The two main parts of a Filter Drier are ,the Filter part which traps any solids/impurities/debris and the Drier part which catches any water/moisture. When the Filter part does its thing by trapping any "stuff" that was floating around the system the Filter part becomes "plugged" which causes a pressure drop that converts into a temp drop which leads to replacing the F/D. When the Drier part does its thing by holding onto any water/moisture traveling around the system and the Drier is then holding onto as much moisture as it can possibly hold, nothing else happens. There is no more moisture removal, and there is no Pressure/Temperature drop due to the Drier being "full" ,but, the Filter/Drier has to be replaced inorder for the new F/D to remove any remaining moisture.
R22- can have an acceptable level of about 50-60 ppm . That is equal to 1 drop of water in a 125lb cylinder of R22. If you have 200ppm in your system that is bad,very bad. A MI is sensitive to that OK level. I don't know of anyone who can tell the difference . MOISTURE INDICATORS ARE SO "IN" THESE DAYS.and they come with a convienent Sight Glass at no extra charge. They are available at EVERY SINGLE supply house that I've been in!