Question

Which individual is the presiding officer of the Senate whenever the Vice President is NOT present?

Answer 1

The answer is president pro tempore.  This refers to the second-ranking officer in the Senate who takes over if the Vice President is absent.  Though limited, the president pro tempore does some of the functions of the Vice President during meetings.  Other functions are appointing committees and officials and receive Senate reports. Apart from that, he can tell the President to have Congress summon back for sessions.

Answer 2

President pro tempore