Old thread, but I just got one of those cheap yellow ph pens which are sold everywhere, ebay, amazon, hydro stores etc.
I found it TOTALLY WORTHLESS. The reading is all over the place and hardly ever "stabilizes" as it says on the package. I haven't even bothered with calibrating since there is no steady reading to even calibrate. When I test my tap, it goes from 8 to 10 and at 10 it's still slowly counting up.
I am using the GHE test kit/drops for some time, while sometimes not easy to read (when the fluid itself is not clear), at least it gives STEADY results. Eg. my tap, at 8.6...and I can mix nutes and add some ph down and then get it "piss yellow" towards the 6ph range.
I don't trust the meter I just got ONE SECOND. Next I might simply get some strips for a few bucks. I am sorta dissapointed since I expected the meter be more precise than the drops, but I could pull numbers from a hat and would likely have better accuracy than this pen. Don't recommend those things at all.