Bruce, ** Re: Calibration of TDR sensors. We have tried several methods of calibration of our `TRASE' TDR, i.e. the instrument supplied by Soilmoisture Equipment Corporation, USA. 1) Field calibration. Following a sequence of (say six) repeat measurements with shorter waveguides (e.g. 20 or 25 cm) at a single position, the waveguide pair was extracted, and bulk density cores were taken using a large-ring (8 cm diameter, 8 cm length) coring tool (also supplied by SMEC), centred on the waveguides. The 8 cm diam. almost exactly matches the `zone of influence' of the TDR field pattern. 2) Container calibration. A container (e.g. plastic bucket) of moist sand or soil is prepared, with known (average) volumetric water content. The waveguides are then installed, and readings taken. The moisture distribution need not be rigorously uniform, since the TDR measures _average_ moisture content along the waveguide length. 3) Comparison with Tektronix system. Measurements were taken in the field, first with a TRASE, and then a Tektronix-based system (both with waveguides of the same length, and centred on the same location). Excellent agreement was obtained. Our calibrations have shown the TRASE to be `on spec'(i.e. +/- 2.5%). In my view the TRASE is a very good, reliable instrument. ****************************************************************** ** Re: Equipment to measure soil water potential (psi). You may recall my earlier posting, requesting info. on equipment which measures the psi of small samples, equilibrated in a chamber. I have now traced full details of this equipment (Thanks to John Annandale in S.Africa). It is supplied by Decagon Devices, Pullman, Washington State, USA. There are two instruments: * The `Tru Psi' measures broadly at the moister end of the psi scale, via either a wet bulb or psychrometer method. * The `Aqualab' measures broadly at the drier end, e.g. dry surface soils. The equipment seems to be very versatile. I am now pursuing funding to buy both instruments, to service a wide variety of research projects, including measurement of moisture status (psi, or `water activity') in soils, wood and timber, plant tissues, compost, foodstuffs.....indeed any composite material ! I can airmail you copies of Decagon brochures, or you can email direct to Bryan Wacker, Soil Science Product Manager, Decagon Devices bryan@decagon.com I hope the above is of some use. Best wishes, Graeme Buchan, Soil Science Dept, Lincoln Univ, NZ ****************************************************************** > Date sent: 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 > Keith Pollock, Graeme Buchan and CSI developers > > Graeme, for your benefit, Keith Pollock suggested I contact you > re shortening TDR wave-guides. (See message below). > > It seems you guys are the ones to talk to. (No need for the whole > of SoWaCS discussion group to get involved - until I can produce > a summary posting). > > My plan (as I mentioned to Keith) is to simply cut my TDR > waveguides shorter than they are supplied, and recalibrate. > I understand that they are below a critical lenght the > electronics will not have the resolution (enough time) to be able > to measure the reflection time. > But I do not know how exactly the Campbell Scientifc Inc (CSI) > CS615 sensor works, so can't speculate on the likely result of > doing the big chop! > Perhaps CSI will help here. > > The purpose of chopping the waveguides shorter is to allow them > to be used to sample [small] samples augered in the field. A > 300mm guide is too long. > > While I am talking to the experts: how is a calibration of TDR > sensors customarily carried out (in practice). Obtaining a > homogeneously wet soil at suitable increments either means a lot > of mixing of soil (and repacking), or a lot of waiting for > natural redistribution of water within a container. Any > suggestions? (I don't really only want one datum point at the > completely dry end and one at the wet end). Perhaps a tension > table would be best for the wet end? > > Regards > > -.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-. > Bruce Metelerkamp SOIL WATER RESEARCH OFFICER > Institute for Commercial Forestry Research, > University of Natal, PO Box 100281 > Scottsville, ZA3209 > Rep. of South Africa Voice:27 331 62314 > E-mail: bruce@icfr.unp.ac.za FAX:27 331 68905 > URL http://www.icfrnet.unp.ac.za/~metele Dr Graeme D Buchan, Reader in Environmental Physics & Environmental Education, Dept. of Soil Science, PO Box 84, Lincoln University, Canterbury, New Zealand