I have been using my di vessel to purify my water for window cleaning for quite a while and always check it with my TDS meter to ensure the reading was 0. Recently the TDS meter has began to read 1 on the purified water. Just wondering at what level should i change my resin to ensure good results with my wfp cleaning?? Also is there any way to get more life out of resin in my di vessel or should i change it straight away? Thanks ?