moisture sensor giving me grief

[geoff]

I've been having all sorts of problems getting my moisture meter to work reliably. After a lot of mucking around I've finally figured out what is causing the problem, but have no idea how to fix it.

With the gypsum sensor in a bucket of water, or in the air, or buried in the soil in a plant pot it all works perfectly. I'm using OWFS to sense the current, and it varies from roughly -1.4 at 100% wet, up to around -0.13 when 100% dry.

However, if the sensor is buried underground then the current readings fluctuate wildly from second to second. It seems to become completely unstable. It will jump from -1.0 to -0.2 to -0.6 to -0.4 etc all over the place at intervals of just a few seconds. It turns out that I can reproduce the problem easily. With the sensor above ground it works fine, if I then just press it lightly into the damp dirt it goes haywire. I have now found that if I sit the sensor on my workbench I can reproduce the problem by shorting the stainless steel mesh on the sensor to a ground (copper water pipe) and it immediately goes into this erratic mode ... remove the short and it works perfectly again.

For a while I thought the problem might be with OWFS (see discussion at https://sourceforge.net/forum/forum.php?thr...forum_id=292718 )

But now I've discovered this grounding effect I'm back to thinking there must be something odd going on in the hardware.

I guessed that maybe C5 or C6 in the schematic may be shorted, so I've tried a series capacitor in both legs of the sensor (made no difference). Also tried changing the time constant by trying a few resistors 10k/33k/68k in parallel with the sensor (also made no difference).

Any ideas ?

 

[makki]

Sorry no ideas but after digging my Watermark-Sensors I have the same problem.
As long as in air/water-bucket everything is fine, when burried readings vary heavily between min and about 50% ?

Michael

 

[geoff]

After wasting too much time I guessed that there must be something wrong with my sensor board and bought a second one. Same problem. Damn!

On the workbench, with the sensor wet I get a current reading of around -1.3 which seems to be correct. If I short the metal case of the sensor to the 1-Wire bus ground then the sensor goes haywire, with readings fluctuating all over the place in the range -1.3 to -0.2 This is the same behaviour as if the sensor is buried underground (presumably because there is then a current path from the sensor back to 1-Wire ground).

I disconnected the sensor and (while still wet) measured the resistance & capacitance between the sensor wires and the metal external case. Infinite resistance but it seems around 0.25 microFarad capacitance (according to my not too professional multimeter). Could this be the issue ? I only have the one sensor, so I'm not sure whether this is correct or not.

I'm determined to get this thing working, so it's time to start my own cct design. I havn't been able to find much guidance as to the "ideal" voltage range / frequency for the gypsum sensor. Does anyone know of any doc describing this ?

 

[makki]

What I found out so far: it only happens when the digged sensors gets wet. As soon as soil gets dryer, I get resonable readings.
Any hints still appreciated but I fear theres some problem related to grounding it, as soon as I put it into an (isolated) bucket with water readings are fine..

Michael

 

[sic0048]

I think your diagnosis is right on. Everything points to a grounding issue in the design of these boards. Either they are not designed to be placed in the ground, or there is something wrong with the overall design IMHO.

 

[geoff]

Fixed it ... I think.

I put a small isolation transformer between the board and the sensor and now (I think) it's fine. The device is a 1:1 600 ohm isolation or decoupling xformer, typically used in modems or other things attached to a phone line. A couple of dollars from your local components shop.

This has changed the calibration a bit, now at 100% wet I read -1.2, hope I'm not declaring victory prematurely ... it will need some time in the ground before I can be 100% sure, but all my problems on the workbench are definitely resolved so it looks good.

 

[makki]

@Geoff: Sounds good!
Can you give an electrical noob a hint what you put in there like Part# so I can dig for something comparable ?

Michael

 

[geoff]

Something like this: link

There will be 4 connectors. Use a multimeter to figure out which pairs are connected. Wire one pair to the sensor connections on the board, and the other pair to the sensor. Unless you are a dab hand with a soldering iron try and find a transformer with longer connecting wires or you risk melting the internal solder connections (voice of experience!)

It might be wise to wait a week, I'll let you know if it really is fixed.

Also re my comment on changed calibration, a typo, I meant -1.02 not -1.2.

 

[geoff]

It might be wise to wait a week, I'll let you know if it really is fixed.

Confirming ... it is working perfectly (at last!)

 

[ericvic]

Confirming ... it is working perfectly (at last!)

Geoff,

Glad to hear you found a solution, sorry you had some much trouble. I will remember this for others that have the same problem.

Eric

 

[makki]

I finally ordered two isolation transformers for my mositure sensors today, hopefully I can fix it also, will let you know..

Michael

 

[makki]

Finally I put this between the board on the Watermark-sensors today and since then it works.
Thanks Geoff!

Eric, as I already had some people asking me for this moisture sensor (as it's a still a valuable solution, there are not that much affordable alternatives..): are there plans to modify the circuit or add such an isolation-transformer to it ?
I'm an electronic idiot but can confirm it was definitely a grounding-issue, both of them worked fine as long as they were alone in a bucket or in air. As soon as both were digged, together with some DS18B20 in my case, values started to waft around.

Michael

 

[ericvic]

We are looking at a redesign in the future but for now we will let people who have this problem know of the solution. We will probably put something up on the website to make users aware of this potential problem before they purchase the unit.

Eric

 

[pete_c]

Eric,

Thats a good answer. Recently I purchased a moisture sensor & leaf sensor. Next I was going to buy a gypsum sensor. I haven't installed anything but reading thread is kind of telling me I wasted my money. I think back how much I have spent on your products - IE Lightning sensor, combo temperature sensor, barometer sensor, 2 dual counters and moisture sensor...lets see that adds up to:

Dual counter 28 X 2 - 56
Barometer 60
Combo 55.75
lightning 32.50
moisture meter 27.50
leaf sensor 2
==============
Total - 233.75 plus shipping

I read originally not to purchase the combo sensor because the light sensor design was flawed, I read that the barometer design was also flawed and caused some issues along with the lightning sensor causing issues on a one wire network. Now I read that the moisture meter is problematic.

I remember writing to hobby boards dot com when my solar- light sensor went out and purchasing the recommended sensor and installing said sensor with a problem and never really getting a response to my concern. Personally that is not really good business etiquette. But maybe hobby boards doesn't care eh?

 

[ericvic]

Pete,

First let me apologize if you wrote to us and we didn't respond. Believe me that Hobby Boards does care about our customers, they are what keep us in business.

Where did you hear that the Barometer or Lightning Detector was flawed? I wouldn't call the solar sensor flawed either. We do recommend against purchasing the combo sensor if you want the best accuracy since what is best for the solar sensor is not best for the humidity and temperature sensors.

If you want to discuss this further please contact me at support @hobby-boards.com.

Eric