Evaporation changing chemical concentrations

Water loss from draining, leaks, splash out and carry out would take with it the same concentration of chemicals (FC, TA, CH & CYA) as found in the rest of the pool. So these kinds of losses would not change the results of a test of the remaining water. Of course filling the pool would then change the chemical concentrations based on the amount and chemical concentrations of the fill water.

But what about evaporation? I would expect the water removed by evaporation to be chemical free ( or very nearly so). So in a pool with high evaporation losses, wouldn't these concentrations build over time? (At least for TA, CH & CYA).

For example, suppose I have a CH concentration of 250 ppm with a full pool and I lose half the volume due to evaporation. Shouldn't I expect a CH concentration of 500 ppm now? If I refill with water having 250 ppm CH, shouldn't I expect a CH concentration of 375 ppm?

I get a lot of evaporation losses. Our pool has NEVER EVER looked better since we started using the guidance provided on this forum. We are currently using cal-hypo to raise CH, but I'd like to know I f my assumptions are right so I don't go too far (or not far enough). Our fill water runs through a water softener and has a CH of 110 ppm.

If you never replace the water and let the water level drop, then yes, the remaining chemicals would become more concentrated. But that doesn't typically happen in a pool. Most people replace evaporated water with fill water which then dilutes the chemicals back to where they were before the evaporation but with some added components. Fill water will normally contain CH and TA so over time CH and TA will climb.

Makes sense. I guess I thought so. Unless fill water CH & TA were zero, the concentration will always climb.

My finish was being etched from low CH & PH, but I didn't know it until I came here. I got my CH up to 250 last week, so I'm hoping the "sand in the bottom of my pool" diminishes and stops.