Best DIY cheap soil moisture sensor
My first version of my cheap soil moisture sensor worked great, but it had a couple of flaws. The first issue was construction. I had great luck on my first attempt, but after trying to recreate additional sensors the thin layer of gypsum between the sensor and the probes made it extremely easy to crack. My success rate on later attempts dropped to about 25%. Beginners luck, apparently.
The second issue was durability. Gypsum suspended in water will eventually break down, and there is not much you can do about that. With my automated grow box handling watering based on moisture readings, I needed measurements that stayed accurate throughout the season. So I increased the sensor size and switched to galvanized nails to prevent rusting. After a few attempts I landed on what feels like a pretty foolproof method.
How it works:
There were a lot of questions in the comments on the previous post, so hopefully this clears things up a bit.
Technically a gypsum block measures soil water tension. When the block is dry, electricity can’t pass between the probes. Infinite resistance. The block is basically an insulator.
As moisture increases, more electrons can pass between the probes, reducing resistance. Fully saturated soil means virtually zero resistance. You read that range of values and you know how wet your soil is. Simple enough once you think about it.
Parts for cheap soil moisture sensor:
- Plaster of Paris
- 2 Galvanized Finish Nails
- 1/2 inch plastic tubing
- utility knife
Construction:
Take your utility knife and cut the tubing slightly longer than your galvanized finishing nails. Try to make the cut as straight as possible, though it doesn’t have to be perfect.
Use your utility knife to cut the smaller plastic tube lengthwise. This lets you peel the mold off the sensor after the plaster cures instead of trying to wrestle it out.
Optional: Make the cut diagonally to prevent a potential vertical fracture line.
If you were careful on your vertical cuts you can skip this step, but to avoid plaster spilling onto my workbench I drilled four holes slightly larger than the tubing. The holes support the tubes upright and catch any plaster that seeps through gaps from less-than-perfect cuts.
Being careful that the tubing fits together where you split it lengthwise, insert the tubes into the holes (or set them carefully on a flat surface). Mix your Plaster of Paris and fill to the top. The friction between the tubing should keep a water-tight seal at the cut. If the plaster is thin and you see it seeping through, wait a couple of minutes for it to set up a bit and try again. At that point it won’t have the viscosity to squeeze through a small gap.
Take your two galvanized nails and push them through a small piece of wax paper. You can also let the plaster set for a few minutes and then float the nails in. I prefer the first method since gravity keeps them falling straight down and parallel. As for spacing, I experimented with different gaps between the probes and my conclusion was: it doesn’t make much difference. As long as they aren’t touching, you should get reliable results.
After letting the sensor cure for about an hour, remove it from the drilled holes.
Gently pull back the plastic tubing and you have a nice clean soil sensor.
Lay them out to dry for 24 hours to cure completely and you’re done with construction.
For attaching the wires you have a couple of options. Soldering is ideal, but you need to get the nail hot enough for a strong connection and my little 15W iron just can’t do it. So I went with wire wrap. Strip about an inch of insulation and wrap the wire tightly around the probe. Since copper can rust and become a failure point, insulate the connection from moisture. A few dabs of hot glue works pretty well. I’ve been meaning to try liquid electrical tape as an alternative, which might hold up better long term.
How to use your cheap soil moisture sensor
You can simply hook up a multimeter and read the resistance. But if you want anything automated you’ll need a microcontroller, something like an Arduino. Apply voltage to one side of the sensor, use a voltage divider circuit connected to ground and an analog input, then measure the voltage coming through. Higher voltage means higher moisture content. That’s the whole trick.
Conclusion
This should give you everything you need to build your own cheap soil moisture sensor and put it to work. I’ve been using mine for indoor grow box watering automation, but the same sensor works just as well buried in an outdoor bed. Hook it into an irrigation controller and you can stop watering on a schedule and start watering based on what the soil actually needs. Tomatoes in particular are sensitive to inconsistent moisture, and anything that helps keep that even is worth the time it takes to build a sensor out of a few cents of plaster and a couple of nails.
