I recently started using a green Hanna checker. Tonight I was surprised by the reading because it was higher than two days ago when I changed my GFO. It was .21 so I rechecked. The next reading was.10, so I rechecked again. I rechecked seven times and had readings from .21-.05 and everywhere between. The only one I got twice was .07 so my only best guess is that is my level. Has anyone had this happen? Any advice? New battery maybe???