Since you mention that it is "used"; you may want to let the probe soak in distilled, storage solution or 7.0 calibration solution for 24 hours and THEN calibrate it. If at any point the bulb tip gets dry for any length of time; it can throw off the calibration a pretty good bit.
Technically, if you pin down the Mfg's of those Pen's; they will say to calibrate them every time you use them. As opposed to a "monitor" style probe that is intended to be continually submerged in a solution; which should be calibrated monthly then.
Fwiw, I keep all my pens in a solution (not just in the cap) when not in use now as it holds calibration much longer (also multiple pens will give similar readings more often).
another $.02 to throw down into the kitty.