Absolutely no recommendation to buy !!!! I ordered five hourglasses which at two hourglasses it always comes back to blockages. The trouble is that all five hourglasses have different cycle times. No hourglass creates 2min +/- 5 sec. Of no use for group work.
Yellow: 1:50 Blue 2:14 Black 2:15 Red: 2:28 Blue (with a thick font): 2:37