Looking at my gravity tube you can see that it's clear. I've never tried to do a frame by frame calculation from game camera pictures, but I have several pictures of a deer eating from the tube alone when the feeder is almost empty (you can see the protein level dropping inside the tube with pictures every minute).
I'm sure if someone did the calculations you could figure out what a given deer ate in pounds by how many inches the tube dropped in the time frame that deer ate from it.
i.e. Deer #1 ate for 5 minutes and the level dropped 4 inches inside the 4 inch PVC. Not exact and you could only do the calculations when the feeder is almost empty, but could be interesting none the less.
This clear tube gave me proof my 500#'s of protein only lasted 2-3 weeks with ONLY two feed ports which led to the purchase of a 1500# Lamco.