# Soil moisture sensor - Interdigital (FEF) vs Coplanar

I am working on the development of a low cost capacitive soil moisture sensor which uses dielectric permittivity changes to estimate moisture.

I found that interdigital capacitor based on FR-4 has more capacitance as opposed to coplanar one of the same size. However, I need some perspective as to the range of electric field each will emit since it determines my sensing range. Insights from practical experiences of users can help me asess this situation better.

A picture of the two probes

Edit 1 :

I am currently experimenting with the operating frequency to be used but it will be in 5-10 Mhz range.What I gather from research papers is that pcb on a capacitor can work as soil moisture probe due its fringing field. The values that I got - two parallel capacitors around 14pf , FEF (interdigital) - around 25pf

Soil thickness I am intend to measure moisture for is about 0.5m

• why such narrow gaps? its the soil you want to measure not a small gap in the soil with large stray coupling on surface of PCB between conductors. What coating dry or wet film? What frequency (s) will you measure C? Will this have a moisture fuel guage or just some arbitrary threshold? Commented Apr 5, 2018 at 21:08
• What were the actual pF values for each design? Commented Apr 5, 2018 at 22:37
• I am currently experimenting with the operating frequency to be used but it will be in 5-10 Mhz range. I see your point, but from what I gather from research papers is that pcb on a capacitor can work as soil moisture probe due its fringing field. The values that I got - coplanar - around 14pf, FEF (interdigital) - around 25pf Commented Apr 6, 2018 at 9:40
• That's pretty linear with length of gap,, if the gap is a bit wider, you can get the same 14pF using the graph Commented Apr 6, 2018 at 11:32

Epoxy (and nylon) absorb large amounts of water - up to several % Since water has a much higher Er, this makes for a significant change in C due to change in the pcb moisture.

There is a reason that rainfall sensors are made like this, but soil moisture probes tend to be two separate metal rods.

If you were doing it with pcb, I would probably be having the plates well apart with a routed groove between them so that soil is the main dielectric

Soil can behave rather oddly at times for some simple sensors - I guess that random gunk + aqueous ionic solution can look like a battery or an electrolytic capacitor, and not just a simple dielectric material

• Strongly agree. At my opinion, two stainless rods is best variant. Also, low frequency (tens kHz) should be more stable and will not interfere with radio communication. DC blocking capacitors should be used to prevent corrosion. Thus, this circuit will measure impedance, closer to ionic conductivity, not just random capacitance at high frequency. Commented Jun 3, 2022 at 21:12

Both are coplanar. This is not a SAW filter or microwave filter but a lumped capacitance based on copper thickness and W/g ratio of the tracks for any pattern. The range may depend on moisture content but will be approx a few times the gap width.

There is an aperture effect on capacitance where the gap is inversely related to the capacitance whose gap is proportional to height where the perpendicular fringe field travels.

Er for water = 80 approx. and 4 to 4.5 for FR4 depending on glass/epoxy ratio and frequency < 1GHz .

But for edge electrodes, it has less impact being only on one side of the electrodes. The capacitance will depend on the total edge length with the other variables being width/ gap ratio (pu) per unit length to some power series below.

Pure water content is 18 to 20 times the relative dielectric constant Er of the FR4 = 4~4.5 depending on glass/epoxy ratio and frequency.

• When conductor width is reduced , capacitance reduces as well.
• The accuracy depends on the soil moisture content being NOT too far below the inverse ratio 1/20x= < 5% moisture. Otherwise the FR4 dominates.
• When the gap is small, it depends on the moisture being close to the board surface.
• thus once the target moisture content is defined for threshold, an optimap W/g ratio can be found.

• Thin tracks are typically 1nH/mm and for W/g=1 , wide (W) tracks will be much less inductance depending on W/length, and thickness.

What frequency do you plan to use? and what soil thickness do you wish to sample as far as moisture content?

COMSOL is perhaps the best tool for simulation, but stripline calculations can be done based on no ground plane ( or really far away) for a single sided board with co-planar electrodes.

Maxwell computed and drew out the lines of Force of the E Field with equipotential values spreading around the edges, which may apply to the edge view of the board.

Notice how it diminishes when the distance is greater than the gap.

I used Saturn's PCB design tool and choose "differential pairs, edge coupled external ground ( no pun intended ). Note that for big ratios or large gaps, the data accuracy declines and is not reliable.

• Consider the effect of FR4 dielectric impedance is raised with more gap and raised with small electrodes making soil dielectric measurements change over a greater soil width but with smaller values.
• This model of the Track Width to Gap or ** ** will affect the capacitive impedance by this trend for a fixed frquency and geometry track length.

• where Zo=(L/C}^0.5 and Zc= (2πfC)^-1

• Coplanar Striplines, No gnd plane
• top side tracks 18um Cu 35um plating over 1 sided FR4
• Thank you Tony Stewart for your answer, It will take me atleast a day to sink in all the information you provided. I was thinking of using COMSOL. I have made some edits - As per frequency of operation, I am planning to use 5Mhz - 10 Mhz range. I gather that since soil dielectric is complex quantity, measuring using the Mhz range can give me an approximate of its real part. Commented Apr 6, 2018 at 9:51
• of course it's the imaginary part that counts. have you considered sequenced multiple coiled L pads that self resonate when moist with 4 inputs using equal gap,width? using ESL , with low Q with a Amorphous Fluoropolymer PCB coating for thin low moisture absorption and low Er Commented Apr 6, 2018 at 11:50
• Hey Tony, finally was able to wrap my brain around your answer. Can you please point me to resources on the internet that will further assist me in deciding the w/g ratio and more about nature of filed lines of such probes. Commented Apr 8, 2018 at 21:47

it easily explains the effective depth of water that can be sensed by sensor.

it goes like so: T=a*sqrt((1+w/a)^2-1)

where

T: penetration length 2*a: gap distance between two electrodes w: width of each electrode