�From:�	Johan Smit ("smit.js")
�To:�		SOWACS@aqua.ccwr.ac.za
�Date:�	Friday, June 21, 1996  11:43 am
�Subject:�	Re: Enclosure file: FUKU 

 SOWACS
> 2/ Development of sensors - should we be aiming at cheap oraccurate sensors?
>
Laboratory accuracy at unaffordably prices are surely of no use to 
the mass of water users? Accurate sensors already exists, and areon 
the market. The need is to cover the earth with managedirrigation. 
> 3/Putting my money where my mouth is : The discussion Icurrently am in
> concerns whether one can simply reduce the length of TDR
> wave-guides and simply correct by recalibration. This appears to
> be limited by the capacity of the particular sensor'selectronics
> to resolve the wave's reflection time. And some sensors have
> internal electronics configured for distances and so cannot bechanged.
Not possible, the time scales involved are minute and usually the 
electronics cannot handle shorter time frames.
Why this emphasis on TDR? TDR, FDR and frequency shift methods all 
measure one basicphysical variable, namely dielectric constant ofthe soil. 
Some are more complicated in measuring permittivity, which is 
dielectric constant as a complex value due to resistivity. Theadded 
functionality is to my mind not worth the bother.
> 4/ Another point I am trying to get discussion on is how to
> calibrate TDR (and related) sensors.
> A simple two-point (oven-dry and saturated) is surely not
> appropriate if the main use for the sensor will always be
> inbetween these. Obtaining uniform water contents at points
> in between will surely involve either a lot of mixing and
> repacking of sequntially wetted soil, or long time periods toallow redistribution of
> soil water added from the top or bottom?
If the soil sample has to be manipulated in any way, surely the 
disturbance of the soil invalidates the whole effort ?

> =============================
> 
> 1/ A transcript (from Japanese) of a message on the
> Japanese Hydrology USENET by Masato Fukumoto of HokkaidoNational Agricultural Experiment
> Station on TDR systems....
> 
> There are two standard TDR system manufactures.>
Whose standard? This is ridiculous. 
> One side, there are cheap or strange manufactures.
Again, any electronics is STRANGE. Cheap does not mean useless. We 
are striving for cheaper (affordable) measurement systems.
> The contents of that message are (1) what method to measure the
> pulse travel time is used by TRIME SYSTEM (that I guessed byreading the
> technical report of IMKO) , (2) I don't know what method tomeasure the
> pulse travel time is used by MOISTURE POINT,
The methods inside the electronics are of no concern to the user, 
results matter.
> I guessed that MOISTURE POINT  may be base on the method by Hook
> et al. (1992, SOIL SCI. SOC. AM. J., Vol.56, 1384-1391). But Ican't
> understand the method.
> Judging by user' talk, I encourage TRIME SYSTEM (this is cheap),
> but don't encourage MOISTURE POINT. Please pay attention thisindividual
> comment.
Mr. Fukomoto should not encourage or discourage things he does not 
understand. I have read some of Dr. Hook's work and seen thesensor 
probes developed on his work. They are perfectly understandable 
applications of electronics. The probes are TDR striplines with 
shorting diodes at various positions. When installed vertically,the 
shorting diodes enables various depths to be measured accurately.
This with a single probe!
In my opinion, the probes have the following disadvantages:
1. Being a vertically inserted intrusion into the soil, water may 
run down the outside of the probe and give a false reading. Thisis 
of course a generic problem with all such probes.
2. The cost of the highly sophisticated electronics is high.
3. At the time I was demonstrated the probes, the softwareavailable 
was virtually non-existant. Soil moisture probes need goodsoftware 
support. 
 
> 2/ I am again sending out Marty's appeal for feedback on whether
> there is a greater need for cheap or accurate sensors:-
> In my opinion, the accurate sensors are available, and are
> being used by those who require the accuracy.
> What is not available are some kind of dime-a-dozen sensor
> for use where a large number of samples are required.
Totally totally agreed. 

Regards all.
Johan Smit