A bit of a math problem

[pedja089]

Take measurement ever XYZ sec.
Discharge rate is First-Second.
Eg do something like this:

RunFirstTime:
ADCIN SensorValue' or whatever
OldSensorValue=SensorValue' initialization at beginning of program
return

SubRoutineToRunEveryXYZsec:
ADCIN SensorValue' or whatever 
DischargeRate=OldSensorValue-SensorValue
OldSensorValue=SensorValue
RETURN

Depending on how often you call SubRoutineToRunEveryXYZsec you can get quicker response, or better resolution(wider range of values). That all depends how quick your process is.