Used TFSE. Found 2 different equations for calculating nitro %, forgot the post #'s, so:
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Once you've got the density, use the formula (which assumes ideal solution behaviour, but oh well):
(2.889*d -2.285)*100
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There's also:
EDIT: I plotted some points and did regression on it... the formula for w/w is:
x = -3.0173*(SG)^2 + 8.6901*SG - 4.9828
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Swim got ~19% for the first equation, and ~25% for the second. Which equation is more accurate? The second one was determined experimentally, the first assumes ideal behavior. If there is no answer, swim will probably go with the lower value since a bit of excess shouldn't hurt. Swim is anal about these things.