Black Water Tank Missing Capacity

Lantley wrote:
How much residual water remains in the tank? The tank does not drain to a bone dry status.

? My black and gray tanks always seem to drain to bone dry status taking into account the bottoms are sloped, and the nipple is formed into the tank so it's invert is even with the tank bottom. I can see a fresh tank holding residual water, but if it's the typical black or gray tank then something else is going on for it to hold enough water to affect overall capacity, like a big loogie jammed into the outlet or something.