One thing worth knowing about PNW winters
If you’re running sensors in outdoor raised beds here in the Pacific Northwest, the near-constant winter rain can actually skew your baseline readings. The gypsum stays saturated for months and your “wet” threshold starts to feel meaningless. I found it helps to pull the sensors out of outdoor beds in November, let them dry out completely, and re-establish your dry and wet resistance values before putting them back in spring. Takes maybe ten minutes and saves you from watering a bed that’s been sitting under grey skies and steady drizzle for six weeks straight.
Also worth noting: the plaster does degrade faster in outdoor soil than in a pot indoors. One season is usually fine. Two seasons and you’ll start to notice the readings drifting. At pennies per sensor, rebuilding them each spring is honestly not a hardship. That might be the cheapest maintenance schedule in the whole garden.
Very very impressive!
Wow, thank you for all these amazing amazing tips and tricks! What very useful info indeed!I will have to stop by frequently and learn a thing or two I am sure of it.Thanks again!
Glad you enjoyed the write up. These are really easy to make and very reliable.
I wonder (actually that much, I'm going to try myself) if you could make capacitive sensors with two wires inside a tube and use the clock of the µC as a source (amplify it with a transistor) and measure the change in capacitance. It is usually quite a high frequency in the MHz-range, so even a small capacitor should show a change.
Ragnar, sounds like a great idea, be sure to let us know how it works out.One other thought I have for these is not to continuously add current to the sensor but instead apply current for a shorter period of time using a transistor and a digital output to break/complete the circuit. This should definitely increase the life of the sensor. Could also use the digital output directly though not sure if it would have enough wattage.
Thank you for this writeup on the improved moisture sensors. It comes at an opportune time, as I have purchased some Plaster of Paris and an Arduino, but haven't made any sensors yet.I was wondering if you used any sort of release agent at the bottom of the hole in the wood. Does the plaster adhere to the wood?Also, I was wondering if different distances between the nails would cause the calibration to be different. Is it important to get the distance the same in each sensor to avoid having to calibrate each one?For turning the power to the sensor on and off, I saw one web site that suggested using a digital pin from the Arduino to provide +5v (sorry, I can't find the link at the moment). With the 10k resistor in series, the current should be well within the 40ma limit for the digital out.
Ken, I haven't had any issues with sticking though adding a little dry gypsum to the bottom could help or a release agent as you mentioned. As for calibration, I had that same concern and did a little experimentation and my findings were it didn't make much difference in the spacing between the probes, assuming they don't touch of course.
Good day, why use gypsum block if we can directly use the two galvanize nails to measure the soil moisture..
fred, the reason you want to use gypsum is one to keep your probes from touching, don't want a false positive that your moisture is just fine because the probes are touching. The specific reason for choosing gypsum is because of soil water salinity. As the block is saturated with water is dissolves some of the gypsum and fill the pore to create calcium sulphate which will provide the same measurement regardless of the salinity of the water. I assuming all inputs are consistent (water, fertilizer, salt content) then you could get away with a couple of galvanized nails in the soil though gypsum will provide better accuracy with these varied conditions.
I made it! After 24 hours saturated in water I made the measurement. With Ohm-meter, I got weird readings, started in 1k and went up to 10k in a few seconds, basically I got an capacitor. Seems like I`m the only one with this sort of reading. After I made the resistor voltage divider, got pretty stable n accurate measures. Had to make sure the "capacitor is discharged" before measuring… got up to 500mV by the two terminals before discharge. Its a month already since I started. I made 2 sensors and read them every couple days, got up to 300mV difference reading between the two, but they are walking side by side!
Luiz, still need to do a post on it but been doing some comparisons between my Vegetronix and homemade sensor and Vegetronix is definitely more consistent though can't beat the price of homemade. Overall wouldn't use this method for scientific measurements though great for knowing when to water 🙂
It would be great if you could post the comparsion between the homemade sensor and the Vegetronix.Merry xmas!//Luiz
Sir,Your idea of the soil moisture sensor is truly truly awesome. I loved the way you've mentioned the construction. Though I have one query that I would want to put forward to you. Can we give the sensor a perforated steel enclosure so that it remains solid over a long period? Just a query, pls do mention the pros and cons of the same.Thank You.
A very Happy New Year to all of you.
