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| Good question. I too initially thought it was % light that was getting through, but the reading is actually % of light blocked. On startup, I think the sensors take a baseline of how much light is blocked and the use that as the effective zero point. The more beans flowing the greater % of light that is blocked=greater population. That is also why seed size (seeds per lb.) and calibration (some function of lbs of seed dropped per revolution of the meter roll) are important. If the sensor knows it has 9 square cm and 50% of the light is blocked by seed flow, it has to know seed size to know how much light any one seed would block. 4.5 square cm of light blocked divided by the cross section of a 2800 seeds/lb bean yields the number of seed between the sensors at any given point. That is all prob way too much info, but that is why it is important that you get that no flow number as low as possible, so it has as much range to work with as possible and there are as few beans in "blind" areas as possible.
That being said, as much as I can tell and get out of my Deere seeder tech guy, any thing under 50% light blocked at no flow will still yield semi accurate pop readings once seed is flowing. Above 50% light at no flow and accuracy falls out of bed rapidly. At 20-30% you were quite good. If you have them down to 7-8% that is excellent. Cleaned ours and the best are down to 20ish. It is a controlled spill anyway, so not too worried about it being dead on. Not sure what a brand new one would read, but my guess is that it would not read 0%.
Edited by jbweston2 4/21/2009 11:10
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Need info on JD 1850/1900 air drill
