^^^^
I was told by a collector that if the person can't go, they are allowed 40 ounces of fluid over 3 hours and obviously you can't leave the facility during this time. Then the collector is also supposed to mark on the form how much was consumed over how long. You have to be able to produce a sample within the 3 hours or else it is a refusal.

Shaggy, sports drinks and not water is preferred because it helps keep up specific gravity.