Damp-proof course v. Moisture meters

[Anonymous]

Hello,
Could anyone please answer this query? Reading website articles + R.I.C.S. "Moisture Measurement in Masonry - Guidance for Surveyors" I note that moisture meters aren't accurate or appropriate for such use but only measure electro-conductivity.
As I have an electro-osmosis damp-proof course installed in my cottage, could this affect the hand-held meter readings & make them higher? I'm not at all scientific or technologically minded!
Thanks

 

[Anonymous]

There are two basic types of moisture meter in general use.

The traditional type uses two metal pins which are stuck into the material. The meter measures the electrical signal between the two pins and gives a result in percentage moisture content. (MC) These meters are calibrated to work on wood and in measuring the MC of wood they are fairly reliable. The problem with them is that some materials produce an electrical charge (i.e. salts in masonry) which can result in a false reading. The other problem is that inexperienced users sometimes take the percentage readings and apply them to masonry. This is wrong, the calibration is quite different.

The other type of moisture meter does not use pins but sends a loop of radio waves into the material and measures the return signal. From this it can indicate if moisture is present, but even then it takes some skill and knowledge to interpret the results. These meters are obviously not affected by the presence of electrical signals.

Both types of meter need to be interpreted by somebody who knows what they are doing.

 

[Anonymous]

"I note that moisture meters aren't accurate or appropriate for such use but only measure electro-conductivity. "

They are calibrated for timber. The reading generated in masonry is not the moisture content of the masonry. They measure electrical resistance, which is influenced by the moisture content of whatever material you happen to use them on.

Does this mean they are useless?

No.

As PC posted yesterday, Graham Coleman's site has a lot of useful information on how to effectively use a resistance moisture meter. Rather than placing complete faith in the reading generated, the comparative profile of readings taken over the wall area will tell you how water is behaving within it. Moisture profiling is a very useful, and non-destructive, technique.

Matt Green

 

[Anonymous]

Given that some building materials rely on having a certain amount of damp to keep them working as they should - as in earth buildings, cob, clay lump etc - then clearly any 'damp' readings should require someone with the knowledge of historic constructions to interpret them also?