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Trevor Finch writes: I was interested in Cliff Highett's comments with regard to the importance of *change* of soil moisture. I have been trying to collect calibration equations for NP's for different soil types, comparing equations from different probes by normalising to counts in a 200 l water drum. (taking standard counts in the shield seems to be pointless...) Calibrating several hundred different CPN 503's in the same four sealed drums of soil show a strong linear relationship between the different probes. Whenever the cross-correlation was non-linear an error was found in the 'dry' drum, probably because of an external influence on the count. The conclusion is that the most reliable way to 'cross-calibrate' a new instrument is to just do a careful count in water. It is also the most practicable way to field check an instrument. If the water drum count changes then the instrument is faulty - do not re-calibrate. However, I couldn't understand the comments... >We found that not only was this counter productive in error terms >(subtracting one value subject to field and calibration curve error from >another point similarly affected by error doubles the calibration curve >error) but as the above reference showed, it actually produced the WRONG >answer BECAUSE of the field error. The result was biased - ie even with >reduced error, the mean was tending to the wrong answer. Could you explain this a little more ? VSW1 = Count1 * Slope + Intercept VSW2 = Count2 * Slope + Intercept Change = VSW2 - VSW1 = (Count2 - Count1) * Slope + Intercept I have obviously missed something, because it seems to me that, arithmeticaly, you get the same result whether you... scale and then subtract subtract and then scale ------ In terms of statistical reliabilty of measuring *changes*, it seems to me that continuous monitoring of a site to measure *changes* is essentially non-destructive testing and the criteria should be... 'If the total soil water has not changed and we measure again, how much change do we get in the reading ?' (and readings taken with a neutron probe down the same tube in stable clay show remarkably constant counts over many years) I would suggest that the recommendation of routine field calibration can lead to errors. Limited soil moisture and bulk desity measurements, especially when not taken over a wide range of soil moisture, can lead to more errors than using a 'standard' calibration curve. ------- Anyway, we are looking for equations for different soil types in the form... VSW = (Count/WaterDrumCount) * Slope + Intercept Typically... Slope = 0.68 Intercept = -0.01 Does anybody have any equations in this form for different soil types ? ---- Trevor Finch Research Services New England 8/16 Nicholson St, Balmain NSW 2041 Australia email: trevor@rsne.com.au tel: +61 (2) 9810 3563 fax: +61 (2) 9810 3323 ----