Water leaves a cooling tower by exactly three doors, and the makeup meter has to pay for all of them: $M = E + B + D$. $M$ is the makeup, $E$ the evaporation, $B$ the blowdown or bleed, and $D$ the drift. All four are flows in one unit — this lesson works in US gallons per minute, because the constants below were fitted in them.

Evaporation is the useful loss and the largest: $E = 0.001\,R\,\Delta T$, read aloud E equals nought point nought nought one, R, delta-T. $R$ is the recirculation rate in gpm — the flow the tower pump puts over the fill — and $\Delta T$ is the range in Fahrenheit degrees, the drop the tower puts on the water. Say the constant in words and it stops being arbitrary: a tower evaporates about 0.1 % of its recirculation for every °F of range. It is a rule of thumb, good to roughly ten percent, and it comes from the latent heat of water being about a thousand times its specific heat per degree.

Blowdown is the deliberate loss: $B = \dfrac{E}{\text{COC} - 1}$. Only evaporation concentrates the water, and the minus one is there because the makeup already arrived carrying one cycle's worth of salt — you only have to carry out the excess. Read what the shape promises: going from 3 cycles to 6 halves the bleed. From 2 to 4 cuts it to a third. The savings are steep at the bottom and nearly flat above 8, which is why chasing 12 cycles is usually chasing very little.

Drift is the accidental loss: $D = \dfrac{d}{100}\,R$, where $d$ is the drift rate, a percentage of recirculation quoted by the eliminator manufacturer. Modern eliminators claim 0.005 %; an old splash-fill tower without them can lose 0.2 %. Drift matters out of proportion to its size because it is liquid water — it leaves at full basin concentration, carrying salts, treatment chemical and, in the wrong circumstances, whatever is living in the basin.

Four letters, one balance, and any three of them give the fourth. That is what makes the survey in this chapter's boss possible with nothing but a clipboard.