Testing Alcohol by Volume: tried the "vinometer"

We're nowhere close to needing to test our 2010 Lodi Cab for alcohol content, as that normally is done before you bottle. But I wanted to test a new item I ordered recently, called the "vinometer". I figured I couldn't go wrong for $3. Well, you get what you pay for. It doesn't work so well. I tested a Missouri Norton I made last year that a lab certified at 11%. The vinometer said 6%. It's a very simple device, I guess too simple.