Cliff and Martin, I am afraid that gypsum block, like all other "small volume sensors" 'see' only the water molecules in their immediate vicinity (max a few mm) and from that point on, like all others media, it's a function of HC. True that TDR and neutron moderation see soil volumes a few orders of magnitude.
The criticism against GB regards mainly their non-linear relationships, weatherability, and huge difference between single units. arie Arie Nadler Dept. of Soil Physics Institute of Soil, Water, and the Environment A.R.O., Volcani Center, Min. of Agriculture POB 6 Bet Dagan, Israel, 50250 Tel: 972-3-9683865 Fax: 972-3-9604017 Home: 972-8-9404161 Email: [EMAIL PROTECTED] ----- Original Message ----- From: "Martin Hodnett" <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]> Sent: Thursday, July 15, 2004 9:51 AM Subject: Re: Gypsum blocks > > Cliff, > I agree about the problem of the small volume in which > measurements are made with many modern sensors. These > are usually of the capacitance type, with small pins, > or the electrodes within a tube. The latter only > measure in a narrow annulus around the tube, which as > you rightly say, is likely to be disturbed, almost > however carefully the installation is done. > > The sampling volume of the sensor raises the question > of "representative elemental volume", or REV - how big > a volume should be sampled to be "representative" of > the soil. Clearly the larger the better - with the > neutron probe in the lead here. > > However, there are modern TDR type sensors, with long > rods, which are spaced a few cm apart, which do appear > to have a larger sampling volume. 300 mm rods, 5 cm > apart would have a sampling volume of about 0.6 litres > (assuming a cylindrical field between the rods). > Perhaps 0.4 l if the em field is more of a flattened > cylindrical shape. There is another TDR type sensor > which is a flexible strip, 3m long, which can be > buried in the plough layer. This must surpass the > neutron probe in volume sampled, but cannot be readily > installed at greater depths. > > Cheers > > Martin Hodnett > > > > > > ___________________________________________________________ALL-NEW Yahoo! Messenger - sooooo many all-new ways to express yourself http://uk.messenger.yahoo.com > This message was scanned against malicious content by the > ARO secure anti-virus and anti-spam system. > Volcani Infrastructure & System Department > > > > This message was scanned against malicious content by the ARO secure anti-virus and anti-spam system. Volcani Infrastructure & System Department