I would like to suggest that the 10K Resistor would do better to go between the sensor and the 5V on your board. There exists the possibility that you could short the 5V supply on your board when you bury your sensor in the GROUND if some kind of insulation fails. Not a big deal if it shorts to ground after the 10K current limiting resistor… I think this was an ingenious method for making “accurate enough” soil moisture sensors. I work in the SCADA group for a large company, and we plan to use them in automating our plot in the employee garden. THANKS! -Rob W.
Copying myself from hack-a-day:
Problem: Galvanized or not, DC current will eventually corrode the electrodes as electrolysis will happen. Measuring conductivity requires a more sophisticated circuit that provides for an AC (ie: 1KHz) current. As a side note, measuring with precision also requires a thermometer integrated into the probe, as the resistance changes dramatically with temperature.
It will work, but you will eventually have to replace your probes. Also applying the current for a short time using an IO pin for the voltage source could make the probes to last longer.
The sensor is a great idea, and a nice design, but perhaps it be simplified:
Why not strip an inch or so of zip cord (18 gauge line cord or speaker wire), leaving a bit of paired insulation on the tip. Stick the bared end into the plaster mold.
The spacing should then be very consistent, and the bit of insulation on the tip would keep the pair of wires straight and from touching.
No problem soldering to nails or having a poor contact from wrapping leads around nail heads…
Ivan has it right, an AC supply will ensure long life. His other suggestion to power the sensor for a short period will also increase the life expectancy. @[those claiming to have made a capacitor] I suspect you have just made a battery
Cool! I’ll definitely use that for my GrowBoxBox v2 🙂
I needed to measure relative humidity with a resistive sensor which doesn’t like DC at all. I believe my algorithm can be used for reading your moisture sensor accurately while minimizing the amount of current that flows through it and bringing the DC component to 0. This will definitely keep the sensor’s characteristics constant for much longer. See if you like the idea:
http://1024.cjb.net/2009/10/rh-measurement-resistive-avr/
http://growboxbox.org/lib/exe/fetch.php/growboxbox.ps
http://github.com/kikcho/gbb/blob/master/src/sensor.c
You’re missing a current-limiting resistor on the ADC input of the Arduino. If these things really do go down to 0 ohms, you’ll be pumping 5V supply current directly into the ADC, which will hurt it.
I would be inclined to up your 10K to 20K and add a 10K on the other arm of the potential divider to limit max current into the ADC, although this will affect the response curve.
Why do you need to use the plaster of paris, why not just use the galvanized nails connected to the circuit by themselves and stick them in the soil???
Have had the occasion to test your probe when the soil is *really* wet (like when it’s been raining for two days).
Does the plaster dry quick enough to avoid delivering a false information (if it’s still wet when the soil is dry) ?
Hi, i am working on precision farming project, I am going to make Soil moisture Sensor will you please help me? How to make Gypsum electrode ? and how to connect external wire to that electrode??
Thanks in Advance..
W, you could get away with a couple of nails in the soil method, would work great for pots, though in regular garden good chance of getting moved around. The gypsum also should give better accuracy between degrees of wetness.
Marc, it seems to get a spike when first “watered” but seems to even out as the soil dries. Haven’t really had any issues with it regulating.
Mahesh, I think much of the information you are looking for is above, please let me know in a little more detail where you are confused.
This is sweet and I will be adding these to my current arduino setup which is monitoring air temp and RH and controlling fans atm.
@Mahesh – Really? did you just reply to a how-to asking how to do what it describes? I’m confused… maybe you just want to be spoon fed?
This is almost exactly the same sensor design, casting and voltage divider circuit that I came up with. 🙂
I made mine using brass rods held parallel with some small pieces of polystyrene sheet. I only use DC voltages (Arduino, 5V) on it but I minimize the variance problem by only applying voltage long enough to measure the resistance 8 times, I then average the results. I do this every 5 minutes. This has been showing extremely repeatable results.
What kind of resistance ranges are you getting? I get about 320 Ohms in fully saturated soil and 480 seems to be about the upper limit in a small indoors pot plant after exactly 3 days without any extra water.
I remember making these at university 30 years ago. The main problem we had when monitoring field soil moisture was cows eating the insulation off the wires.
So ,are you leaving them(the probes) permanently in the bed at a certain depth,or just lowering them in the bed as needed? If they are in the bed all the time,how long before the plaster of parts dissolves?
