proudly sponsored by
|
archiving of SOWACS proudly sponsored by |
|
SOWACS
Ken Uzzell's calibration of the Cambrone sensor is done at 100%
in water, and 0% in air. (See below.)
I asked Ken if the soil's heat capacity would not affect this
[soil-less] calibration.
He replied:
>>Our thermal probe lives in a matrix environment. The
calibration is to sync 100% and 0% to the matrix environment.
After this, it does not matter what type of soil the probe in the
environment is installed into. The thermal device measures the
matrix which is a result of the surround soil moisture.
<<
Gotya, but my point is still that a 100% wet sensor matrix (in
water bucket) is not necessarily the same volumetric water
content as a saturated soil. Only if the matrix adequately mimics
the soil's pore size distribution will this be the case.
But as long it is a relative change for an irrigation tool, then
this is fine.
Your other observations were also most interesting...
>>Situations where calibration would change...
* Change in soil compaction.
* The movement of crop roots around the probe.
* Slight ground disturbances (surface tilling).
* Soil organisms - worms/ants etc. making a home near the device.
* Soil cracking.
A thermal probe without a matrix will alter its calibration when
any one of the above situations occur, which is very regular in
most cases I have seen.
We experienced much heartache to go to all the trouble to
calibrate the probe to the soil then install it and to then have
natural soil conditions void the calibration a week later.
<<
IF all of these things change the calibration of a thermal probe,
then what is it that one was calibrating?!?
*What* gets changed by compaction, disturbance, cracking etc.,
etc?
These point to the tricky nature of point SWC measurement. And
they indicate that the care required during sensor installation
that was being discussed previously on sowacs.
I tried a heat dissipation probe, but found inadequate resolution
at the dry end. In addition the problem of long lead wires
required for the thermocouple make it inappropriate for any field
application. (Does the Cambrone avoid this by converting
temperature to another signal electronically at the sensor end?)
The sensor I tried was also too fine a matrix for the sandy soil
I was using. Could this problem not be avoided by providing a
large range of pore size distributions on each sensor (and making
the sensor small enough not to retain a lot of water on drying
out?)
>>Next to developing the thermal probe, designing an environment
that would truely reflect soil moisture with fast response times
(ie. quicker than the 15 minute resolution of the probe) was our
most important work. For without a good matrix environment, a
thermal probe for field/crop use is almost useless.
Our primary task for developing our probe was for use as a
scheduling/monitoring device for irrigation systems.
Regards
Ken Uzzell
k_uzzell@geko.net.au
<<
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 Private Voice Mail 0881236337
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
>>>
From: Bruce Metelerkamp (bruce)
To: k_uzzell@geko.net.au
Date: Thursday, January 16, 1997 12:02 pm
Subject: Calibration of thermal probes
Ken - correct me if I am wrong, but shouldn't your Cambrone
calibration of 0 and 100% water be done in the soil itself?
Does the thermal conductivity of the soil not affect the
heating/cooling time?
>>from your web page
Blue = Soil Moisture
100% = Top of thermal curve (void of detectible air particles)
0% = Bottom of thermal curve (void of detectible water
particles)
To achieve 0% calibration, the moisture probe is hung up in the
shade on our patio. Next I will place the probe in a jug of
water to make the probe move to 100% water content. Following
this the probe is removed from the water and allowed to
dry out to test/re-adjust the 0% point.
<<
So 100% is equated to when no air is present, only water? And 0%
is when there is only air and no water. What happens if your
probe at 0% (in air) is put into air dry soil. If the reading
changes, then your method is incorrect..?
By the same argument, when the soil is saturated, there is a lot
less than 100% volume of water in it. So would your sensor not
register something less that 100% in saturated soil?
--
Regards
Bruce Metelerkamp SOIL WATER RESEARCH OFFICER