Tutorial: How to measure moisture in a wall / wood with a multimeter

[SeanB]

http://www.youtube.com/watch?v=u1LhGfzJc-A


A nice little demo from Martin on moisture measurement basics.

To measure conductivity you generally use a standard cube, dimensions of which are actually not needing specification, just that all 3 must be the same dimension, and the measuring electrodes must be on opposing side of the cube. Thus if you take 2 square electrodes 1cm square and have them 1cm apart you have a standard conductivity cell. Round, square, rectangular ( to certain limits) makes no difference, just the area. Then you measure the desired substance by placing between the electrodes, with the outside being insulated from the material.

Typically for long period measurement you use an AC excitation voltage, as this reduces the effects of electrolysis on the electrodes and the material being monitored. For short term use and single measurements the DC voltage and current from a typical DVM will have little effect, provided you are using an electrode that has an area that will not corrode appreciably during the test.

Conductivity is normally measured for things like water for purity ( normal tap water typically has a conductance over 200 microSiemens, or under 5 kOhm) or for dissolved salt concentration. you can get purified water to incredibly low conductance, lower than 1 nanoSiemens, and this is often used as a high voltage resistor in extra high voltage labs as a current limiting resistor connecting the apparatus to the source.  Normally in a 1-2m long glass tube with ultra pure graphite connectors to keep the water from being contaminated. You can touch it while energised provided you are standing on 2m of glass insulator and get the unique feel of all your hair standing up straight away from you, but if you touch ground at the same time ( or are close enough) then you will not have a good day.

[MJLorton]

Thanks for adding that valuable input Sean!

[SeanB]

Still have to figure out how to embed the actual video, looking for tips on how to do so. Must be something simple I am missing.

[G7PSK]

Your leak might not be due to a bad joint, I had a similar problem due to cement is alkali and will eat through copper pipes, they should be covered with plastic such as lagging foam and where they come through the tiles it is a good idea to wrap them in clear tape to prevent contact with the grout. Off topic sort of I know.

[SeanB]

Most copper pipe here is direct buried in cement. Will only be a problem if the wall is permanently wet, the oxide layer that develops on the pipe does protect it quite well. Corrosion is only a problem with high sulphur containing gypsum boards though, which destroy the copper piping and wiring. I have seen piping that has been buried for decades without problems. Most is buried to prevent theft, as open copper is a magnet for thieves ( brass water meters are actually cheaper than the plastic ones, but are much better as they do not crack with time) and will be stolen very fast.