Are all your pens the same model or do you also have any brand-name ones? Not blaming the versions but they likely lack the tight manufacturing tolerances of a brand-name model. I still haven't checked my own pens for accuracy since I hate the thought of wasting a cartridge just to test. Maybe I should pick up some inexpensive ones elsewhere to run trials.
Would someone be able to help me figure out what's happening here? I got some pens and decided to check their accuracy by sticking an insulin syringe into the pen, dialing up units, pressing the plunger, and seeing if the syringe reading matches what I set. The odd thing is it underdoses the first time every time, yet later tries are exact even at just 2 units. Steps I followed: put the cartridge in, attach the pen needle and keep dialing until the plunger touches the stopper and liquid starts coming out, set to 2 units, flick like a brand pen to clear air, push and confirm the 2 units came out so everything should be ready, remove the needle, insert the insulin syringe, dial 20 or 60 or 80 units while holding everything steady, and press. It underdoses. Then pull the syringe out and squirt the liquid away.