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Here are the other factors that affect the accuracy of an hourglass:
The amount or volume of sand used
The size and angle of the glass bulbs
The quality of the sand or granular material. It must be fine, dry and consistently formed so it can flow smoothly. (Some substances used in the past were fine grain sand, powdered eggshells, and powdered marble.)
The width of the neck
A tight seal so no moisture can get into the chambers. Moisture can add weight to the sand or clog up the neck.