Using multi-meter to check damp in plaster

[Ronald Etherington]

I have a damp area in a plastered wall and having remedied the cause I want to check if the wall is drying out. I am using the resistance option on my multi-meter and connecting the leads to 2 masonry nails part hammered into the damp area - about 1 inch apart. When I hook up the leads the multi-meter displayed value starts to climb and it takes several minutes to finally stop changing. Can anyone explain why it does this please?

 

[Geoff1946]

Interesting idea. At a guess some electrolytic action between chemicals in the damp masonry and one of the nails. That is, the current from your meter will effectively create an opposing voltage by chemical action.

As an aside my experience is that the plaster never fully recovers from serious damp, and will continue to exude salts for a very long time.

 

[Phoenix]

Aldi have got some moisure meters in the middle aisle atm, think they are about six squids, https://www.aldi.co.uk/ferrex-moisture-sensor/p/803173437477202 it says no longer available on the website, but I picked on up on thursday evening (probbaly last weeks stock, but seemed to be plenty hanging about at the time)

Probably not amazing, but a slightly better solution than the mutimeter as the distance between the probes is fixed, so when you input the material type, it can give you the % of moisture, rather than results that don't mean a lot as the spacing is always hanging if you are jabbing multi meter probes in

 

[awarner]

Get a damp meter they only cost around £15 for a simple unit on Amazon, I use one for checking my firewood, make sure you have it on the right setting though as different materials and different woods etc will give different readings. Very useful tool.

 

[UNG]

Aldi have got some moisure meters in the middle aisle atm, think they are about six squids, https://www.aldi.co.uk/ferrex-moisture-sensor/p/803173437477202 it says no longer available on the website, but I picked on up on thursday evening (probbaly last weeks stock, but seemed to be plenty hanging about at the time)

Probably not amazing, but a slightly better solution than the mutimeter as the distance between the probes is fixed, so when you input the material type, it can give you the % of moisture, rather than results that don't mean a lot as the spacing is always hanging if you are jabbing multi meter probes in

They do work well for the price I think I got mine from Lidl's middle aisle has a cap that allows you to test it is working correctly at 2 different levels before you use it

 

[Ronald Etherington]

Thank you for the comments. I have a moisture meter but it is calibrated for wood - there is help online in the form of a rough indication of what the wood readings mean when used with plaster. However this moisture meter shows the same 'ticking up' display as the multi meter. Since I am comparing readings over a period of time either instrument will suffice but I was hoping that an insight into the ticking process would help me understand what is going on. The reason I have nails driven into the wall is so that the measurement is not just the surface and also to ensure the readings are taken from the same place every time.