I replaced an old SPI Observer 300 because the audible alarm was no longer working. My cellar is slightly damp and the old alarm was made of metal. I liked the fact that this one was all plastic but sturdy. It was very easy to install since it was similar to the original one. I tested it and the audible and alarm light worked great. I like that this one has battery back-up, as the old one did not. A very good product. Thanks.