Phenomenal Travel Videos explains the working thus:
The sundial has 8 major spokes that divide 24 hours into 8 equal parts, which means that the time between two major spokes is 3 hours. There are 8 minor spokes as well. Each minor spoke runs exactly in the middle of 2 major spokes. This means that the minor spoke divides the 3 hours in half, so the time between a major spoke and a minor spoke is an hour and half or 90 minutes.
Now, at the edge of the wheel, you can see a lot of beads. If you observe carefully, you can see that there are 30 beads between a minor and a major spoke. So, the 90 minutes are further divided by 30 beads. This means that each bead carries a value of 3 minutes. The beads are large enough, so you can also see if the shadow falls in the center of the bead or on one of the ends of the bead. This way we can further calculate time accurately to the minute.