Wow I feel your Situation,
First off Ph is sometimes difficult to deal with as your experiencing.
I also have the Milwakee ph-600 pen paid like 20-25 for it delivered off of e-bay and I like it. I also got a bottle of General Hydroponics 7.0 calibration solution and it also serves as a storage solution. The pen works really well but does need to occasional tweek(calibration) mabey once or twice a week. but once its dialed in its great. I also have the general hydroponics ph test kit. but i ditched that after seeing my nutrient solution when mixed resembles a tea like color and thats at 1/4 strength. But my point is a color test kit is useless because you need to check the nutrient solution after you add the nutrient's to the water and thats why ya need the ph meter. I never tried FF and dont know if you have the same color change as I do. but if you do for now try to calibrate the ph pen with your color ph test kit. get your water use the test kit for a reading and then use the pen in the same water and with the little blue screwdriver it came with dial it in to the color test kits reading. Mind you doing it this way is not as accurate but you will get your pen back in range and be able to fix your problem, do this till you can get some calibration solution and for the ph-600 milwakee says you need a 7.01 cal solution which i cant see why since it only reads to the tenth not the hundreth. Like I said earlier i got the GH 7.0 cal solution and i swear by it all.
420barista Reviewed by 420barista on . pH problems.. read lots of info, still need help! Your Equipment: .1) Type and wattage of lights. 250w MH .2) Distance from tops? was very close.. <1 ft, I raised it up to 2ft this week .3) Reflector type? enclosed .4) Is there a consistent fresh air supply? yes .5) Do you have an exhaust fan and a circulation fan? yes .6) What are the bulb wattages, kelvin ratings, and schedule? 24/0 250w MH conversion in an HPS ballast Your medium: .7) Specific brand and type of soil, FFOF straight out of the bag Rating: 5