Water hour glass timer

Frustrated and had not kitchen timer .... *cracks knuckles* ... make my own timer using two 2L water bottles left lying around. Basic concept is to join the bottles together with a elongated nut that has a solid plate in it with 2 tubes that allow the air to flow one way and the water in another at a controlled rate.

My experiments have resulted in about 1000ml of water will give you close to 10 minutes of time. this is greatly effected by the print quality as it slows things down if there are zits and ridges inside the "flow tubes"

Print have fun and if there are any people that are good with fluid mechanics that can help me calculate the exact aperture of the holes to make it 1000ml // 10 min love to know about it.