I only have used them indoors, the small ones usually will last a season and I normally start with new ones each year. Outdoors with hard rains could reduce their life though.
hi, I made a soil moisture sensor by myself, but I use regular powder gypsum plaster of paris instead. Then what ratio gypsum powder and water to make a block of gypsum? I use a regular digital multimeters resistance acquired but why can not a constant? Are there other alternatives? I have very limited knowledge of electronics, thanks for the solution given.
Thanks for sharing, this is perfect for my arduino-controlled drip irrigation system!
What about the warning on the Plaster of Paris box about the product causing cancer (apparently only in California)?
I’m assuming this is from the silica. I’ve done a few searches but can’t quite find the information i’m looking for. When it sets, do I have to worry about my delicious tomatoes becoming carcinogenic? I was under the impression that the warning pertained to inhaled plaster of paris, but can’t find anything reliable to confirm this.
Any thoughts?
I have seen that label as well and I have not been able to find too much more information about it either. Though there are also similiar warning for perlite though in that case I know it is based on the inhalation of the dust I would presume Plaster of Paris probably is the same. But given I live outside of California apparently I am immune 🙂
Awesome. Brilliant. I haven’t made any yet, but I’d like to suggest the following design modifications which I plan on implementing. Not sure if they’re worthwhile, but anyway …
1) Prepare the nails and wiring before putting it in the plaster. Lie the nails in position on some paper and string a bit of hot glue between then at each end to keep them evenly spaced. Attach the wires to the nails before hand. Submerge the entire nail assembly in the plaster so that only the insulated wires are sticking out. This may help to avoid spikes when the sensor is completely wet,
Also, you could insert the nail assembly in the tube before pouring the plaster, then plug up the end where the wires are hanging out with hot glue, then pour the plaster in the ‘bottom’ of the tube and leave it to set. No mess. May make getting the tube off harder, unless you go with suggestion [2] as well.
2) Make a permeable, semi rigid sheath (for longevity). I haven’t figured out what to use yet – perhaps punch a bunch of wholes in plastic tubing> Seems like a lot of work though? Flyscreen material, maybe?. Of course, then you have to figure out how to plug (and then unplug) those holes – wrapping electrical tape around the whole thing would probably work though.
phlipped, I agree any extra insulation protecting the connections should provide longer life though given the metal is still frequently saturated with water so it will fail eventually though if you can get an extra season out of the technique I say got for it. Let us know how it works our for you?
I would use stainless steel nails or such instead of the standard jacketed nails. Stainless will hold up better, but it will be a pain connecting the wires securely. I think that shrink wrap would also be of benefit to hold the plaster in place, and holes can be added for permeability. Shrink wrap may be a good way to connect the leads to the nails, with a little hot glue holding it on for good before submersion in the plaster. Thank you for the good ideas!
There is the Watermark gypsum soil sensor, that uses some sort of metal with holes around it that makes it more durable.
It would be nice to see the sensor teared down to see how its constructed.
http://www.specmeters.com/Soil_Moisture/Watermark_Soil_Moisture_Sensors_and_Meter.html
I tested my first successful sensors this evening and they seem to work! I’m going to have to do some testing over the next week or so to get a feel for how the voltages correlate to soil moisture content (or tension). One thing I recommend is using a pair of really good scissors instead of a utility knife. It makes cutting the tubing so much easier. I actually used a pair of gardening scissors to do all the cutting and they worked much better and faster than when I tried using my utility knife.
Thanks for a great article! Your site is like a gateway drug for geeks like me to really get into gardening. In fact, this article is the very reason I have begun to take it so seriously (serious fun!).
Garrett B. Thank you for the many compliments there and will have to try using scissors though I have been throughly impressed with the Vegetronix soil sensor I have been using lately.
the design is really amazing.. im gonna use this in our project.. but there are still questions that i want to ask:
– how would you calibrate the sensor so that it would give accurate soil moisture percentage?
– my sensor reads only 1kohms when it’s already super dry, is it correct?
– can i use this sensor in the circuit shown in this site: http://www.faludi.com/2006/11/02/moisture-sensor-circuit/
CVG you are really gifted! im from the Philippines and i am thanking you so much for the information you have shared! hope to see more cheap but very useful projects from you!
