I’ve only had my apex since Xmas (Older UK style as we don’t have the new one yet...looks unlikely we will either) and my orp this morning got up and over 450.
I know it's an old thread but for me its a good indicator for nutrients. when it starts to fall I need to check and then usually either raise my phos, nitrate or usually both. Lot easier to look at Apex than to do tests. Just hope Apex doesn't fail me!