I'm curious: "A group of students needs to determine the time constant of a water tank. FIrst, the student establish an equilibrium level, with a known flow rate into the tank and both valves (solenoid and hand-operated) fully opened. One of the students reduces the input flow and the group records the change in water level as the level drops. after the level reaches a new equilibrium level, the student closes the hand valve. The group then recods the data as the level rises. analysis of data yields that the time constant for the tank when the level is falling is 9 minutes, while the time constant for the rising level is 11.5 minutes." Why is that so? why is the time constant value different?