I have two ph pens the same, just cheap ones. also have a roll of expensive narrow range ph paper and lots of ph6 and ph4 buffering solution sachets. calibrate frequently and always test with both ph pens to make sure one's not out of whack. test with a bit of the ph paper if there's any doubt.
Sounds OTT I know but it's cos I'm growing an auto strain that is extremely fussy about PH. feed it wrong once and you lose a ton of weight at the end. Nothing wrong with those cheap chinese PH pens though, I've never had one break before I lost it.