What diameter of outlet tube does a biosand filter use?

The early versions of the biosand filter (v8 and earlier) used a ½” PVC pipe for the outlet tubing.  This is essentially ‘weir flow,’ meaning that the water will exit through the tubing until the water level drops to the same level as the bottom of the upper bend in the pipe (see the first diagram below). 

When the biosand filter was redesigned to make it lighter weight, the outlet tubing was changed to ¼” ID flexible tubing so that it would fit within the thinner walls.  In consequence, the water flows out of the v9 and v10 biosand filters as ‘siphon flow.’  This means that the water level will drop until it reaches the level of the bottom end of the spout, as in the second diagram below. 

Either flow mechanism works fine.  The important factor is knowing where the water will drain to, and ensuring that there will be enough water over the sand surface during the pause period.