always trust the drops,,if you can ,buy some 7.0 and 4.0 solution to check your pen,,they do get out of wack,,you also need to keep the pen in water or the solution i talk about,,ebay or amazon has it cheap,,but keep it in there ,you dont want the tip to dry out,,there are some home remidies,for same thing,i heard folk talk about,,like mountian dew for a 7.o but dont qoute me on that,,i trust the solutions,,get some that and bet your pen needs calibrated,with the swing you talk about,,if your in soil best ph to start with is 6.5,but soil buffers most tap water,im not that lucky my tap water is very alkaline,,so i catch my water from my ac condenser in a 55 gal barrel,,in my vegtable garden i dont have a choice but to use the alkaline water,and you can sure see the difrence,,so go with the trusty drops till you calibrate your pen