Delta T
Delta T is calculated by using return air temperature minus supply air temperature, the value is called Delta T. For example, if the return air temperature is 75, and the supply air temperature is 65, then the Delta T is 75 - 65 = 10. The normal range for delta T should be between 18 and 22. The higher Delta T normally means your fan is too slow, that is, air sits around the evaporator too long, lower Delta T normally means your fan is too fast, that is, air sits around the evaporator too short, had to no chance to condensate, so the fan needs to speed up a bit. Notice that Delta T gets affected by many factors, in some cases, Delta T of 15 is probably perfectly ok. but 20 may not be ok at all.