Is there a word complementary to "ullage"?

"Ullage" is a useful name for the empty space that often remains in a container of liquid after it has been filled as much as is practical. (Think of the small space at the top of a sealed bottle of wine.)

Is there a name for the complementary situation, where a vessel has been emptied as much as is practical, or its design allows, but there still remains in the vessel a quantity of liquid that is inaccessible? I can think of "dregs" but wonder if there is another word that does not have the connotation of "the worst part" as "dregs" has. A word that could be used in the following sentence without disparaging the quality of the liquid that has been lost.

"The poor placement of the submersible pump made dregs unavoidable."

Solution 1:

In trucking we call it the heel. In some cases it's referred to as "tank heel." As sited at (id.energy.gov/EIS/Chpts/ch_7.pdf), "A tank heel is the amount of liquid remaining in each tank after lowering to the greatest extent possible by use of the existing transfer equipment, such as ejectors."