A blog post

The electrical moisture meter and salts

Posted on the 21 July, 2011 at 4:58 pm Written by in Blog, Damp, Surveys

When one starts to trawl through articles and the Internet, it becomes very clear that to some, the whole existence of the damp proofing industry, and indeed any person suggesting rising damp occurs, is based solely on the use of an on the electrical moisture meter. The electrical moisture meter is commonly blamed for ‘misdiagnosis’ of damp problems and one individual even suggested that it should be banned!

Protimeter

Scores of people persistently come forward waving, for example, a piece of clinker block and show that this material when fully dry causes good responses from an electrical moisture meter and therefore, hallelujah, we lesser mortals can all be fooled into misdiagnosing the problem. Of course we all know the same effect can be gained by sticking the probes into their heads! It appears that only those that have read the manufacturer’s instructions and brought our attention to this known phenomenon have ‘the knowledge’ which will lead the unknowing out of the darkness into the light!

They also point out that electrical moisture meters do not measure moisture but electrical resistance, capacitance (impedance) and radio wave reflection. Hence, put any electrically conducting material on the electrodes and you will get a ‘result’. Mind you a carbide meter doesn’t directly measure moisture either-it measures gas pressure in a sealed space.

Those ‘in the know’ usually point out that conductance meters also respond to certain salts, especially hygroscopic soluble chlorides and nitrates, which of course is indeed a fact and is also in the manufacturer’s instructions. Now they have ‘discovered’ the presence of these particular salts in tap water, salt, animal waste, flooring materials, washed sand and so on. And as these salts are so widespread in building materials we are told they can easily ‘fool’ the unwary if one relies on identifying their presence alone as an indicator as to the source of water ingress.

However, let us take a look at the reality and the science.

The table below illustrates responses of conductance type moisture meters to air dry materials:

Response of moisture meters

When a competent, experienced surveyor is investigating a building it is known that a very few types of material such as certain clinker blocks, and some black ash mortars, will cause electrical moisture meter to respond in a significant manner; this is well documented. However, it is highly unlikely that most investigations are undertaken by directly sampling such materials with a moisture meter should they even exist in the property – in other words these situations are rare. Most electrical moisture meter readings are taken from surfaces such as plaster, render and possibly exposed brick and mortar. If one does obtain moisture meter readings throughout the whole property then any moderately competent surveyor should readily identify that there is a particular problem and this may well be due to an electrically conducting substrate. Nevertheless, most investigations are made incorporating the use of moisture meter on surfaces such as described above and not these exceptions.

So what about the composition of some commonly encountered materials in buildings?

Mains water: Legislation dictates that mains water contains up to a maximum of 50 ppm nitrate but much higher levels of chloride are allowed, probably up to around 250 ppm. Groundwater may in certain geographical locations contain higher levels. The ratio of chloride to nitrate will vary according to the water authority and groundwater in which part of the country one samples. Look up your local Water Boards’ own analyses. What becomes obvious is that you cannot specifically rely on the ratio between the chloride and nitrate in diagnosis between mains water and ground water as their origin in buildings.

As far as building materials are concerned. Water used in their manufacture (bricks, etc.) or for gauging will have used one of the sources of water, probably ground (pond/river/well) in older buildings. Thus, even the small amounts of chloride and nitrate present in such water are insignificant with regard to affecting electrical moisture meter readings, and of course they are diluted even further when mixed with mortars, limes, plasters, brick, etc. As such they will not cause an electrical moisture meter to respond significantly: in this situation their presence is of no consequence whatsoever.

Where mains water has a long history of wetting masonry directly, not via soil, then such salts may build up to a greater level where they alone can start to cause an electrical moisture meter to respond to a greater extent in the absence of free water that lead to their origin-but this takes a considerable time and is usually localised to the vicinity of the leak.

Similarly, it is frequently argued that rising damp is the result of mains water leaks in the ground. If so, (1) for water to rise the wall must be sufficiently permeable and therefore any soil water can rise, (2) mains water would pass through the soil before rising up the wall and also pick up groundwater salts in its passage, (3) any rise of water into the wall will take a long time before any significant levels of salts accumulate from whatever origin given the rate of rise of water.

