How to wire for resistance /moisture


Active Member
I posted this in HS forum but thought I would do it here also. I'm finishing my sprinkler system tie in with Elk and David menioned this. But I'm not sure how to wire this up?

"You can measure the moisture in the ground with ground rods and Rules using the analog input. You will read the analog value on a timed interval in the Rules"

Okay back in the day I built worm shockers!!

But I have a question How would I wire this? I'm trying to locate a few sites which talk about this but haven't yet. I finished wiring up the zones and mostly done. It's pretty neat walking around with your house phone turning the sprinklers on/off.
I'm not sure but I think the reisitance between the ground rods would change based on the moisture of the land. I would imagine you would place two say copper ground rods in the soil (not sure how far apart) and connect each to a positive and negative of a zone.

How can the Elk measure resistance? I believe there is an "analog" input option that will give you eight bit accuracy (0-255 bits) (going off my memory discussing this with David once). So you would just setup a "rule" to query this input and when it got to be below or above a certain "bit" value you would perform an action.

The trick is how to calibrate this? Maybe saturate the area with water, then let it dry up and see the difference in resistance/bit value between the two. Also, I'm not sure if you need to actively "bias" this setup.

Sorry I can't be more specific as I don't own an Elk (I just play like I do on CocoonTech ;) ). But, I did stay at a Holiday Inn Express last night so hopefully some of this is correct and maybe an Elk owner can take it from here!
I know nothing about the Elk but if you try to measure soil moisture using DC then you can get electrolysis on the electrodes and it will skew the readings, this is a non-reversable condition. With DC you won't get very acurate readings and they will get worse over time.

I'm trying to keep it reliable as the Elk is.

As far as measuring it I plan to do averages over a period of time. I'm looking for a rough estimate on wether there is enough moisture in the ground. I've seen blocks that you bury but the go for around 30+ each.

I really like MCSsprinklers but I don't think I can justify the price and the local weather station doesn't report rainfall. Also prefer to keep it on the Elk.

I've thought about 1 wire solution but again my Elk m1 hasn't crashed.
Is there any way to use/hack those cheap soil moisture probes Wal*Mart sells? The look like a typical temperature probe you use to check the turkey in the oven.

I did some googling, and found several companies selling these type of probes/sensors, but no pricing information.
Most of the probes I've seen have only 1 wire.

The two wire probes are for industrial irrigation and make it difficult to find. I did find someone on eBay that is a dealer and have contacted him.

But thought maybe some cocooners had a idea....