I have the same one as Machog. It is NOT waterproof. Water got into mine and not only did the display look weird, the voltage reading was off by a full volt.
Stranger yet, as I thought what a waste now that it is ruined, the next day the display was slightly better and so was the true volt reading. After several days it dried out and was good as new.
When dried, I cut a clear Plexiglas that overlapped the meter on all sides by about 1/4 inch, put double sided tape on one side of the Plexiglas, stuck it to the top of the meter and then put in caulk under the overlap to make it completely waterproof. Has been fine since even in hard rains.