session updates

[eluser]sawatdee[/eluser]
Does anyone know why CodeIgniter changes the session_id every 5 minutes by default? Is there a security reason for this? If not, is there a way to make session_id remain the same and only regenerate a new one when the previous session has expired? I need to log some information for each session, but if the user reloads the page then the session_id changes and I can no longer identify what session it is.