Arie The actual resistance measurement is indeed within a "small volume sensor", but I think the point that Cliff was making is that the gysum block equilibrates with the matric potential "field" in which it is placed, and as such, has a larger effective "sampling volume". There will be a lag time while it adjusts to the changing potential, which will depend on k unsat of the soil and the contact with the soil.
A capacitance sensor, especially one inside a tube, is not equilibrating with the potential field in the same way, and has a very limited effective measuring volume as a result. GBs do indeed suffer from a range of disadvantages, but they are cheap, and in "benign" soils can give good service, especially in non-scientific applications were great accuracy is not required, and some drift can be tolerated. Martin Hodnett > 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 > > > > ___________________________________________________________ALL-NEW Yahoo! Messenger - sooooo many all-new ways to express yourself http://uk.messenger.yahoo.com
