I have a Hanna grocheck (ph and EC) which is wall mounted like yoursand a cheap pH pen and bluelabtruncheon. All three are calibrated and I use them all to ensure my meters are accurate. Truncheon although does not have the two decimal places PRECISION, it still measures entirely accurately, I've tested it all through my grow with a range of concentrations and is as ACCURATE.

You had me doubting it again so I just tested. If you want I can show you a photo of before, adding nutes and the truncheon reading after to prove to you its not shit. but alas i cant be arsed and know my instruments are accurate