Introduction: Comparison of Capacitive Soil Probes
This is not an instructable as you will most find here, it is a comparison between two projects -capacitive soil sensors- found here on instructables and as such it does deserve a place here.
In february I published a capacitive soil moisture probe in which the capacitor consisted of some scrap pcb that effectively formed a double sided PCB.
Then in May I came upon another capacitive soil moisture probing project by acolomitchi. His probe design was a more 'interlocking' pattern etched on a PCB.
We got into a discussion on the best design and he graciously offered to send me one of his probes to test.
He used an oscillator based on a the good old '555', in a simple astable mode, whereas I used a feedback oscillator based on an HCT14 which is basically a Schmitt trigger inverter. There probably isnt much difference between those two oscillators but we were mostly interested in the difference in probe design. For both oscillators T=0.67*RC. (for HCTversion) (and 0.8*RC for the HC version)
When I was 'designing' my probe, I just wanted a cheap one quickly and therefore the intricate design of interlocking pads as used by others was out of the question. I hesitated between two designs: a single sided piecen of pvc with two seperate 'plates' on it, or a double sided piece with the two capacitor plates at opposite sides. As I figured the dielectrum formed by the pertinax or polyester in between the copper would be less than that formed by the surrounding soil if humid, while with a single sided design due to the small gap the capacitor might be more sensitive but have a smaller range, I chose for the dubblesided design.
The picture as shown by acolomitchi tells the opposite with the dubbelsided design with plates of opposite polarity back to back, most of the capacitance would be formed by the pcb material between the copper and a much lesser part by the surrounding soil.
This was what actually sparked our idea to do a comparative test. Colomitchi was kind enough to send me one of his probes as well as a pcb for the circuit. This arrived soon enough.
The probe was well made, rather sleek and measured 19x97mm. Definitely smaller than my probe and I thought it would be unfair to compare those two sizes, so I made new one, same size as the colomitchi probe.
Obviously, mine had more copper surface, but as I was testing two designs, I thought it to be fair to test comparable PCB sizes.
Before my comparative tests could start I first had to find a way to coat my new sensor again. That in itself proved to be an entire undertaking.
- heat shrink
I had covered my previous,bigger, probe with heat shrink, but when i did that with a smaller probe, the heat shrink proved too thick to provide for a decent capacity of my probe. - Spray on laquer
Initially that seemed to work fine, but halfway through the test the laquer seemed to start leaking. Not being deterred and after some back and forth with Adrian, I tried 2 coats but with a decent curing by the sun in between. That held a bit longer but also started leaking - nailpolish
Nailpolish seemed promising. After all, i use that too to touch up a PCB before etching. However even after a thorough bake in the sun that didnt hold up when put into water. I tried another nailpolish that specifically required curing under UV light, applied that, baked it in full sun but no avail. Also that gave in to being immersed in water. - laquer
I finally tried a laquer that I had previously used on wood. I think it was a regular Flexa high gloss wood laquer, but it was of the 'old' variety. Meaning that it was not water based, but with a decent solvent. I immersed the probe in the laqcer and hung it out to dry in the sun for a day and gave it another coat. It formed into a rather tough coat that indeed turned out to hold well in water. - PCB soldermask/paint
A bit too late for this test, I got my hands on some soldermask to coat PCB's. After proper curing this also held up pretty well.
The testing could begin. As we had exceptionally much rain, testing it outside was not really an option, So I decided to test in a 20 cm diameter (3.7 liters) pot, filled with regular potting soil. But first some open tests to set some baselines.
Fully immersed in water both probes measured around 4 nF. In open air it was verging on the limit of my measurement range, but it might have been around 50pF.
The soil that I used is only slightly moist when it comes out of the bag. In that environment my probe measured 300, His probe 500pF. Then started to add equal amounts of water. 50 ml at the rim of the pot (so at about 10 cm distance. No clear immediate effect. I waited and then something interesting happened. "My" sensor started to react after about 30 secs, but only minimal. It became 400pF, but after about 40 secs Colomitchi's sensor kicked in. Mine had gone up to 450 by then and his went to 630 and then both stayed around that value for a while.
Added another 50 ml and as the path obviously had been wetted by the previous amount, both probes went up. His to 1.2 nF, mine to 900pF. adding another 50 ml sent his up to 3nF and mine to 1.6nF. Till now I was pouring the water all at one side, directly facing the plates. So, as the soil at one side was rather wet, decided to pour 50m at the otherside. That had a dramatic effect on my probe, while his went up to 3.2 nF, mine went up to 3.5. adding another 50 ml at that second site made both probes go up till about 4. adding more water did not have a significant effect.
HIS MINE
500 300
50ml 500 320
500 400 (after 30 secs)
630 450 (after 40 secs)
50ml 1200 900
50ml 3000 1600
50ml 3200 3500 (poured at opposite side)
50ml 4000 4000 (poured at opposite side)
Then I decided to repeat (my floor really has become a mess) the test with fresh soil, but this time pour the water facing the edge of the probes. Starting values again were practically the same. Pouring 50 ml again each time.
HIS MINE
500 320
50 ml 500 320 after about 1 min 550 -450
50 ml 600 450
50 ml 1300 1200
50 ml 2500 2600
50 ml 3900 3900 (climbing to 4nF)
That prompted me to try some rather dry and compact sand and pour water at one side: facing the broadside HIS MINE
-- --
50 ml -- -- waited 5 min... same result
50 ml -- -- waited 5 min : 400 --
50 ml 590 --
50 ml 800 --
50 ml 1000 --
50 ml 1700 300
50 ml 2000 350 Waited 10 min 1900 350
OK time to pour some water at the opposite side
50 ml 2000- 400
50 ml 2100-1700
50 ml 2250-2100
50 ml 3000-3000
50 ml 4000-3900
Well which probe is better???? :-)
Not completely surprising my probe is very directionally sensitive. When water comes from one side, his probe already has half of its plates being able to 'act', whereas my probe doesn't. Now if the soil is a bit moist, that isnt such a problem as my probe's backplate takes part in the dielectrum as well, but in very dry soil there is just no decent dielectrum between my probe's two plates (or better around the my probe's two plates) even though one side of the soil may already by moist. So... when it rains.... I think both probes go up equal (but then there is no need for irrigation) but that if irrigation comes from one particular spot, my probe may let irrigation on for longer than necessary, but ofcourse that is all a matter of choosing the right level to switch off. However his probe's linearity is better. for my probe the same amount of water can cause a jump from 350-400 or from 400-1700 So I think my probe can definitetly see a good difference between dry and wet, but if the soil is very dry to begin with it may have some trouble differentiating between wet and very wet. Now obviously with an irrigator in place the soil is not intended to get very dry, but still my probe may be a bit better suited to differentiate between, "yeah it is time to irrigate" and "it is really really time to irrigate."
I am still a bit baffled by the first result in which my probe seemed to earlier sense the water, but that could well have been an artefact.
Conclusion
It is hard to say which probe is 'better'. But my guess would be acolomitchi's probe. However, the most important thing is that both probes can distinguish between humidity levels pretty well and they are not saturated in the range they are supposed to measure. My recommendation though would be acolomitchi's sensor rather than mine. Mine may seem cheaper, but colomitchi's probe, coming from a fab house, is already covered with a proper coating, whereas for mine you would need to coat it yourself and you may have a few misses there.
