Soil Moisture Sensors

[Sacarino]

You guys have any experience with moisture sensors?

I'm ashamed to admit that I had an old 1-way Rain8 (in the box, unused, for about 6 years now) that I finally put to use. I'm only using this to control my seasonal garden irrigation at the moment (sprinkler valve mod and a soaker hose for the win!) and while it's working great, I have the potential problem of over-saturating the garden. I briefly looked at the irrigation system rain sensor/relays but that wouldn't really help me as I don't care about rain so much, but rather the moisture needs of the soil.

To that end, I have started digging around in the moisture sensor/probe world but I'm not sure who is good and who is not. I discovered an outfit called Vegtronix with two probes - one 3.3V to 20V input w/ 0V to 3V output, the other 2V to 20V w/ 0V to 1.8V output. (link is here) These seem like the would be right up my alley, and at $30 it wouldn't break the bank. I also discovered a really interesting blog called The Cheap Vegetable Gardener (looks like he's a Microsoftie) who made his own probe, which seems ingenuous.

The only hesitation I have to building my own is I wouldn't want a manufacturing materials flaw to create an error-prone probe. Between the restricted flow from the soaker hose and the auto-timeout from the Rain8, the worst that I can do is drown the plants from an unnecessary every-other-day watering. I have a funny feeling that it would greatly decrease WAF, however.

I doubt that I'm the first cocooner to have this problem so I'm looking forward to your thoughts!

 

[mustangcoupe]

I also discovered a really interesting blog called The Cheap Vegetable Gardener (looks like he's a Microsoftie) who made his own probe, which seems ingenuous.

Great find, I think I'll be making a few of these this week and test out as soon as I can, this will reduce my over watering I think(hope)

 

[electron]

I purchased a few of these with the intention of hooking up a DS10A to it, but never found the time to do this project. They make a noise whenever the soil needs watering.

 

[hucker]

Before you start building your own stuff you might have a look at this site:

http://www.sowacs.com/sensors/index.html

Gives the pro/con on the various sensor types, it is a sales site so you might note that as a source of bias. Didn't speak too highly of the gypsum based sensors.

BTW it does sound like a fun project...

 

[Sacarino]

Gives the pro/con on the various sensor types, it is a sales site so you might note that as a source of bias. Didn't speak too highly of the gypsum based sensors.

That is an incredibly handy website - thanks!!

I am hoping to get some time this week to investigate further... maybe even find the time to do a howto!

 

[FrankMc]

Hi Guys

Im currently testing a probe from a circuit i got on the internet ...

http://www.redcircuits.com//Page18.htm

Have added some components to the led section to convert the output to open collector and filter out the led pulses....Have it connected to a ds10 (open collector compatible) and it appears to be working ok....Have the test probe mounted in a plant pot and will monitor it as it drys out ...If it works i,ll probably make a little pcb for the probe....

HTH
Frank

 

[electron]

Keep us posted please!

 

[FrankMc]

Hi Dan

Its looking promising at this stage...I had 2 prototypes of the circuit built but hadnt implemented the x10 rf sensor (im actually using a HA129)...Have now got both proto,s connected to a HA129 ....When the moisture probe led starts flashing (soil dry) it sends a x10 security normal message .....When the led stops flashing (soil moisture ok) approx 20 seconds later a x10 security alert message is sent ...The reason for the delay is due to the rc filter i added to prevent the flashing pulses from sending multiple messages...I do have a DS10E sensor and it appears to be open collector compatible......Im going to get some electrical plastic fittings to house the prototypes and i,ll use 3mm stainless threaded rod for probes approx 150mm long....

The HA129 seems to be slimmer than the DS10E and i picked some up for $20;00 AUS each which is cheap normally around $50:00...;-(

http://www.winplus.com.au/automated-controllers.html

Frank

 

[hult]

You guys have any experience with moisture sensors?

You guys have any experience with moisture sensors?
.....

I doubt that I'm the first cocooner to have this problem so I'm looking forward to your thoughts!

I've used the Watermark brand of improved classic gypsum block sensors made by Irrometer for hydrologic research and personal purposes since before they went commercial in the mid 1980's.

See www.irrometer.com/sensors.html

The most important installation consideration is "Location, Location, Location" -- especially depth. I suggest that you consider installation at two different depths ( one in root zone, one below root zone) at least at two different X- Y locations (shade, sun; hillside, flat; lawn, garden etc).

The most important electronics/wiring consideration is that you should use an AC (not DC) excitation and thus measure device impedance, not resistance.

Important considerations in interpreting the data are that:

1) The sensor output is temperature dependent. One can correct/compensate for tis by installing a temperature sensor at each probe and

2) The ouput is dependent not only on the amount of moisture, but the salinity of the water because the conductance of the water ( conductance == 1/resistance) is proportional to the salinity (=~ concentration of dissolved solids , principally Na, Ca, Mg, CL, SO4, and/or HCO3 ). Depending on your circumstances, this can be relatively unimportant or a major challenge.

It is also part of why I suggest that you install a probe below the root zone as well as in the root zone if you plan to irrigate -- and if not, why are you measuring moisture ? . To avoid building up dissolved solids in the root zone, one should irrigate until the applied moisture/water also shows up at the lower probe.

(A completely different approach to determining when to water is through the use of gravimetric lysimeters. In essence, one buries a scale in the ground and measures the weight of the soil + plants + water in the volume above the scale. Discussion will have to wait for another day though ... ;-)


Hope This Helps ... Marc [email protected]
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Marc_F_Hult
www.ECOntrol.org