Throw that thing away. When I first started growing i used one of those and thought everything was hunky-dory. Then all sorts of shit happened. What was the kicker for me was that I would stick that piece of shit in all different soils and it would read the same. Half my plants were locked up.
They sell ones that look the same but read moisture-those work pretty well.
Invest in a 50 dollar digital pen, get some 4 and 7 ph calibration solution and never look back.