10 min jar hourglass timer

This is a 10 min hourglass timer. I use it in my class to time snacks and activities.

You'll need the following.
2 Pint Masson Jars
1 pt fine sand
1 3d printed coupler
2 part epoxy rated for plastic and glass.

Print the coupler, and carefully test fit it into the necks of the two jars. If there is a proud spot sand it down. DO NOT force this into the jars, that may cause cracking.

Fill the bottom jar with fine sand, leaving about a cm of headspace between the sand and neck of the jar.

Apply a liberal amount of epoxy to the flange of the coupler, making sure there is no epoxy in the hole in the center of the coupler.

Press the coupler into the neck of the sand-filled jar.

After that carefully glue and press the second jar on top of the coupler. Twist the two jars to make sure the labels and measurement sides of the jars line up.