I'd recommend you buy a monitor instead of a pen
I bought this
Milwaukee Waterproof pH pen a year ago @ Christmas
Calibrating was a pain and not adjustable
I didn't start using it until May, stored it as per instructions, and by Fall it was pooched
The probe is not covered by warranty and too expensive to replace ($50 shipped), so I tossed it
If I ever want something portable, I'll get one of these
American Marine pH Monitors instead. Good quality probe used and replacements aren't too expensive
Calibration is easy (I've read)