From: Keith Pollock (POLLOCKK) To: bruce@icfr.unp.ac.za Date: Tuesday, June 18, 1996 11:17 pm Subject:Re: http://www.icfrnet.unp....metele/sowacs/may32.arc Bruce We are using the Trase System I which can use stainless steel rods (the waveguides) from 15 to 70 cm. We have made up lengths within this range. The operator has to enter the waveguide length which remains in effect until changed. The system, if you are not familiar, relies on measuring the apparent dielectric constant (Ka) for a particular waveguide length (i.e., the Actual length / apparent length). The soil moisture %Volume is then estimated from a table of %vol vs Ka supplied in the instrument software. The manual states quite clearly that waveguide lengths less than 15 cm are too short to get an accurate readings, i.e., not enough time, and lengths longer than 70 cm will lose too much in signal attenuation. This model we use does not allow the operator to select shorter or longer lengths but if you were desparate, and patient, you could probably use shorter rods and work out your own calibration relating the instrument displayed Ka, set for a 15 cm waveguide, to known %vol of calibration samples. If the instrument displayed Ka is erratic or otherwise unreliable then one last resort may be to examine the graph (signal strength vs transit time) to calculate Ka, but this is getting pretty tedious. Graeme Buchan here is shortly to use the Tectronics system which allows a different configuations of wavequides including small spirals in a printed circuit. His Email is Buchan@lincoln.ac.nz. Keith Pollock Field Service Centre Dept. of Plant Science PO BOX 84 Lincoln University ph. (64) (03) 325 2811 (64) (03) 325 3808 ext 8129 fax (64) (03) 325 3880 PollockK@lincoln.ac.nz From: Keith Pollock (pollockk) To: BRUCE@mailgate.icfrnet.unp.ac.za Date: Thursday, June 20, 1996 1:22 am Subject:Re: TDR waveguides length discussion, calibr Date: Wed, 19 Jun 1996 12:14:39 +0200 From: Bruce Metelerkamp Subject: TDR waveguides length discussion, calibr To: CRAIG-C@CSIUS.COM, RUSSELL@CSIUS.COM, BUCHAN@ono.lincoln.ac.nz, POLLOCKK@whio.lincoln.ac.nz I have never done calibrations except to check that clean water is close to 100 %vol. A case of blind faith in the manufacturer's conversion table, and no indication, for my level of precision, that I need to do a calibration for our situation. But with detachable waveguides as in the Trase system the problem is one of using lots of rods in lots of different samples of known bulk density and water content then connecting the measuring instrument to one pair of waveguides at a time. If your sensors (I assume waveguides plus some lead wire and +/- some electronics) are not identical then individual calibrations look to be rather time consuming. You would have to check with CSI that their instrument can resolve the signal for the shorter reflection times - Sorry I can't be more helpful. Keith Pollock Field Service Centre Dept. of Plant Science PO BOX 84 Lincoln University ph. (64) (03) 325 2811 (64) (03) 325 3808 ext 8129 fax (64) (03) 325 3880 PollockK@lincoln.ac.nz