My dude, pH is so important to me that I own this:
http://www.omega.com/pptst/PHCN37.html
and this is the probe I bought to go with it:
http://www.coleparmer.com/1/1/70796-cole-parmer-ph-electrode-line-double-junction-100-ohm-rtd-w-yo-27001-70.html
That thing is the absolute shit. My other pH gadgets went to the garbage. They sucked. Hannah.. garbage.. Blue lab...garbage.. they were inaccurate.
This has temperature sensor and two junctions == made to test pH constantly to two decimal places.
What does that mean to you, other than I spent a load of cash on pH gear? well, I did it because I know and care about water chemistry.. I have had probs with well water.. probs with RO... probs with pH down .. and I have been setting pH at 5.5 and allowing it to naturally drift upwards to where you like it.. and then I adjust down to 5.5 and let it drift up...
I have found that the less adjusting I do, the better it is (more stable).. there are buffers in the nutes that want to stabilize the pH. I let them do it. I have been damn close to 5.53 for this whole last grow.. the final week I upped it to 5.8. It is so stable because I don't add up or down unless i am waaay off.
anyhow, that is my twenty cents on this topic..
tommy
reference: WHAT pH IS BEST FOR GROWING PLANTS HYDROPONICALLY?
Answer: The ideal pH range for most hydroponic crops is between 5.5 and 6.5.
general hydroponics website