The number of baffles is usually 4; the number of impeller blades ranges from 4 to 16 but is generally 6 or 8. Special situations may, of course, dictate different proportions from those listed above; it may be advantageous, for example, to place the agitator higher or lower in the tank, or a much deeper tank may be needed to achieve the desired process result. The listed "standard" proportions, nonetheless, are widely accepted and are the basis of many published correlations of agitator performance.