jsmith said:
Anyone have a simple design approach for a moisture meter for house plants?
I used a commercial IC from National Semiconductors to measure the
resistance with AC. Despite using AC, metal electrodes dissolved in a few
months. This was soved by using carbon rods from old D size batteries.
It has been discussed before, see:
Hi I have been given a project to create a sensor and measure the
humidity in soil in Plants.
I have an idea of stikking 2 pins in the soil and there by measuring
the capacitans between the stiks. What do you think and how do i
measure the capacitans. Is there anyone who can hellp me.
Thanks in advance.
The standard low-cost way of doing this is with a "Gypsum block"
normally a pair of wires cast in a gypsum block. Strip about 20mm of
insulation off some wire, then cast them in a plaster of paris mix. A
35mm film cannister makes a good mould. The theory behind this is that
the gypsum takes up the same equilibrium moisture content as the
surrounding soil. Just putting probes in the soil gives inconsistent
results depending on soil pH, soil texture, fertilisers added, etc.
You must use AC when measuring the probe impedance, otherwise
electrolysis gives you very wierd results, although a number of newer
commercial systems use short DC pulses of a few mSec with good
results.
There's many thousands of these things buried in orchards, etc. They
have a typical life of 4-5 years.
There's also a number of other techniques used, all much more complex.
See
http://www.bae.ncsu.edu/programs/extension/evans/ag452-2.html
Barry Lennox>
Succes, Wim Ton