Because I have airflow measuring gear and a bit of expertise I decided to measure the air flow of the rear fans to see if my HEPA filter Mod significantly slowed the air speed.
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Here are the results measure in L/s for 4 filtering arrangements for two speeds (50% and 100%).
Uncertainty in measurements is +/- 0.2 L/s.
First observation is 50% is not half the airflow of 100%, going from 50 to 100% increases flow by about 20- 30%
Removing the front (inside) Grille (FG) or the Carbon Filter (CF) makes little difference, as does adding my small HEPA filters.
The biggest difference is obtained by opening the door, about + 18%.
FWIW the airflow meter is a TESTO blue tooth linked to mobile phone instrument. I use it mainly to measure air flows in wood and metal shops.
The main constructor to air flow is likely to be the size of the small holes on the rear exit. When My warranty runs out I may drill these out a bit.