^^
Hi. Thanks for the instructions. I am a hydroponics farmer and am using your probe design in my plant bags.
The problem some of your posters are having with inconsistent readings when measuring with a multimeter is caused by a certain amount of electrolysis caused by the DC probe voltage from the meter.
I drive mine with a high frequency AC signal. The electronics amplifies and rectifies the signal, giving me a 0-5V dry-wet signal which is then routed to my automatic watering/time system.
KiLLua, sensors of this nature are better suited to reading soil water tension than they are soil moisture percentages. The sensor has to move water in and out of the material, a process that is affected by soil water tension. Soil specific conversions exist for taking a given soil water tension (expressed in Centibars or kPa) and translating it to percentage of moisture, but for accuracy it requires a site specific calibration. It would also first require that the resistance measured from this sensor could be equated to a soil water tension. A published calibration for manufactured gypsum blocks would probably be somewhere in the ballpark, but there are a lot of factors in the construction of the sensor and how it is read that will determine the real relationship.
These sensors will give you a great relative reading of soil moisture for a low cost, but to be able to equate the readings to published values for accurate measurement or comparison, you might be better off with a manufactured sensor that has an established calibration.
Very good! And simple!
You have a graph showing moisture x time? Making the sensor is completely wet and then drying naturally. How do you think would? How should really be?
Thanks!
Hello. I made a sensor is the same as you, I am embarrassed by a high resistance, about one 1MOhm in wet ground and about 500KOhm in the damp earth. What is your distance between the electrodes?
I have used electrodes made of stainless and discovered an interesting thing, the sensor operates as a galvanic cell for about an hour i am fix by multimeter voltage about 40 mV
And one more question, how do you recieve the signal from the sensor? On the line connecting the input to the ADC resistive divider? Such an approach does not provide the full range of ADC, what do you think the account of the operational amplifier together with the sensor?
I like your method of making gypsum blocks. How do you make the plaster consistent from batch to batch?
Also, you can drive the sensors through a capacitor with a square wave. This would eliminate the corrosion by ion migration cap.
Commercial gyp block readers use a 50 Hz square wave through a 4.7 uF ceramic cap.
Thanks for publishing the great job you’ve done.
I normally make a batch of 6-7 of them at a time and then calibrate wet/dry with the first one I use. Provides me with sensors for at least a couple years.
I love your suggestion of using the ceramic cap to generate the 50Hz square wave. I will have to try that out, been having great luck with my vegetronics moisture sensor but would like to see if I can make these even more accurate.
How is it possible to eliminate the corrosion with a Square wave through the capacitor? Could someone explain that? I do not understand how a square wave eleminate the corrosion.
Luis, the concept is creating an alternating current. By passing voltage through the capacitor is sort of a cheap way to simulate this.
Why use a wax paper??? Can it be used another paper??? I do not understand how the wax paper will help ensure the nails fall straight down and parallel to each other. Could someone explain that?
Luis, I used wax paper because I could see through it and it would not stick as easily to hardened plaster. Though you are open to use anything to help with the placement (plastic, wood, paper, etc)
Use a screw type connector (two conductor Chocolate block type)to hold the probes and connect the wires reliably before setting in the plaster. Repeatable, reliable and replacable.
thats great! Thank you! I assume you do not completely put it into the soil? Does it make any difference if i build it longer so i can get deeper into the soil?
thank you!
tm
brilliant idea.. i made a soil moisture sensor for my final yr undergrad project. i replaced plaster of paris by a block of wood. 2 by 2 by 1 inch. pierced the nails through. insulated the nails. they worked just fine. wood is much easier for making the sensor, compared to plaster of paris. just a suggestion.
How can I use the sensor on a battery powered drip timer like this one?
http://secure.misterlandscaper.com/products/307-misterfloweasyblacktimermechanical.aspx
I really just need to know how I can hook this up to where when it test the ground and finds the soil lacking water in it I want it to turn on the drip line.
Anyone help?
Matt, this is a great idea. It should be pretty easy to set this up using one of my moisture sensors and IC (or Arduino) to determine when the analog value reaches your watering threshold. Once this point is acheived you just send a digital HIGH out to override to the circuit that opens the valve. Couldn’t tell you how to specifically do this without having one of those timers in front of me to test but seems definately viable setup to me.
