I love this tool. But yesterday I encountered something I want to share with other potential buyers. I was under my house to check a faulty circuit and it was really humid: probably 100% relative humidity. Anyway, I turned on my Fluke and started moving it from wire to wire to make sure it was working. Nothing. No beep, no constant light. So what's wrong? When I got back out and went into the garage I tested it on an exposed wire running into an outlet box. Sure enough, it beeped and the light came on. So what gives? Well, I did a little research, and the specs indicate it only works up to 95% relative humidity. Who would have guessed. Anyway, this is a minor drawback to me and I'd rate it 4.75 if I could.