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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 The use of clay slurry close to the tube in a sandy soil environment would produce some VERY strange results indeed. I would have used local soil if at all possible. If there is a problem with water flowing down the loose soil zone near the tube then I suggest a thin band of bentonite slurry (say 10mm deep) inserted with a tube at about 15 cm depth. (It may be possible to locate this band such that it is not 'seen' by the sensor - ie if you measure at 10 and 20cm depth then place the band at 15 cm. I have suggested an alternative approach to to work around this problem to several makers of devices such as this. I repeat it here. I hasten to point out that this technique is time consuming and NOT justified if you just want a wet/ dry indication. On the other hand, in a precision irrigation application it would give a VERY good indicator of plant water status - better than an indirect measure such as water content. For short measurement range devices such as the Sentek and Diviner the best method would be to dig a oversize hole (eg 10cm diam) and fill with a 'soil' compounded to give a very even moisture release curve. A thin layer of bentonite should be placed midway between measurement points to isolate each measurement depth. Then the sensor will only measure the water content of the compound soil filler. Knowing the moisture release curve of the filler, the suction in the filler and therfore the main soil body can be determined . If you actually need the water content of the soil, then you will have to get a moisture release curve for it. In other words I am suggesting these devices be used as a tensiometer. Yes, I know that there are all sorts of problems with hysteresis. But they are known and understood. The errors associated with 'fillers' around holes are just that - errors. 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 > > I recently acquired a Diviner2000 moisture probe. The supplier installs the > access tubes by drilling a hole of 10mm bigger OD, inserts the PVC tube and > settles it in with a clay mixture (similar to potters clay - the exact > compostion I am not sure of), to ensure perfect contact between tube and > soil. > > We work with sand (as in sand dunes) with moisture holding capacity as low > as 45 mm/m. > > I am wondering what the effect of the clay band in direct contact with the > tube could be. > Can I assume that the sand-clay will reach equilibrium, in a short enough > time to give me the correct response. > > Your comments will be appreciated. > > Thank you: Jac le Roux > jac@besproeiing.co.za -- Cliff Hignett Soil Water Solutions 45a Ormond Ave Daw Park South Australia 5041 pH 61 (08) 8276 7706 WWW.SOILWATER.COM.AU