Why there is always timing errors of sand timers?

People who buy a hourglass in purpose of a timing tool do curious about how accurate is a hourglass? how precise is a sand clock?but actually the hourglass countdown still cannot be as precise as digital clock or stopwatch,with modern industry technology,the deviation of a hourglass timer is around plus/minus 10%,let's give you a example to help you understand how much is that,say you want custom a 2 minutes sand timer for your board game set.2 minues is 120 seconds,so the deviation is ±12 second.in other words,on average,the sand clock duration you have will be from 1min 48s to 2min 12s.

why is that happen? what cause it?

here are some of our viewpoints based on years of production experiences:

1. The sand beads friction between each other,as you know,we use artificial glass beads as sand in modern sandglass,original sand we purchase is in white color,we can add color powder to get different colors we like it be.the size of the glass beads we use range from 80 meshes to 600 meshes.but if you magnify the beads,you will see they are not perfect spheres,so when thousands of beads moving together,there will be friction,especially they are not a perfectly spheres.

2. Static electricity between glass beads and glass bead with the inner side of the glass bulb.

according to our experience,static is the main reason caused the counting not precise,and some other problems caused by it as well,such as sand stuck when dropping down,sand beads stuck on the glass bulb inner side.of course we have our methods to Eliminate the static,but it just cant be fully eliminated.

3. have you guys notice one problem on hourglass? each hourglass two countings actually,the counting from side A to side B is different to from Side B to Side A.Although the difference is not big,but it is there! why is that?