I bought this dampmeter recently after we had some building work done recently. There was some white stuff on the outside of the new plaster and I suspected that there was some damp because the work hadn't been done properly.
So, to check this out I ordered this dampmeter. It is very light and requires a 9v battery. There are two buttons on the side, one to check that the battery is working and the other to use when you are testing for damp. In this case you just throw the second switch and press the two metal pins on the wall you wish to test. The meter indicates a percentage damp and there are 3 colour-coded regions. Green indicates normal (< 10% to 16%), Yellow indicates attention required (16 - 20%) and red indicates Action required (20-28%).
For the wall I needed to test, most of it was fine, although there was a small patch that was at the top end of the yellow scale. I also used it to test the cellar. There were some damp patches here as well.
This is a very useful tool to have to test the state of your own house, as you can probably find out where the water is coming in. I t is also particularly useful if you are thinking of buying a property, as you can easily test for dampness within a few minutes around all those places where dampness is likely. at under £15 it is reasonably cheap and well worth the money.