Fill a hose with water, bend it into a U-shape so you have a connected body of water inside. Then attach the fan to one side (so it blows directly onto the water surface inside the hose, and measure how much lower that surface now is compared to the other side of the hose.
The more pressure the fan can generate, the more it will push the “water column” down on its side. If it’s for example 3mm of difference, that’s a 3mm H2O fan.
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