Half Life: the amount of time it takes for one half of a given amount of material to decay
The way to compute the replacement rate required to maintain a constant volume with a given half-life is
r = ln(2) / h
where ln is natural log, h is the half life and r is the replacement rate required to maintain a steady state.
So, in order to maintain a given volume of oil with a half life of three days and a continuous rate of replenishment, the effective daily rate must be ln(2) / 3 or 23.1% of the total volume.
Some rates for reference:
half life replacement rate
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