Further information about affecting factors...

About sand hourglasses: Learning more about what determines the passing of time, and especially the accuracy of passing time, the factors can be further narrowed to 3 areas of focus - the size of the particle, the size of the aperture the particles move through, and the angle at which the particles slide against the container's wall toward the aperture. The Mechanics of the Sandglass lays out a mathematical formula for determining accuracy. The math focus on these 3 areas and even when approaching the dilemma in a practical, day-to-day fashion, common sense seems to concur.


If the hole is too small for particles to fall through easily, then their descent is slow or perhaps get stuck. Even if one could squeeze through a hole that was just big enough, all the particles would have to be the exact size.  With just one that is too big to easily pass through, there would be a back up.

Left shape - sharp, almost vertical wall
encourages more sand to fall toward the hole whereas
the right flattens out the angle discouraging particle fall.

Another issue concerns the angle of the container wall. As the wall becomes more  horizontal, the particles would have less gravity promoting the descent through the hole. This becomes a problem with hourglasses built from plastic bottles or jars using a horizontal pieces of board or plastic between the two openings with a punctured hole in the middle for the sand to fall through.