TDR PURCHASE AND APPLICATION
k.stee...@abdn.ac.uk
Iam looking into buying TDR equipment for the measuring of soil
moisture content above the phreatic surface level in coarse-grained
beaches in the laboratory environment.
At the moment I don't know much about TDR. What does the equipment
involve exactly (oscilloscoop, interpretation software, TDR probes,
cables??).
Would the TDR system give good enough results in a coarse-grained
envrionment under influence of high flow velocities above the bed to
establish moisture content change in time within the bed?
Can somebody recommend me a particular quick response equipment (I
have heard of Campbell and Tekronex systems) and has anyone have an
idea on the price of a TDR system?
Thank you very much in advance
Kind regards
Kate Steenhauer
Oscar Cainelli
we acquired one year ago the Champbell TDR system in order to use it in
laboratory tests in a infiltration column.
The system comprises:
the TDR100 box (wave generator + oscilloscope)
one 8-channel multiplexer
8 TDR probes, 7.5 cm long with 5 m cable
and a AC/DC power adapter (220V AC in/12V DC out) (I bought this separately)
The multiplexer allows you to attach to the system up to 8 TDR forks (one
for each channel), yet you can buy more multiplexers and set them in cascade
up to 3 levels and this allows you to reach up to 512 probes.
The cost of the whole package was around 6000 euros (I guess some 4000
pounds), but this was the price we got from the italian supplier,
maybe if you buy it directly from the Champbell Sci. you might get it for a
lower price. But it depends on how many multiplexers and probes you buy
(500-600 euro for a multiplexer and 60-80 euro for each probe).
With the TDR system they also supply the PCTDR software that allows you to
read from the TDR100 continuous or single measurements from a single
channel. The problem is that this software is very simple and does not allow
you the automatic monitoring of your TDR system at regular time intervalls
and for each channel. We had to write a LabView code that automatically read
in turn from each channel of the multiplexer connected with the TDR100 the
waveform and the moisture content monitored by the selected probe (the
TDR100 system evaluate automatically and quite accurately the moisture
content from the waveform once you set correctly some parameters. The manual
is very exaustive in explaining how it works). If your choice is to buy this
system and you need this software I think I can provide it to you, I need
first to ask my supervisor but I don't think it will be a problem.
Regarding the suitability of the system for your test cases I think there
are no problems. I had to use rods 7.5cm long because my laboratory
apparatus is 20 cm diameter and the material I use is a very conductive sand
(pore size from 0.075mm to 2mm). I recommend you to use longer rods if your
apparatus allows you to do so and also if the material is coarser than mine.
The problem I see is the representativeness of the readings in case of very
coarse material and short rods. The reading is the average moisture content
of the materiale lying between the rods, therefore the longer the rods the
more representative is your measurement.
So far I found no problems in using this TDR system. I know it is quite
expensive but it is very accurate. Unfortunately I have no idea on the
performance of other systems. You should find informations also on them and
then decide what is the best for your objectives.
Hope I have been clear enough, If you need more informations simply email
me.
Good luck for your work
PhD ing. Oscar Cainelli
Dipartimento di Ingegneria Civile e Ambientale
Universita' di Trento
via Mesiano, 77, I-38050 Trento
ITALY
tel: +39 0461882637
Lauren Bissey
A capacitance type system should give you similar accuracy and response time
as the TDR system. I won't write a long response regarding this, but let me
know if you are exploring other types of soil moisture technologies and I can
give more detail regarding the different types and companies that you can
contact about this (including ours).
Lauren Bissey
On Mon, 16 Jul 2007 14:14:28 +0200, Oscar Cainelli wrote
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Rodney Thompson
A consideration, may be salinity effects on the sensors being used. If your beach sands contain salts, you may encounter salinity effects. In TDR systems, salinity can cause reduced attenuation which could make measurement difficult. In capacitance (FDR) systems, it can result in overestimation. If you are working with elevated salinity, you may consider checking out with the suppliers how your salinity values would affect their sensors, and if there are means to reduce such effects. Or maybe, you just wash the salts out and avoid the problem.
Rod Thompson
ROD THOMPSON
Universidad de Almería,
Dpto. Producción Vegetal,
Edificio CITE-2B,
La Cañada
04120 ALMERÍA
SPAIN
Tel: ++34 638140123
Fax: ++34 950 015939
E-mail: rod...@ual.es
Verstricht Jan
we just ordered the Campbell System. Their TDR100 box (3429 EUR) is
somewhat similar to the Tektronix 1502B TDR, but then without the screen
and manual controls.
If you connect the TDR100 to one of their dataloggers (e.g. CR1000, 1407
EUR), it should be quite straightforward to make automatic measurements
(we are currently doing it with a Tektronix connected to a Campbell
Datalogger, but the TDR100 should be more convenient, especially in the
field). You can e.g. specify to store the waveform (reflectogram) to
make a custom analysis (to derive both moisture content and
conductivity), or it can also be performed by the datalogger.
Coarse sand should be quite easy to work with regarding calibration
(i.e. the response in sand corresponds well with published
relationships). In clayey materials, things are much more complicated
(as we learned), most probably due to the complex interaction between
clay particles and water.
If you take measurements during day and night, don't forget to take also
into account temperature influences.
There is of course a lot more to say about this, feel free to contact me
if you need more specific info.
Kind regards,
Jan Verstricht
jan.ver...@sckcen.be
EIG EURIDICE
c/o SCK*CEN
Boeretang 200 BE-2400 Mol
tel +32 14 332786
fax +32 14 321279
www.euridice.be
www.sckcen.be
Hello Kate,
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