Capacitive moisture sensor ideal frequency

[hal9001]

How to calculate the ideal frequency to measure moisture in soil by capacitance? The paper from nih.gov chose 430 khz because this is the fastest a 555 can output but what is the best frequency for most sensitive measurement?

An integrated circuit (IC) oscillator, the 555 IC, was selected to be used in a stable mode that outputs a square wave frequency of 430 kHz with an approximate duty cycle of 55%. This frequency was chosen to suit the maximum frequency that can be generated from the low cost 555 IC that was selected and to align with the cut-off frequency for the low-pass filter.

https://pmc.ncbi.nlm.nih.gov/articles/PMC8399650/

[Sensorcat]

In capacitive moisture measurement, you have a capacitor with some amount of water in the dielectric material. Since water has a very high relative permittivity, the capacitance of electrodes put in soil is dominated by water concentration. The good news is: This is true for a frequency range from very low (Hz) to at least 1 GHz. So, nothing to worry about, moisture will in principle be detectable at any practical frequency. In consequence, you can choose your measurement frequency entirely from circuit considerations.

Depending on the geometry of your sensing capacitor, capacitance will perhaps be quite low. Hence, choosing a high frequency f has the advantage of a large voltage drop U across the capacitor C at a given measurement current I: ω = 2πf, Z = 1/(ωC), U = IZ.

OTOH, measurement at high frequencies is generally more difficult, especially if long wires are involved and you have to take parasitics into account. This is less of a problem if the measurement circuit sits close (mm) to the capacitor.

Not knowing the circuit you have in mind, the distance between circuit and capacitor, the electrode geometry, and the actual application, I would say that a freqency of several hundred kHz is not a bad choice. If you want a more precise recommendation, its necessary that you provide more detail. Do you already have capacitor and circuit, or do you want to design them?

[hal9001]

In capacitive moisture measurement, you have a capacitor with some amount of water in the dielectric material. Since water has a very high relative permittivity, the capacitance of electrodes put in soil is dominated by water concentration. The good news is: This is true for a frequency range from very low (Hz) to at least 1 GHz. So, nothing to worry about, moisture will in principle be detectable at any practical frequency. In consequence, you can choose your measurement frequency entirely from circuit considerations.

Depending on the geometry of your sensing capacitor, capacitance will perhaps be quite low. Hence, choosing a high frequency f has the advantage of a large voltage drop U across the capacitor C at a given measurement current I: ω = 2πf, Z = 1/(ωC), U = IZ.

OTOH, measurement at high frequencies is generally more difficult, especially if long wires are involved and you have to take parasitics into account. This is less of a problem if the measurement circuit sits close (mm) to the capacitor.

Not knowing the circuit you have in mind, the distance between circuit and capacitor, the electrode geometry, and the actual application, I would say that a freqency of several hundred kHz is not a bad choice. If you want a more precise recommendation, its necessary that you provide more detail. Do you already have capacitor and circuit, or do you want to design them?

Thanks for explaining. I want to understand these cheap capacitive sensors. The schematics I find online also show a 555 at 430 khz. The sensing capacitor is 2 plates on the pcb, probably low capacitance. I wonder if a higher frequency than 430 khz will give better sensing for this same pcb but 430 khz is used because 555 is cheap?

[Whales]

I wonder if a higher frequency than 430 khz will give better sensing for this same pcb but 430 khz is used because 555 is cheap?

Most likely.

Too low of a frequency: not much current flows, so it's hard to measure and sense.

Too high of a frequency: design gets more complicated and expensive; and too much current might flow (which might saturate your measurement method).

There are some materials and circumstances where capacitance vs frequency is nonlinear, so you want to be aware of those and avoid them.  This is common in situations where your electrodes will chemically react with their environment.  Best way to find out is to try.

[Sensorcat]

I wonder if a higher frequency than 430 khz will give better sensing for this same pcb but 430 khz is used because 555 is cheap?

Because it's cheap, everybody knows it, and there are a ton of descriptions how to make it an oscillator.

The PBC-based sensors are actually not bad, because the copper is usually covered by solder mask. This makes another 2 capacitors in series with the measurement capacitor. Since solder maks is thin, the capacitance is high and the whole sandwich will be dominated by the soil capacitance. Solder mask is a very good insulator, so any DC component is blocked - and copper is not wetted with soil. This avoids corrosion and copper ions in the soil. I do not know if the latter is important, but in general, plants can be poisoned by copper ions. Next steps would be to look at the schematic and the surface area of the electrodes, such that the capacitance can be estimated, it not already given.

[floobydust]

You need to know what the dielectric is, as far as an upper frequency limit. OP you can search for it with soil+water.

[TC]

As others have stated... higher frequencies can make the circuit more difficult. Parasitics come in to play at some point.

But there is another factor to consider and that is ... how do you plan to implement your sensor measurement?

Are you going to drive the sensor with a fixed frequency and measure the amplitude and/or phase?
Are you going to use the sensor capacitance in an oscillator circuit and measure the frequency?

I think that when you consider the circuit details of making the sensor measurement you might see that higher frequencies can make the circuit more difficult to design. It depends on your measurement approach, the accuracy you hope to achieve, the quality/expense of the circuits you implement, and your skill as a designer.