From: Bruce Metelerkamp (BRUCE) To: SOWACS@aqua.ccwr.ac.za Date: Monday, August 5, 1996 3:11 pm Subject: Gypsum blocks homepage creation & & Sowacs.. Gypsum blocks, radiotransponders and NEW homepages (Dynamax's and Campbell Scientific Inc.) For info on sensor & logger setups connected without wires, take a look at: http://www.icfrnet.unp.ac.za/~metele/sowacs/wireless.html There are two new (to me) homepages worth noting: Campbell Scientific has an excellent page which I have only just discovered: http://www.campbellsci.com Dynamax's page has been around, but I have now added it to my list: http://WWW.pernet.net/~dynamax Dynamax supplied me with additional details about the DeltaT thetaprobe ML1 which are available at: http://www.icfrnet.unp.ac.za/~metele/sowacs/thetainfo.html =================================== I also thought it about time to start a discussion on GYPSUM BLOCKS and other such marvels of pre-industrial revolution technology! This text appears as the entry for GYPSUM BLOCKS on the SoWaCS web page: (which you can read there if more convenient) http://www.icfrnet.unp.ac.za/~metele/sowacs/gypsum.html I will welcome comments, corrections and ammendments. GYPSUM BLOCKS BASIC PRINCIPLE The principle of operation is that the resistance of a pourous block is proportional to its water content. Thus, the wetter a block is, the lower the resistance measured across two embedded electrodes. The catch is - one has to use an AC ohmmeter - which are still available at a price. This type of sensor is suited to various irrigation applications where only "full" and "refill" points are required. For more exacting work, gypsum blocks tend not to have the range, sensitivity or reaction time required. Upon drying, they may become uncoupled with the soil solution. ADVANTAGES Inexpensive allowing many replicates. DRAWBACKS All such types of blocks suffer from hysteresis (more resistance to wetting up or drying out (or vice versa) at a set water water tension). The range is usually only down as far as 100 kPa tension. The sensitivity in the dry range is usually very flat (a large change in dryness reflects small changes in measured resistance). The gypsum was an attempt to buffer soil salinity changes which does work to some extent. The result is, however, that the block will degrade over time, and eventually dissolves completely into the soil solution. The time this takes may be in the order of a year depending on conditions. The more extreme the water content - the quicker this occurs. GENERAL In attempts to overcome some of these problems - various modifications have been introduced. Watermark (trademark) blocks have gone someway to solving some of these problems. To improve the characteristics of gypsum blocks, the pore size distribution may be matched to the soil texture being used. Smaller pores will allow the block to register to lower water contents, but reaction time will be reduced. LITERATURE: A link to landmark papers to follow when found.. Manufacturers; DETAILS; PRICING; CONTACT details etc See the web page for details when they become available: http://www.icfrnet.unp.ac.za/~metele/sowacs/gypsum.html 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 /SoWaCS.html /RR.html Host of SoWaCS (Soil Water Content Sensor) Discussion List. SEND info sowacs OR subscribe sowacs TO majordomo@aqua.ccwr.ac.za