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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 Greg, With most models and makes of neutron probe, the count that is displayed is less than the count that is measured. That is, the measured count is divided by an integer before display. This is done to keep the count to a reasonably small value. The divisor used may vary with the count time as well. If the displayed counts are not converted back to actual counts before the Chi ratio is calculated then the Chi ratio will be wrong. This may be what happened. To know more we would have to know the firmware revision number of the probes involved, the number of counts you took to compute one standard count and Chi ratio, the number of seconds for each count, etc. Although the 503DR has been in production for many years, the firmware, and too some extent the hardware, have been upgraded several times. Thus, the newer probes are quite unlike the older (early 1980s) versions of the same model number. For at least the firmware version 503TDR.6 and up, there is a FACTOR setting under the MENU. It allows the counts to be displayed in a counts per 16 second format, which was used by the older 503DRs, or a counts per 60 second format. One way to get the proper counts for calculating a Chi ratio by hand would be to set the FACTOR to FACTR 16 (counts per 16 seconds) and then set the counting TIME to 16 seconds. I believe that this should give counts that are not modified by an integer division. Doing this gives me standard counts and Chi ratios much closer to the gauge's automatic values than you are getting. Another consideration is that CPN changed the way that standard counts are taken in the newer gauges. The new gauges take 240 1-second counts. This increase in the number of samples increases the probability that each Chi ratio will be close to unity. The older gauges took 32 8-second counts. Unless we take 240 counts for a standard we are not likely to see the stability in Chi ratio values from one standard count to the next that the gauge reports automatically. Best wishes, Steve At 06:32 AM 4/18/2000 +0000, you 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 went through the exercise of calculating the "Chi ratio" of the >standard count procedure using the equations in the CPN 503 DR neutron probe >manual and comparing them to the "Chi ratio" displayed on the instrument. Two >different neutron probes were used and three standard counts were done on each >instrument. The results were : > >Instrument number 706 >Automatic Manual >Chi ratio Chi ratio >1.07 1.51 >0.98 1.39 >0.88 1.28 > >Instrument number 3521 >0.97 1.37 >1.23 1.74 >1.01 1.43 > >"Automatic Chi ratio" is the reading on the neutron probe display and >"Manual Chi ratio" is the calculated value. > >I would be interested in comparing notes to see if my situation is unique. > >Gregg Dill > >_______________________________________________ >> Gregg Dill, P.Eng., Irrigation Specialist >> Alberta Agriculture, Food and Rural Development >> Agriculture Center >> 100, 5401 - 1st Avenue South >> Lethbridge, Alberta, Canada T1J 4V6 >> Telephone +1 403-381-5136 FAX +1 403-382-4526 >>______________________________________________ > > > > > ___________________ Steve Evett, Soil Scientist USDA-ARS, 2300 Experiment Staton Rd., Bushland, TX 79012 USA 806-356-5775, FAX: 806-356-5750 srevett@ag.gov, http://www.cprl.ars.usda.gov/programs/