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[Thanks Chuck for an excellent, detailed and interesting post.
No doubt this will raise much discussion, particularly requests for the
design details.
If you are willing to share these, I will make them available on a sowacs
webpage.
Bruce]
Rick Allen,
We use the system you describe with three gypsum blocks, instead of
Watermark sensors. The gypsum blocks are placed at different depths , 6,
12, and 18 inches. We designed our own data logger, and AC driver for the
gypsum blocks. Everything is enclosed in a plastic welding rod container,
with screw on lid. The wires to the blocks and upload cable pass through
holes drilled in the container, and sealed with RTV. Two 9 volt batteries
power the system and last our whole irrigation season, which is from late
June to the end of September.
The container is buried about 15 inches deep. The upload cable, which is a
25 foot, six conductor, flat telephone cable with plug, is routed to the
edge of the field if possible. If too far from the edge of the field we
tape a plastic bag around it and bury it a few inches. The spot is marked
with a plot flag.
The data logger takes readings every hour. We have 32K EEPROM to save the
data. Other than the data of the three sensors we keep the total number of
readings since the device was turned on. We have enough memory space for
over 300 days. This season we intend to measure soil temperature at the 6
inch depth, also.
Our cost is well below the $100 range you mentioned. Of course that doesn't
include design, programming, and testing time. The most expensive part of
our set is the welding rod container.
Our farm has only one soil series. Which made it easy to calibrate the
gypsum blocks. But, I see your point that relative changes over time are
more important than absolute values for irrigation management.
A laptop with an adapter lets us connected the upload cable to the parallel
printer port. The entire data logger readings from the time it was turned
on is saved in a file. We import the file to a spreadsheet template and
graph the data. All this can be done at the site.
We solved the AC problem by using an H-Bridge to drive the gypsum block.
The bridge is driven by a square wave from the data logger.
I understand that even with AC it's better to take a gypsum block reading
in just a few msec.
We've used this system with Watermark sensors also. However, on our
non-irrigated fields they seem to lose response as the soil dries, before
the gypsum blocks do.
Thank you,
Chuck Mancuso
>
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|| Chuck Mancuso || i Univ. of Idaho i ||
|| Sandpoint, ID USA || {|} Sandpoint {|} ||
|| cmancuso@netw.com || {{|}} R&E {{|}} ||
|| || {((|))} Center {((|))} ||
|| || {{{{|}}}} {{{{|}}}}||
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