[Scoundreller]

Dumb question I never thought about: do the circulation/filtration pumps get turned off during races? And for what minimum time before hand to let things settle?

If so, I guess this would be a serious competition only thing because you wouldn’t want them off for hours.

[dredmorbius]

FINA competition pool standards have recirculation minimum standards, and pumps run continuously. Further, "water distribution has to be such that no appreciable current or turbulence is created. 'Appreciable current' is defined as water movement that can move a floating basketball (filled with 6 litres of water to obtain the right buoyancy) in one direction for more than 1,25m in 60 seconds."

<https://resources.fina.org/fina/document/2022/02/08/77c3058d...> (PDF)

Most competitive swimming pools have a large number of inlets with diffusers on them, laid out every 2m or so across the pool floor.

Those are circular disks about 10cm in diameter, looking vaguely like this:

  __________
  \________/
   |      |

    ^^^^^^

The carets indicate inlet water flow beneath the diffusers. The effect is that water entering the pool largely moves perpendicular to the pool floor, and slowly diffuses upwards. Water return is through the (large, wide, deep) gutters.

Because the gutters are continuously removing water from the pool, circulation needs to be on to maintain a consistent fill level.

[sandworm101]

They do, but circulation cannot be totally stopped. This is a greater problem in outdoor pools but any pool will have some sort of temperature gradient that will inevitably result in circulation. Any water movement means slower times, at least in those events longer than 50m (1 length).

[OJFord]

Surely that has far less effect than that of all the other swimmers? Naïvely I would think we should be running time trials of a single swimmer at a time before caring about any of this?