage output. If it is not part of an automatic control system then you can avoid electronics completely.

We cannot teach you electronic engineering in one thread. If you do not have the tools and understanding necessary to assemble a circuit-board then you should not be attempting to build an interface circuit. You should instead buy a finished module or solution. You might consider an Arduino based system. Search the web for things like …
https://www.instructables.com/id/Arduino-Soil-Moisture-Sensor/

You will need independent sensors at different depths to get a vertical profile. A long thin electrode will not give meaningful results since the bottom may be underwater while the top is quite dry.

 

[sabishaw]

I fail to see why you are pursuing this project, judging by your insistence that you have a voltage output to a multimeter. An automatic system would work better, over a wider range with a frequency output rather than a lower resolution voltage output. If it is not part of an automatic control system then you can avoid electronics completely.

We cannot teach you electronic engineering in one thread. If you do not have the tools and understanding necessary to assemble a circuit-board then you should not be attempting to build an interface circuit. You should instead buy a finished module or solution. You might consider an Arduino based system. Search the web for things like …
https://www.instructables.com/id/Arduino-Soil-Moisture-Sensor/

You will need independent sensors at different depths to get a vertical profile. A long thin electrode will not give meaningful results since the bottom may be underwater while the top is quite dry.

thanks for your reply...
ya ...indeed it all about automation ...i just want to create a long soil moisture sensor... while get the data via my nodeMCU (microcontroller)...
i don't want to go into electrical engineering either... i just want to make a sensor all by myself...
i already used these toys ...but ya...coding part...
so is the circuit, i uploaded the pic, workable..
thanks again
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