Balancing grades, and what an out-of-balance impeller really costs
G6.3 or G2.5 is not a slogan. It sets a permitted residual unbalance, and the force it allows rises with the square of the speed.
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Why the sound rating you were quoted depends on where it was measured
Sound power and sound pressure are different quantities, and inlet, outlet and casing-radiated levels solve different problems.
Reading a fan curve and picking the operating point
The fan runs where the fan curve crosses the system curve, not where the catalogue extract says it does.