Hi, i have problem with my RO water. My ph doesnt ...

Always pH adjust after you add fertilizer. There is no point to doing it before. Is your probe broken? Re-callibrate it. Sounds like equipment issues. RO should be ~7, but maybe when there are so few dissolved molecules, it doesn't read accurately. This should not be a problem after adding fertilizer, if that is the case. Also, save yourself some money. Just use some 5% white distilled vinegar (acetic acid).

You can’t PH test straight RO. There’s no PPMs in the water the PH tester has nothing to read so the values will just aimlessly drift. Like m0use said you need to add your nutrients and then adjust the PH. This is why I don’t mess with RO anymore. It’s more work than it’s worth and you can’t PH it when you’re doing just a straight water with no nutrients irrigation. However I’m lucky because my tap water is ultra clean by industry standards. I’m at 60-61 PPM. So if you have used your PH tester in straight RO water, I suggest you do a recalibration on your device. Again like m0use said already, using your PH meter in straight RO can cause damage to the device.

Fuck, had a nice answer typed out and the page refreshed and killed it all. In short, test your solutions and ph probe. Testing ph probe by using vinegar as ph down in a glass of tap water. Use a few drops in 250ml or 1cup and measure the PH drop as you continue adding drops of it into the solution while stirring. Make notes, Do this again with your PH down phosphoric acid. Depending on the concentration of your PH down it should drop quite raptly. The stuff I have is 80% and I only need a few drops to alter 1Gal or 3.7L of water. If you need more it means its not as concentrated or its not fresh anymore. Neutralize the solutions with some baking soda before pouring them down the drain. If both solutions of tap water with either vinegar or phosphoric acid seem off, its likely your PH pen needs to be replaced or calibrated. Pens should be calibrated before any grow and around every 3-4 months. I find its best to let them sit in storage solution KCl for 24hrs before calibrating if they have not been used in a while. Sometimes you need to use a toothbrush and soapy water to clean the probe tip if its got buildup on it. Its always good to have some PH strips as backup. Can find them in hydro stores or pool section of big box stores. The good ones have 2 or 3 indicators to help resolve the PH. I have a feeling your PH down solution's is just ageing and loosing its strength. Or your pen is ageing and loosing its ability to monitor PH. Do the above and find out. Also RO water should have a PPM of 0-25. distilled is 0-10ppm. Might be time for a filter change? Low TDS waters like RO and distilled are bad for PH probes. Best not to let them touch it till after all nutrients are mixed in and it has some stuff dissolved into the mix. Good Luck!