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I am starting up an algae project, and I am getting a bit confused about some details in calculating density. For example, I take a sample of 250 mL, and concentrate it down to 25 mL with a sand filtration apparatus (factor of 10). I am using a gridded sedgwick-rafter chamber to enumerate the density of algal cells on a cells/mL basis using this formula from Standard Methods 10200F: cells/mL = [(# organisms counted)* (1000 cu. mm.)/ (field area)(field depth)(# fields counted)].

Once I calculate cells/mL, should I DIVIDE that result by my concentration factor (10)? My instinct is to multiply, but wouldn't that be appropriate only if I diluted the sample?

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Your original sample is more dilute than the sample you counted. The concentration is less and therefore you should divide your counted concentration by 10 to get the concentration in the original sample.

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