Re: TDR questions

[Marty Grogan]

Marty Grogan writes:

The CR10X datalogger uses an extremely high input impedance.   Your installation
should probably limit this with a parallel resistance (~10k) for noise reduction
purposes.  The CS615 probably has a voltage type output with an impedance of 1K
ohm or so.  It may be available in a current mode output as an option.  For long
wire runs, the wire resistance can have a substantial effect on calibration
since it is in series with the sensor output.   Current output sensors are
preferable for long runs.  The overall loop impedance can then be reduced to a
few hundred ohms which provides excellent noise immunity and eliminates
resistance sensitivity.  If you have no electrical noise problems and can use
the logger without a terminating resistance, the wire resistance should not
degrade accuracy.  Many other factors have far greater effect on system
accuracy.  The SOWACS archives have some additional comments on accuracy that
may be of interest to you.

M. G.

[Bruce comments: 
I have been through the archives found several appropriate examples

on accuracy, see for example
www.sowacs.com/archives/older/msg00051.html

There are also some temperature dependance related posts 
www.sowacs.com/archives/jul7.txt
and calibration related posts: 
www.sowacs.com/archives/jun5.txt
Bruce Metelerkamp]

owner-sowacs@aqua.ccwr.ac.za wrote:

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> Hi All
>
> Currently I am using a TDR system supplied by Campbell Scientific and
> comprises CS615 sensors and a CR10X logger.   I need to measure the soil
> water contents on different silvicultural treatments. Now these plots are
> really large and I will need to have cables up to 200m in length between the
> logger and sensor. Does the cable length affect the accuracy of the reading
> and what is the maximum cable length that can be used with these sensors?
> Second, I suppose like the neutron probe the best way to calibrate the
> sensors is gravimetrically?  Anyone else using the same equipment?
>
> Any comments or suggestions?
>
> Regards
>
> Luke
>
> Luke Esprey
> BSc (Hons), MSc (Hydrology)
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