proudly sponsored by
|
archiving of SOWACS proudly sponsored by |
|
sowacs
Now THIS is what sowacs was started for! Real discussion on real
sensor problems!
I have used the 229 sensors, but only calibrated at 10, 20 & 30
kPa.
I was busy assessing what heating time interval to use to get the
most accurate indication of heating rate - and so matric
potential. I tested 4.8s, 10s and 30.9s.
It seems that 4.8s was marginally better than the other two, and
looked to be equalising out as the tensions increased.
You talk of an air entry potential below 10kPa (do you mean
wetter than this? ie closer to 0kPa?) (You say the response was
adequate for the 20 to 400 kPa range.)
I thought that the air entry potential was when the tension rose
(drying out) to a point where water columns break and air rather
than air is sucked into gaps. This would occur drier than 200kPa
and surely not when _wetter_ than 10kPa.
The poor response wetter than 10kPa must be due to some other
phenomenon. Any suggestions?
I gave up using these sensors due to the highly accurate
temperature changes that I would need to measure at the very dry
water contents I wanted to cover, and because of cost of numbers.
There are other sensors available that work on heat dissipation,
(but I am not recommending any, or putting any down).
This is the list I have so far on the sowacs Heat dissipation
page:
http://www.icfrnet.unp.ac.za/~metele/sowacs/heatdiss.html
DRW's :
http://www.cohort.com.au/drw/asensor.html
Cambrone sensors by Ken Uzzel :
http://www.icfrnet.unp.ac.za/~metele/sowacs/heatdiss.cambrone.html
Claude Phene manufactures a model,
and the ones we are discussing:
The Campbell Scientific Inc. heat dissipation 229 sensors.
PLEASE ADVISE ME IF I HAVE MISSED ANY MAKES HERE.. Thanks
brooz@pobox.com
_______
Re: CS615 (TDR, I think) sensors:
With the default calibration values, I got an acceptable range of
water contents (when wetting up a soil from oven dry).
The only hiccup I had was a jump in the trace - which I have yet
to figure out (or repeat). I am busy running the same wetup, so
will expand on this. (See
http://www.icfrnet.unp.ac.za/~metele/pix/view03.jpg )
I have heard that this sensor is:
a/ extremely sensitive to soil salinity, and even low salinities
render it in error.
b/ that the sensor is more sensitive to water at the far end (I
think it was) rather than having the claimed uniform
"intergrating" measuring effect over the entire 300mm probe
length.
Problem a/ can be addressed apparently simply by insulating the
steel probes with a suitable material. Heatshrink has been
suggested, but how this would affect sensitivity, I do not know.
But if it gets rid of salinity problems - which may be what your
190% problem, is, then it is worth a try (and the probable
necessary recalibration.)
Why the manufacturers have not done this, or at least
investigated this I do not know! [Give us some feedback if you
have, guys, thanks!]
Regards
-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.
Bruce Metelerkamp SOIL WATER RESEARCH OFFICER
Institute for Commercial Forestry Research,
University of Natal, PO Box 100281
Scottsville, ZA3209
Rep. of South Africa Voice:27 331 62314
E-mail: bruce@icfr.unp.ac.za FAX:27 331 68905
brooz@pobox.com
Bruce.Metelerkamp@pobox.com
URL http://www.icfrnet.unp.ac.za/~metele
../SoWaCS.html
../RR.html
Host of SoWaCS (Soil Water Content Sensor) Discussion List.
SEND info sowacs OR subscribe sowacs TO
majordomo@aqua.ccwr.ac.za