This looks great, but I imagine the gypsum will break down eventually…
why not just stick 2 nails straight in the soil and use an insulator (rubber or foam perhaps?) to keep them at a constant separation?
And I thought I was the only one crazy enough to hack together a DIY grow box! Was trying to figure out how to construct a reliable soil moisture sensor for an automated watering system when I came across your site. Nice work. Will probably give this a try once I’ve procured the materials. 🙂
Pj, great point! You guys should have a look at this site!! Really cool info:)
http://gardenbot.org/
To make consistently spaced electrodes (and also to prevent them from shorting when you push them into the plaster), you can use a tiny bit of hot glue to secure the two nails to each other, one near the top and one near the bottom. It does not have to be a big gob (in fact, it’s better if it’s just a tiny drop), and if you make more probes, the resistance skew will be (for all intents and purposes) the same, so you only need to calibrate one (given that the other factors are also highly similar).
Instead of hot glue you can use basically any (even water soluble, as by the time the humidity dissolves the glue, the plaster will have firmly affixed the nails anyway, though having glue in the mix may have unwanted effects) glue, like a drop of epoxy (this might be easier to apply than hot glue that solidifies relatively quickly).
Do not coat the nails with the glue, just a tiny drop between them to secure them together.
This method can be perfected ad infinitum (tiny rings of thick-walled heat shrink tube for spacers, thin walled, bigger diameter rings to attach the two nails together, …)
I hope you could distribute many more articles, myself as well as my children take pleasure in your site as well as really feel were far better advised soon after browsing.
good idea of capacitive sensing and nice and accurate Arduino circuit. I got Mega 2560. If the sensor is far from the Arduino you can attach transistor to it to amplify the load from the sensor.
Mine turned in to a capacitor. Not sure what to do about that.
the design is really amazing and so simple but i just wanna know in which depth i can put this sensor ??
Do you really need nails? Wouldn’t it work if you just bared the tips of two wires, then cast them in plaster? That would solve the problem of connecting the wires to the nails, and also the problem of insulating the exposed wires.
Francois, reason you normally go with nails is that they are galvanized and will hold up to water. You could simply stick copper wires into the ground but unprotected they will rust pretty quickly. I have heard of people using galvanized wire with good results, but normally you can’t purchase a few feet of galvanized wire.
Thanks for designing this and providing the instructions here. I have an analog moisture sensor that I got from the gardening store, which says it doesn’t use electricity – I wonder what principle it uses. I want to monitor moisture levels, but I worry that sticking something in right near the root ball that is conducting electricity or dissolving plaster might have an effect on the plant. It’s amazing that you have a watering system working with this. Are you familiar with self-watering pots that use vacuum seal based valve sensors to regulate moisture automatically? I’m planning on using one of these but want some data on moisture levels to help me tune the moisture levels by getting the placement of the sensor right. Also, I think it makes sense to monitor it as a backup, so I could be notified if the valve breaks or malfunctions. If you have any experience or thoughts on this method I’d love to hear about it. Thanks!
Extremely impressive! I have an engineer friend who will be making one. I can’t wait to see it work.
Sir, Awesome! I am a new and a humble beginner for these types of exposures. However, I am impressed and want to incorporate this in my 5 acre lands for growing Onions and Maize. What do you advice me? As I am sitting about 200 Kms away from my lands and want to operate through Arduino. Kindly respond to my this request.
Gnaneshwar GS
Bangalore, India
hi, we are working on this project using 8051 in place of arduino wit adc0808. we have some basic doubts.how do we measure voltage readings from the sensor? how do we transfer it to the 8051 using adc0808. kindly help us out ..
Hi, I built a sensor with two zinc coated nails. I put them in the gipsum casing, as you show it. I was surprised to observe a power of about 100 mV between the two nails without any external source. Is this
an effect of different metals to be in contact among them? Of course this small power affects the resitance values between nails.
