I have an airspeed indicator in my hand, not connected to anything. I can blow into the AI's pitot port and quickly bring the indicated airspeed up to any value I want. When I quit blowing into the AI, the needle very slowly goes back to 0. It takes 60 seconds for the needle to go from 120mph indicated to 0 with both pitot and static ports open. Is this normal or does the AI likely need repair?