That does make sense...but periscope depth from what I've heard is way to shallow for that to mean keel.
Keel depth always. There are several depths that are of major concern for a submarine and each varies from class to class. Test depth for certain (max normal operating depth), the depth in which you broach the sail making the submarine visible to radar and visual contacts, periscope depth(most masts and antennas also work at periscope depth with the exception of the induction mast that takes sea conditions into account), and the depth at which you can efficiently dewater the ship in the event of flooding.
Some other tidbits for you: any tank exposed to sea pressure is considered a hard tank and can be pumped to sea at any depth, but under normal operating conditions for safety reasons normal ship keeping functions are restricted to shallower depths. Trim tanks, depth control tanks, and aux ballast tanks are hard tanks pumped to and from to balance the ship and fine tune bouyancy. Main ballast tanks are not exposed to sea pressure since they are open to the sea at the bottom of the ship at all times. Trash is compacted and weighted, then put in a mini tube and flooded to sea. Sanitary tanks on my boat were pressurized with air and blown to sea.
