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Peter Toome writes to the sowacs list: ===8<============== Bruce et al MEA is the Australian Distributor for Delta T products. I would like to make a correction to Steve Evett's comments on the Theta Probe. Frequency domain devices (FDR), such as capacitance sensors, use the change in dielectric constant of the soil as water content changes to alter the resonant frequency of an oscillator circuit (with the sensor forming the capacitance component of an L-C resonant circuit). The frequency of operation is then measured and, by use of a calibration equation, related back to moisture content. Factors such as the thickness of the access tube in which the sensor is installed, air gaps between the sensor and access tube, air gaps between the access tube and soil, and variations in the propagation of the electric field into the soil as frequency changes all create problems with the accuracy of FDR sensors: they are oft quoted as highly repeatable but very imprecise. Hence the importance of the calibration. The Theta Probe is not a frequency domain device - it often gets incorrectly lumped in with the FDR sensors because people do not know how to classify it. It operates at a single frequency (around 100 MHz). Like the other devices it does however determine volumetric soil moisture content by detecting changes in the soil's dielectric constant. The pins of the Theta Probe and the soil between them become part of a Radio Frequency (RF) transmission line - much the same as the coaxial cable feeding an antenna on a radio transmitter. (The pins are about 60mm long and enclose a volume of soil 60 by 40 mm). Normally a radio transmitter feeds into an antenna and you "tune" the antenna for the best impedance match. In place of an antenna, the signal from the Theta Probe feeds into an open circuit (the soil at the end of the pins)- creating a deliberate impedance mismatch. Rather than all the RF energy being transmitted, some of the RF energy propagates into the soil and some is reflected back into the transmitter - a Standing Wave is thereby generated. The amount of energy transmitted versus the amount reflected is measured as the Standing Wave Ratio or SWR. When the dielectric constant of the soil changes (i.e as moisture changes), the impedance of the soil changes hence the degree of mismatch between the transmitter and its pseudo antenna changes, changing the SWR. It is this change in SWR which the Theta Probe then measures and relates back to moisture content. The Theta Probe's single frequency operation and the fact that the measurement is dominated by the soil contained between the pins which allow it to measure accurately in a wide range of soil types. Better than 5% accuracy can be achieved using the standard calibration and 1% accuracy is available after a 2 point (wet and dry end) gravimetric calibration. The calibration generates two coefficients which are used to simply shift the position & slope of the calibration curve. The Theta Probe is also very tolerant of salinity, continuing to record accurately at levels where the attenuation of the transmitted pulse would have rendered a TDR unit unusable. We can supply information on the calibration process and performance of the Theta Probe is anyone wishes to access it. Peter Toome Marketing Manager Measurement Engineering Australia (MEA) Data Logging Applications and Measurement Systems Engineers 41 Vine St., Magill, SA 5072 ph 08 8332 9044, fax 08 8332 9577, mob 04 1883 2422 Web www.mea.com.au E-mail peter.toome@mea.com.au -----Original Message----- From: Bruce -owner of sowacs [mailto:bruce@sowacs.com] Sent: Monday, 18 August 2003 6:40 PM To: Bruce Metelerkamp Subject: Fwd: Re: SOWACS: request info - Campbell CS616 probes v theta probes 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 At 08:03 PM 8/11/2003 +1200, you wrote: >Craig Barton writes to the sowacs list: >===8<============== >Does anyone know the relative merits of the Campbell CS616 and Delta T Theta >probes. Or how either of them compare with the Campbell TDR100 system for >monitoring soil moisture. >Thanks >Craig > >_______________________________________ >Dr Craig Barton >Forest Research and Development Division > State Forests of NSW >PO Box 100 >Beecroft >NSW 2119 >AUSTRALIA > >Phone: + 61 (02) 9872 0199 >Fax: + 61 (02) 9871 6941 > > >===8<============== Steve Evett, Soil Scientist, Lead Scientist - Crop Water Use and Irrigation Research Team Soil and Water Management Research Unit USDA-ARS, P.O. Drawer 10 (2300 Experiment Station Road), Bushland, Texas 79012 USA 806-356-5775, FAX: 806-356-5750 srevett@cprl.ars.usda.gov, http:/www.cprl.ars.usda.gov/programs/ ===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>