Length of TDR rods - is 7.5cm too short for sandy-silty soils?
List Owner - BruceM
=========================
Good morning to everyone
I have a question concerning TDR probes. I am about to buy a TDR system
in order to use it in a column experiment but my problem concerns the
lenght of the rods to be used. In the brochure of the TDR system i will
buy it is given only an upper limit for the soil conductivity for which
a specific lenght for the rods might be used. Since my column
experiment will be 20 cm diameter I was thinking to use a 7.5cm lenght
rods but I was wandering if these might be too short in order to have
accurate readings of water content. Is there any experience you might
report me in using so short rods, any problem encountered, expecially
in using them in sandy-silty soils.
Thanks to all of you
Regards
Oscar Cainelli <cain...@ing.unitn.it>
tr...@campbellsci.com
APS_CSL
is no definitive answer. What can be said about shorter probes is that
with shorter rods you decrease the resolution of the measurement to a
limited extent and many other errors, such as those incurred in
defining the probe offset, and "end effects" become more significant.
Those downsides happen roughly, but not necessarily directly, in
inverse proportion to the ratio of the lengths of rods, i.e. the longer
the rods the smaller the errors.
On the upside there is less signal attenuation caused by soil
electrical conductivity, so you can get a measurement from a shorter
probe in more saline soils.
A full copy of the CS probe manuals can be found here where some of
these issues are discussed a little more.
ftp://ftp.campbellsci.com/pub/outgoing/manuals/tdr-probes.pdf
As to the sand/silt soil type: whether you can use a standard
calibration or you need to find or derive a more specific calibration
for the soil will depend on the nature of the silt. It may be a case
of try it and see.
Perhaps you are best to wait for further responses from real users.
However, we have only been making the shorter probes for a some months
so there may not be that many that have used them in anger, although
there maybe other people who have used short probes from alternative
supplier or who have built them themselves.
Yours sincerely
Andrew Sandford
Campbell Scientific Ltd (Europe)
torr...@eel.upc.edu
I am not sure if I'll anwser your question. I will try to be short:
1) Usually, conductivity of sandy soils is low (compared with lime or
clay).
2) TDR pulse speed is v=c/sqrt(k) where c is the speed of light and k
is
epsilon relative of the mixture of soil (k=2-5) and water (k=80). It is
an
average (e.g. k=10).
3) Your system samples at a rate of M GSps (Giga Samples per second).
4) Aditionally risetime (tr) of your pulse lowers the effective number
of
samples you detect inside your soil sample.
Example: k=10 => srt(k)=3 =>v=(3*10^8/3) m/s, assuming tr=0, M=1
GSps=(10^9) Hz
distance inside soil sample= v*t/2 (/2 because pulse goes and backs),
only one
measurement inside sample=>(3*10^8/3)/(10^9)/2=5 cm
At least, you'll need 4 or 5 samples inside, then your minimum length
should be
about 20 to 25 cm. Life is not so easy (tr) but you have a rough
number. Then,
you should check M and tr in your system.
Sincerely,
Josep M. Torrents
ps. A colleague told me about a software in internet that do these
calculations, but I never checked.