proudly sponsored by
|
archiving of SOWACS proudly sponsored by |
|
NOTE: To get off this list, send email to majordomo@aqua.ccwr.ac.za with the body of the message containing the line: unsubscribe sowacs Dear Ioan, I have never used (or seen) an Adcon C-probe, so you should interpret my comments in that context. However, you mention you wish to use a sensor (probably capacitive) in an expanding clay, so I repeat the comment I made to another SOWACS correspondent on May 4th on the problem of finding out what the water content of expanding materials is - by ANY method. If the c-probe is capacitive, then it measures volumetric water content. If you take the normal definition of volumeric water content then you want the amount of water in a known volume of soil. In a swelling material - this can depend on the scale of the sampling. Clays can shrink with cracking or without. The difference is wether the cracks occur internally to your sample size or externally. If you are using a sensor, the sample scale is very small and the answer is going to be quite different if the sensor is located adjacent to a crack or in the centre of a clod and if the soil cracks on a small or large scale. Consider the case (which I once encountered) where a B horizon soil swelled but possibly due to a small organic matter difference or past history, some areas had fine cracking (on a scale of 1mm or so) , some areas cracked on a scale of 20cm or so. The first soil is easy - to measure the water content just take a volumetric sample with a sample ring about 5 to 7 cm diameter and handle in the usual way. The second soil was much harder. A 7 cm diameter sample is far too small to 'average' the cracks. To get a good answer we had to use a sample of 50cm diameter. Unless the sample includes a fair average number of the cracks, the answer you get will be quite wrong - and who deliberately samples cracks? This is VERY relevant to sensor technology. Most modern sensors sample a VERY small volume. A TDR (for example) set in the centre of a 'clod' of perfectly swelling clay which is drying should (and probably will) show the same water content until the clod gets to near air dry and starts to crack internally. This is not a problem with the TDR, this is what is actually happening. If you could repeatedly take a volumetric sample from the centre of such a clod, you would get the same answer. This is because the volume of such a clod can shrink EXACTLY in proportion to the water loss over most of the drying range. The gravimetric water content is decreasing because the mass of water per mass of soil is decreasing, and the density is increasing because the mass of soil in a fixed volume increases until cracking begins internally to you volume of sampling. So think carefully about what a sensor reading means in such materials. Cliff.hignett@soilwater.com.au owner-sowacs@aqua.ccwr.ac.za wrote: > NOTE: To get off this list, send email to majordomo@aqua.ccwr.ac.za > with the body of the message containing the line: > unsubscribe sowacs > > Dear Bruce and List-Members: > > I will appreciate very much your help in recommending published > literature on laboratory calibration of Adcon C-Probe across a broad > range of soil physical characteristics. Research results on laboratory > calibration using 2:1 clay expanding minerals (as montmorillonite) are > of particular importance. > > Manufacturer's calibration equations versus independent calibration > equations for different soil physical characteristics and clay > mineralogy types, will improve our understanding of multisensor > capacitance probes, and telemetry system's contribution to real-time > soil water content dynamics monitoring over large areas. > > Best regards, > > Ioan C. Paltineanu, Ph. D. - President > PALTIN Intl. Inc. > 6309 Sandy St. > Laurel, Maryland 20707 > U. S. A. > Voice & Fax: (301) 725-0604 > Email: icpaltin@bellatlantic.net -- Cliff Hignett Soil Water Solutions 45a Ormond Ave Daw Park South Australia 5041 pH 61 (08) 8276 7706 WWW.SOILWATER.COM.AU