This is a repost:
You can't go hydro without a PH meter. Ph changes alot (sometimes daily).
I have a total of five different PH meters. I had more, but I tossed out the real cheap-o's. The ones that are $14 are worthless. You get what you pay for.
I have two Hanna meters and one monitor:
The Hanna HI-98129 combo (the black one below) is great for TDS/EC, but with most Hanna meters the PH readings seem to drift all over the damn place. I cailbrate all my meters before ever grow. The HI-98128 PHEP (red one) works ok, but it takes a while to get a reading and never agrees with any of my other gages. The three Hanna gages I have never agree on PH (ever).
The monitor (I can't find the exact picture, but it's in one of my grow pictures), was great for two grow cycles. It died half way through the third.
The Milwaukee SM101 is a great PH meter. It is very low tech though. You have to calibrate your temp knob every time you use it to get a correct temperature adjusted reading. Most other meters will do that for you. I have to say, this meter gives you a reading as soon as it hits the nutrient. **Update..... The Milwaukee probe went out in less then 9 months.
The EXTECH PH100 is the one I use most. It agrees with the Milwaukee and gives fast results. I did have to replace the probe one time in the two years I have owned it. It costs more then half the total price of the whole meter. It has never been wrong about PH (like the Hanna).
Most the meters above will cost you $70 or more. Every company makes a high end and a low end meter line. Buy one good one.
TDS/EC is important to most hydro growers, except me. I use the Lucas formula. I don't do "add back", I dump every two weeks and make a new batch.
The Hanna combo meter works real good for the TDS/EC. I use it to check my RO water. It lets me know when to change my filters. EXTECH makes a combo meter. I had one and dropped it bad. It broke and I never replaced it.