Unwashed sand: almost all ‘pit sand’ is washed – it is washed in water from which it was removed! This is to remove fines (clay and silt), not salts. If unwashed ‘sea sand’ is used then distinct and significant levels of sodium chloride would be present and this would have been readily identifiable for years causing problems throughout the property in which it was used. Analysis would reveal chloride alone.

Water additives containing chlorides: the additive is usually calcium chloride (deliquescent), which form insoluble complexes with cement and as such they do not result in hygroscopic problems or detected by simple water extraction. If, however, they are mixed with gypsum plasters then hygroscopic problems do distinctly arise. But this is a use for which they are distinctly not intended.

Seawater exposure: this will occur obviously in coastal areas and any surveyor worth his salt (excuse the pun) would be aware of this. Nitrate levels in sea water are very low and contamination would effectively lead to readily identifiable levels of chloride.

De-icing salt: this would only be present to lower part of walls immediately facing adjoining treated roads. The contamination would be effectively solely chloride-sodium chloride.

Animal waste: this usually contaminates buildings where animals are kept i.e. agricultural buildings. It may only directly contaminate lower parts of walls but it can provide ‘enhanced’ groundwater salt concentrations into a wall through rising water via contaminated ground due to the waste: a lot of chloride should be expected together with ammonium salts. Nitrate does not appear to be detected in urine (human) by commonly used techniques, i.e., those used in identifying nitrates in building materials; nitrite in fresh urine indicates a medical problem.

Stored fertiliser/salted food, etc: this is rare and in most cases probably present in agricultural buildings, old meat processing areas, old bakeries, etc. It is also likely to be very localised.

Chimney flues: the result of long-term burning of fossil fuels cause the build up over a long period of time of soluble chlorides and nitrates; these are often associated with staining of a chimney breast and it will also occur on upper levels and adjacent to the chimney structure. One usually finds excess chloride to nitrate ratio in these situations, and also possibly ammonium salts. If there is a sufficient build up of the salts they will also cause visible dampness and high electrical moisture meter readings due to their hygroscopic nature.

Washing-up liquid: this was distinctly not available at the time of building houses with lime mortars: there appears to be no free chloride or nitrate present in diluted ‘Fairy liquid’ or Morrison’s, but I have no idea about Tesco, Sainsbury’s, etc. own brand. If present they would have been significantly diluted by the water and the mortars/renders into which the water was added and as such they would be of absolutely no consequence.

Electrical cables: These do not affect electrical moisture meter readings as some claim – unless you penetrate through to the wire, and you won’t need a meter to tell you that!

In relation to the presence of certain soluble salts, it should be appreciated that soluble chloride and nitrate are almost certainly present in most building materials, especially where groundwater (including rivers and ponds)/tapwater are used in their manufacture/application. But at such levels they are of no consequence and for practical purposes can be totally ignored! It is not a case of presence -v- no presence but being present at sufficient levels beyond a ‘background’ level; in other words they will have been introduced following construction from some source.

In most clean building materials, including old bricks/mortars, they are present at levels of less than 0.01% wt/wt, often far less. In almost all cases where high levels are present, sufficient enough to influence moisture meter readings, they are introduced most often in solution and over a period of time. Some seem to suggest or imply their presence at any level is akin to, “All elephants are grey; if it is grey it is an elephant”

Finally, the most important feature is that when using an electrical moisture meter or any other method of potentially looking at salt/water problems, the area should be ‘profiled’-that is how the meter readings are distributed (This also applies to any moisture and salt profiling by destructive lab methods) – it is the level and distribution of the salts that are effectively the definitive key to diagnosis. For example, if using an electrical moisture meter all readings are related to the chimney area only, it may be due to a salt problem related solely to the chimney. If the high readings are restricted to lower parts of walls only, salt and/or moisture such as found in long term rising damp may be the result – further information may be obtained by the nature of the profile. If the meter readings are obtained throughout the property, this may possibly be inherent in the building materials, etc.

Basically, any investigation is the combination of simple common sense together with using ones’ eyes, and experience – realistically a very high proportion of significant electrical  moisture meter responses will be water ingress and/or reasonable hygroscopic salt contamination, neither of which should be in the material and must therefore have originated from somewhere subsequent to construction.

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