I do not have a moisture sensor but I saw your plight and might offer some
suggestions....
I read where someone, maybe you?, were able to get decent readings if you
placed the sensor in a pot or when the ground was dryer.
I suspect the problem is ground currents which are present virtually
everywhere. If the sensor data/power lead goes back to a source that has a
reference to real ground then a unwanted current could flow if the sensor
itself had conductivity to the ground. Since to measure the moisture it would
have to have earth contact, unless it was isolated in a tub or pot, this could
be a problem.
US electrical power is referenced to earth ground. While a ground return is
suppose to always be present depending on connection quality and ground
conditions, considerable currents can flow in the ground. In other words a
small portion of you AC power supplied to your house could seek a ground return
through your sensor to a ground return elsewhere - you neighbor or the nearest
power pole ground or water system. Every home should have a ground rod at the
main electric box and sub panel but these grounds are often compromised. Even
if your ground and system were operating without any ground leaks your
neighbors may not be. In short ground current exist everywhere, and vary
greatly over time based on your and your neighbors power draw.
The best way to solve the problem is to isolate the sensor. My choice would be
to use separate power and data and use an opto-isolator on the data leads. It
is ashame that they did not realize this could often be a problem when the kit
was designed and build this on the board. Power could be supplied from and
isolated source with neither power lead have any reference to ground.
I am sure others will pipe in but there is a weather sensor company that sells
or includes with their kits a USB/1-Wire optocoupler adapter WSI603B that
provides optoisolation and lightning surge suppression. There may be others
available or a schematic of how to build one out there.
In general it is a good idea to do this for ALL sensors that are outside or on
long lead runs. Besides power line current leaks and surges, lightning and
static can also cause erroneous readings and damage. I believe the opto device
I mentioned isolates the USB/1 wire so that your entire one wire could be
isolated as long as supplemental power were also isolated. BUT if you had more
than one moisture or ground connected device in the system you might still have
a problem. Since these currents flow in the ground, if you put a probe in the
ground at two locations there could be a current difference between them, with
changing currents over time.
This is often a problem when putting in wire video surveillance system. AC hum
from these currents can show up in the picture. I had a system where a camera
was mounted on a metal frame building. The recording device was several hundred
feet away. The frame of the building, although grounded, had a voltage
differential between it and the ground at the office where the surveillance
recording equipment was installed. A quick fix was to isolate the camera on a
piece of wood. A fully isolated balun would also probably have worked.
Hope that gives you some ideas. Good luck.
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