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The Onset computing Hobo 4 channel external logger is being used with
Aquaflex 4-20mA sensors by many different organisations in New Zealand
The major disadvantage is the inability to turn the sensor on
before measurment, which then requires a car battery or similar to
power the sensor (a solar panel removes the need to swap batteries
every month or so). It still is a cheap method of getting good
reliable continuous volumetric water content. If you use the hobo 2
external and RH and temp logger with external soil temp sensor you
can get Air and Soil Temp, Rh and Soil Moisture at a very cost
effective price.
Blair
Date sent: Sat, 11 Mar 2000 21:04:10 +0000 (GMT)
From: owner-sowacs@aqua.ccwr.ac.za
Subject: Re: Onset Hobo dataloggers; Watermark retrofit
To: john@kea.lincoln.ac.nz, brightj@kea.lincoln.ac.nz,
MILLEB@tui.lincoln.ac.nz, BUCHAN@tui.lincoln.ac.nz,
woodhead@kauri.lincoln.ac.nz
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John Johnston,
Your recent Email indicated that the tipping bucket is the only soil-
water related application thus far from Onset Corp. What about the
company's retrofit of the Hobo datalogger to facilitate the
measurement of four WaterMark sensors? My understanding was
that all engineering had been worked out on this and that it was
merely a matter of production. The product would be a datalogger for
around $150 that is weather proof and that can measure four WM or
other resistance-based soil sensors. Can you bring us up to speed
on the status?
As I've indicated to Onset in the past, if one can produce a low-cost
data logger (in say the $100 range) that can record a near-
continuous record of soil water content or potential, one may see
these planted by the tens of thousands as farmers utilize them to
monitor how well they have been managing irrigation or other land
applications. The system must be cheap enough that it can be
cultivated out by a 15 year high school kid on a tractor, but then
replaced by the kid's father, because the cost is relatively low as
compared to the benefit.
In this type of situation, the absolute accuracy of the reading is of
much less importance than is the relative change in recorded signal
over time. This is what a farmer will evaluate at the close of an
irrigation season. He can look back over the record of soil water
content or potential (relative readings) and evaluate trends and
changes over time as they are impacted by how he has managed the
system. He'll use this information to improve his management the
following season.
It would be even better to have a display on the logger that can
provide a graph of the last four weeks or so of measurements at a
glance, for example like that used on the AM400 system being
produced by M.K. Hansen Company of East Wenatchee, WA. (
mikeh@nwi.net ,2216 Fancher Blvd. East, Wenatchee, Wa 98802
1-509-884-1396, fax 2218). Hansen's system can read up to six
Watermark probes at a time (using AC excitation) along with soil
temperature and sells for approximately $250 in bulk (I believe),
which isn't a bad price. The graphical display is invaluable for
allowing a farmer to quickly review the recent trends in soil water
change and the need for any on the spot actions. The logger can
retain a year's worth of data in memory and downloads to a notebook
computer.
Regarding the less expensive Hobo datalogger, I can send a three
page writeup on how to connect the Onset Hobo datalogger to
WaterMark or other resistance based sensors to anyone interested.
Send a request to RALLEN@Kimberly.uidaho.edu . The Hobo
logger uses DC excitation in its reads which can cause some
electrolysis effect (i.e., micro gas bubbles) at the electrodes. This can
bias the electrical conductance and therefore the resistance reading.
However, my understanding is that the retrofit that has been reengineered by
Onset for soil-water applications will shorten the time of excitation on
each of the four channels so that the magnitude of the electrolysis effect
may become insignificant.
Thanks.
Rick Allen
Water Resources Engineer
Univ. Idaho
> John Johnston writes
>
> Hello Again SOWACS
>
> I was formerly with ESI in San Diego, CA, and occasionally gave inputs on
> the use of TDR in soil moisture applications. About 20 months ago I joined
> Onset in Pocasset, MA, but until recently haven't had anything useful to
> present to SOWACS. Early this year Onset offered its first product related
> to soil water in the form of a fully integrated tipping bucket rainfall
> logger. http://www.onsetcomp.com Please take a look at it, and if you have
> comments let us know on our site registration page under "Request a
> Catalog". I had our internet site administrator add a source code
> selection called SOWACS under the links and newsgroups pulldown, so that
> you could leave your "tracks"..
>
> Although I haven't been in a position to make a useful contribution, I
> have continued to monitor SOWACS as a learning experience. Thanks again to
> Bruce and everyone for continuing my education on this continually
> challenging instrument application.
>
> John Johnston
> Onset
> Web: www.onsetcomp.com
> (800) 564-4377
> (508) 759-9500 Ext 158
> (508) 759-9100 FAX
>
>
>
>
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