The Vice President if they are available,otherwise The Speaker Of The House.
The Senate President Pro Tempore
When the US Vice President is absent or acting as President, his/her Senate duties fall to the US Senate President Pro Tempore, who by tradition is the longest-serving Senator of the majority party.
The 'President Pro Tempore' is the President of the Senate, when the Vice President of the United States is absent.
The Vice President. If the Vice President is absent, the Secretary runs the meeting.
The Vice President of the United States serves as the presiding officer of the Senate. In this role, the Vice President can cast tie-breaking votes but does not participate in debate except to break a tie. When the Vice President is absent, the Senate appoints a President pro tempore to preside over sessions.
The president pro tempore serves as the head of the senate in place of the vice president- APEX
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the president pro temp
the vice president serves as president of the senate
President Pro-Tempore
The President pro tempore is the second highest ranking official of the United States Senate . He is a senator who is elected to his position by the Senate.In actual practice it is not unusual for the vice-president and pro temp to both be absent. In that case another senator or, I think, one of the staff serves as time-keeper and keeps track of who has the floor.
The secretary conducts the meeting.