Does this work on plaster walls?

The ET140 Moisture Meter is designed to detect moisture in drywall, masonry, hard and softwood. It is not calibrated for plaster and will not provide an “exact” value of the moisture content, but it can be used to do a comparative value test. To perform a comparative value test with this meter, first measure an area of the material that you know is dry and record the value. Then measure the area you suspect has moisture and compare this value to the previous measured dry spot value.