Calibration usually holds 6 - 8 weeks, though you should start to varify it after 4. I have worked with lots of PH
probes back when I was into ground water cleanup, and I seriously doubt there is a probe that needs no calibration, however technology changes daily so who knows!
Here's to swimmin with bowlegged women.
Reef with 30G Sump/Fuge Combo
-30G LTAnemonarium with mated porcelain crabs, 10 sexy shrimp, and 2 pink skunks all tied into the other 135G in my system.