MrStinkyanswered grow question 3 years ago You can keep the solution, if you can keep it totally free of contamination.
If you use meter every day or so you will probably not need to recalibrate very often.
Best just to get the powders and some dionized water and recalibrate in between each grow.
Optimal pH for autos in soil - 6.0-6.5 - I go with 6.38 every time.