Wall Cavity Wiring.

This video shows inside a wall cavity, for those who wonder what we are talking about. There is thermal insulation there in this case. Notice the condition of the old orange coloured Black and Decker electric drill flex.

https://www.youtube.com/watch?v=Ez_qTOiREWM


Z.

Testing for damp internal house walls using electronic damp test meters.


A historical report.

https://www.youtube.com/watch?v=9E63vIgV9Oo


Z.

Damp meters may produce false readings and cables bridging cavity walls be blamed, but have a look at about 11 mins onwards in this video.

https://www.youtube.com/watch?v=XORucdJ5zQw


Z.

Zoomup:

Damp meters may produce false readings and cables bridging cavity walls be blamed, but have a look at about 11 mins onwards in this video.

https://www.youtube.com/watch?v=XORucdJ5zQw

A bit of thread drift here, but that was interesting. The tradesman appeared to know his business. We have some damp in two bay windows. I feel sure that it is due to condensation because the bays are thinner and, therefore in winter, cooler than the main walls. It would be interesting to take some temperature measurements next winter.


I don't think that damp meters are at fault (tho' some are better than others): they will show where the walls are damp (but you can see and feel that) but not why.

Some years ago I was plumbing a kitchen sink that was under the centre of the window,  there was a double socket either side of the window. 


I had installed the kitchen and having set the sink into the work surface and having plumbed the hot and cold supplies to the tap I drilled through the back board of the sink base unit then started to vote drill through the cavity wall to run the 1 1/2" waste pipe out.


Having drilled through the inner block well the core drill hit the socket circuit cable that was hanging forming a centenary loop in the cavity between the sockets either side of the window. It was one of those WTF moments as there was a flash and a bang in the cavity of the wall with me hanging onto the drill with a core and a 600 mm extension on it.


Bearing in mind the kitchen installation was virtually complete I then had to waste half a day getting a replacement cable installed. 


There is absolutely no way anyone could have foreseen that drilling that hole halfway between the sockets under the window could possibly result in a cable being damaged in the wall cavity,  


Life's like that, people pull cables into the most unlikely places then they get damaged. 


Andy

So much for safe zones!

A so called 'damp meter' will only tell you when a wall is conductive That is the rate of ions reaching each electrode, which is a product of the number of conductive ions in a unit of the material and the mobility, which is a measure of the moisture content. Pure water is a pretty good insulator, (megohms across a unit cube 1cm test cell), it is the fraction of conductive material dissolved in it that makes it conducting.


So a high conductivity reading may be a symptom wall with a coating high in conductive salts, like lime plaster or certain types of whitewash and a low % moisture, or it may be a modern gypsum plaster , and a lot of moisture - the electric test cannot tell.  (cured gypsum plaster is almost insoluble - it crumbles but does not dissolve, in the way that say salt or sugar dissolve )

Hence the professional use of a drill to get a core sample, and the carbide pressure cell test - calcium carbide (a white powder) gives off acetylene gas when combined with water, and will wick water out of brick dust to do so, so for a known weight of dust, the pressure rise in a closed test tank after mixing the core sample power and the carbide and closing the screw cap,  is an accurate proxy for  total water content of the sample, and so a more reliable method than the 2 prong meter

LOL.


When I was in the Venture Scouts in the 1970s calcium carbide could be ordered and collected from Boots the chemists to use in acetylene lamps when we went caving and if you ran out of water you could always wee in your lamps tank.


Andy