Just wondering a few things if anyone would like to answer it would be greatly appreciated… 1. what if you wrapped the probe in something like pantyhose or stuck it in a sponge, would that help the life of the probe? 2. Is it possible to daisy chain them? Thank you in advance for any help you can give. I am going to get some plaster of paris tomorrow and I am going to try few different things… thank you to the poster this is an excellent idea
Hi, thanks a lot for the detailed explanation about your project and congrats for the idea!
My only curiosity is about the gypsum used in the sensor… Is it good for the soil? Since, as far as I know, gypsum has chemicals and those chemicals are probably unhealthy to the soil. Am I wrong?
In second if we just use the galvanized nails with some support that grants us always the same space between them, which would be the difference between both sensors?
Thanks in advance for any answer!
Gypsum is calcium sulfate dihydrate a naturally existing mineral, basic processing to “gypsum plaster” is to dehydrate it by heating it to 300F to remove 2 of the water molocules. Now if the manufacture adds more chemicals after this process that is something to keep an eye out for but in the pure manufactured form we are just talking about basic minerals here.
As for difference between galvanized nails and gypsum blocks both can provide you with pretty accurate water measurement numbers. If you just want to know the soil is dry I should water now either option will give you this. Using the gypsum block will give you additional accuracy especially if you happen to water with fertilizer (coventional or organic) as well as an overall more accurate and conistent reading. So if you really need a smaller specific range of accuracy I would go with gypsum.
For me personally, when growing seedlings where I just want to make sure soil is always moist I go with nails, when I am growing more mature tomatoes/peppers I want to have a much more consistent watering pattern and accuracy where I switch to gypsum.
I completed the sensor today (I used gypsum instead of the paris plaster) and I was now checking the voltage ratings exactly like you have in your sketch above!
The voltage drop I got with the dry gypsum was around 0.50V (I did the conversion Vdrop=measure*5/1024). After I placed the sensor inside a glass of water and Vdrop value was 0.63 after waiting around 15 mins.
The questions I have, is:
– Is it normal this small voltage drop? Isn’t possible to increase resolution by inserting a smaller resistance so we could get a higher current?
– Second, is any way (not the empirical one) to tune correctly our sensor?
Thanks a lot for the help!
NICE! Love to see DIY instructions. Great idea for soil sensors. I’m a hands on guy myself also because I own a mold removal company in Toronto so I’m all about fixing things yourself! Thx again!
Thanks for finally writing about > Best DIY cheap soil moisture sensor – The Cheap Vegetable Gardener
< Loved it!
Hi,
A little suggestion on replacement of nails with another material : graphite rods.
Where to find graphite rods?
Dry batteries!
Before alkaline batteries were popular, all carbon battery built in with graphite rods.
Try find one. Break it open. Clean it up.
There you are, a graphite rod.
Measure it’s resistance. It is low.
To my knowledge this stuff doesn’t rust nor afraid of heat.
I didn’t try to make the sensor with it yet.
I would like to hear feedback or report if you ever try it.
Michael
Would graphite rods actually be any better?
Does the plaster keep the non-stainless steel nails from corroding, or does it only minimize the effects of soil salinity?
If the nails aren’t stainless steel, aren’t they going to rust, and wouldn’t that affect the readings? Has anyone torn down one of these sensors after it’s been used for years to check the condition of the nails? If so, what did you find?
Would using copper nails be better, so as to avoid a galvanic reaction with the copper attachment wires? If not, has anyone found a good way to waterproof the connection between wire and nail so that the point of connection doesn’t corrode?
What are the trade-offs around how much plaster to embed the nails in? It would seem that the less plaster, the faster the response to change, but at the price of a lower lifespan. Has anyone experimented with different plaster thicknesses, and if so, is there any apparent sweet spot for this trade-off?
Lastly, does it make any difference what length the nails are? What are the trade-offs around that, and what’s optimal?
One more thought: thin nails would be more feasible to solder the wire to. Do thin nails last as long as thick ones in this application? Does it make any difference? Any reason *not* to use thin nails?
Perhaps using small dixie cups would offer cheap and standardized form for casting the plaster. After the plaster sets, you could simply peel off the cup. Not sure if there’s any downside to this approach, but it seems like it might be a little easier than using plastic tubing (as above).
Hi, does anyone have data on the lifetime of such sensors? How often must they be replaced?
Thanks
Nice work,
Could you please send me arduino code
Best Regards