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Jim Bilskie writes to the sowacs list: ===8<============== Greetings: I have little experience with the Theta probe, but brought the CS616 to market and so have much experience with its operation. The linearity of the response is discussed in section 6.2 of the current CS616 manual. In general, the response is close to linear though not strictly linear. The effects of describing the response linearly compared to precisely is shown in this section also. As Steve states, the closer the response to linear, the easier to calibrate. The CS615 was more sensitive to signal attenuation and the response is 'less linear' than the CS616. The reason for the change response for the CS615 and CS616 is attenuation of the signal that is on the probe rods. Signal attenuation can be caused by presence of free ions, organic material or some clays. High compaction can confound the situation. When signal attenuation exceeds some level, the response changes. So if a user derives a good calibration that characterizes the response of one of these probes in a high EC soil, it is likely that the response in other soils with the same EC will be different because there may be different amounts of organic matter or different clay amounts. This must be considered if absolute water content measurements are important. My observations show that the CS616 is a time domain device. The principal difference between the CS616 and TDR devices such as the TDR100, Trase or Tek1502 is that the latter have elaborate sampling circuits that capture and digitize the reflection which is then analyzed with an appropriate algorithm. The CS616 generates a fast risetime pulse and couples it to the rods. The electromagnetic energy of this pulse travels the length of the rods, is reflected from the rod ends and returns. The travel time of the pulse along the rods depends on the dielectric permittivity of the material surrounding the rods. The reflections of 'traditional' TDR devices and the CS616 are actually a superposition of reflections. In the CS616, instead of capturing the reflection, the reflection is detected by a special circuit and used to initiate the next pulse. Consequently, the time between pulses is directly related to travel time along the rods. This time is scaled and buffered to be read by a datalogger or other suitable device. I've captured CS616 reflections with a high speed storage scope, and the reflections are actually quite similar to the commonly observed reflections from passive TDR probes. Another difference is that the risetime of the TDR100 and other devices pulser output is usually less than 200 psec while the pulse of the CS616 is a couple of nanoseconds. Hope this helps. My regards, Jim Jim Bilskie, Ph.D. Soil Physicist Campbell Scientific, Inc. On 11 Sep 2003, at 20:59, Bruce -owner of sowacs wrote: > > Roland Stenger writes to the sowacs list: > ===8<============== > Recently, Steve Evett wrote that > "Of the two, the CS616 output is linear with water content, while the > Theta probe output is nonlinear with water content. This makes > calibration of the CS616 much easier. Both devices should be > calibrated for the specific soil in which they are to be used." > > Is there anyone around who can address my follow-on questions: > > 1. Has anyone shown that the CS616 probe is linear with water content > for a wide range of soils (different texture, different mineralogy, > different bulk density)? > > 2. Has anyone on the list experience with field or laboratory > calibration of the predecessor model CS615 and is willing to share it > with the list? > > Cheers, Roland > > =============================================================== > Steve Evett writes to the sowacs list: > ===8<============== > Craig and all, > > The CS616 and the Theta probe are both frequency domain devices, while > the TDR100 system is a time domain reflectometry system. The TDR100 > captures a wave form from each probe in the system and analyzes it for > travel time, > > which is then used to estimate water content. Alternatively, the wave > forms can be stored and analyzed separately. > > My judgement is that the CS616 works better than the Theta probe. It > is certainly more robust. Of the two, the CS616 output is linear with > water > > content, while the Theta probe output is nonlinear with water content. > This makes calibration of the CS616 much easier. Both devices should > be calibrated for the specific soil in which they are to be used. > > Steve Evett > > > ******************************************************************* > Roland Stenger > > Environmental Scientist (Research) > Dr. agr., Dipl. Ing. agr. (TUM, Germany) > > Lincoln Environmental > Ruakura Research Centre > Private Bag 3062 > Hamilton, New Zealand > > Phone: (64)-7-858-4844 > Fax: (64)-7-858-4841 > Email: stenger@lvlham.lincoln.ac.nz > Homepage: > http://www.gsf.de/iboe/expertn/Authors/stenger/ > > ********************************************** > Some countries have oil, New Zealand has soil. > ********************************************** ===8<============== NOTE: To get off this list, send an email to list@sowacs.com with the subject line: unsubscribe_sowacs For full instructions see this page: <http://www.sowacs.com/subscribe/index.html>