Rule 10: damp meter

Damp meters don’t measure water.

They measure the flow of ions through a liquid, such as water.

H2O, water, itself only measures less than 9{f898b7ccba74aedaf6f9f165b1651d39d829939f1b62e7036dcda8fdc0245a9d}WME on the Protimeter damp meter and so according to the Protimeter damp meter, water is “dry”.

A Protimeter Surveymaster II is used to detect potential damp in “search mode” across all walls and up chimney breasts, and where elevated readings noted, the area is checked more accurately in “conductance” mode. In addition, a sample of at least 10 damp meter readings are taken in conductance mode. Conductance measures the passage of electrolytes (mainly salts) in solution, between two pins.

Damp meters are designed to detect dampness in wood. Greater than 18{f898b7ccba74aedaf6f9f165b1651d39d829939f1b62e7036dcda8fdc0245a9d} Wood Moisture Equivalent (WME), is sufficiently elevated to need further investigation, but wood requires a minimum of 28{f898b7ccba74aedaf6f9f165b1651d39d829939f1b62e7036dcda8fdc0245a9d} moisture and oxygen for rot to form. Surface timbers, flooring and skirting boards are sample tested in conductance mode.

Use of meters

Damp meters are used for two simple reasons;

  1. They are used by RICS surveyors in the mortgage valuation surveys, so to address evidence uncovered by the RICS survey the same wall needs to be measured and photographed with a damp meter and a explanation put forward,
  2. Because they are very quick and easy to use, so long as more emphasis is put on low measurements than high measurements.

Although meters are useful tools for identifying areas of potential damp in masonry, the meters can give very misleading information, mainly because some substrates are high in salts, such as a gypsum plaster, very common and made from calcium sulphate. Only about 9{f898b7ccba74aedaf6f9f165b1651d39d829939f1b62e7036dcda8fdc0245a9d} of the WME reading is caused by water, the rest is caused by salts, see a YouTube clip.

Damp meters should be used as qualitative tools, not quantitative. Low readings are as significant as high readings and are reported. Walls are tested on both sides where possible.

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