Dose Achieved from Chemical Added

Run the feed equation backwards and you get the after-the-fact question every technician is asked: I dumped a pail in. Where does that put me? Ten pounds of product into a 5000 gallon system is 10 lb spread through 5000 × 8.34 = 41,700 lb of water, which is 10/41,700 = 240 ppm. In metric the same idea is even simpler, since a cubic metre of water weighs about 1000 kg: one kilogram in ten cubic metres is 100 ppm.

The number that wrecks this calculation is never the arithmetic: it is the system volume. Nameplate volumes on closed loops are optimistic, and towers with deep sumps, remote basins or long buried runs routinely hold 30–50% more than the drawings say. If accuracy matters, measure it: add a known mass of an inert tracer such as lithium or a fluorescent dye, let it mix for a full turnover, test the residual, and solve this same equation for V. That trick is the basis of every tracer-controlled feed program on the market.