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I have data displaying the grind time for a sand sample and the resultant screen size in microns at which 80% of the mass falls through. we do this quite a lot so it will form part of a template.

I would like to know how to get excel to report what grind time I would need for the 80% point to be at a set particle size e.g. 80 microns. This is not a lineir plot. I can do this manually by looking at the graph but would like it to done using a formula to minimise human error.

X Values (80% pass point microns) 619.4, 172.1, 89.4, 74.0, 52.6,

y Values (grind time min) 0, 5, 15, 20, 30,

Many thanks

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you can use the solver add-in or do a what-if analysis to see what different X values result in Y values. Or are you looking for the formula that will calculate Y from X? Or are you looking to create the graph? I'm sorry, you might need to clarify your question. –  Raystafarian Mar 28 '12 at 11:17
    
If you plot ln(x) vs y, the last three points are very close to a straight line. I suspect this is generally true if all your tests are similar. You could fit a line to the last few points then compute the 80% value, but it may be sufficient to interpolate between the points that straddle ln(80). –  W_Whalley Mar 28 '12 at 14:35

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