Hi, Dabber.
I buy a new, cheap $10-$15 pH pen every two years or so. With the new pen comes foil packets of pH powder to make the calibration solutions. I use my analytical balance (+/- .001g) to weigh out just a portion and save the rest to mix for future calibrations. I only calibrate the pen to +/- .1 pH as I think +/- .01 pH is overkill for the grows.
Any degreed chemists in here who can get into the pH solution "weeds"?
I had a coco grow two years that was slow to to vegetate. Yep. Bad meter, pH off. A new meter fixed everything.