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Electrical moisture meters -v- carbide moisture meters

In the recent press there has been considerable 'sniping' against the use of electrical moisture meters for diagnosing dampness in masonry substrates. The snipers rightly point out that the meters do not measure dampness but show changes in electrical resistance, capacitance or radio wave reflection/absorption in the substrate, depending on the type of electrical moisture meter in use.

They also tell us that certain salts and electrically conducting materials including foil-backed paper will give erroneous and misleading results: all the above, of course, are true. On the other hand the Carbide meter, they tell us, will give a true quantitative measure of moisture in the substrate, and that it is a recommended method of moisture measurement described in the BRE Digest 245.


But let's have a closer look at these two fundamental methods of diagnosing dampness/damp related problems more closely. Please note that I use the word 'diagnosing' carefully and specifically because diagnosis is the identification of a problem, and this should be definitive. Once diagnosed, however, there may always be arguments over what treatment is necessary, if any at all.


Electrical moisture meters:

Depending on the type of meter in use electrical moisture meters measure changes in the electrical resistance, capacitance, etc, in the substrate. In over 99% of cases this change is brought about by water and/or certain salts. Clean uncontaminated substrates that are electrically conducting are rare, and foil-backed paper is usually readily identified and of course if present it is usually because of dampness problems in the first place.

As far as salts are concerned, efflorescent salts alone do not cause electrical moisture meters to respond; hygroscopic salts do! Again in over a 99% of cases hygroscopic salts in building materials arise through long term rising damp and perhaps around chimney flues from burning of fossil fuels; in a few cases they may arise from other sources. Basically, clean and dry mortars, plasters, brick, etc, do not cause electrical moisture meters to respond. However, very low levels of free moisture in some materials e.g., 0.1-0.2% free water in permeable lime plaster for instance, can cause very high meter readings as can relatively low levels of contaminant hygroscopic salts. But neither free moisture or the salts should be present - they must have come from somewhere (remember, we are only talking diagnosis - not treatment). The question is from where have the free water/salts originated?

Careful and proper use of the electrical moisture meter can guide us, and experience has shown that over 95% accuracy can be obtain in diagnosis using an electrical moisture meter properly and with common sense.


Electrical moisture meters


Unfortunately, the majority of surveyors do not appear to understand how an electrical moisture meter should be used, and the manufacturers of the meters do little to educate the user either. With most surveyors the meter is simply stuck into the surface of the wall just above the skirting, the indicator goes into the red and, hey presto, rising dampness!


That is certainly not the way to use electrical moisture meter and using it like this will almost certainly lead to a high number of wrong diagnoses! Similarly, it is not a valid exercise to directly compare readings from one material with that of another in respect of moisture content. (also see 'A cautionary tale!')


In most cases only surface readings are taken, and lets face it, the surface is of most interest since it is here that the problem becomes manifest. Furthermore, with one major exception, condensation, what is happening at the surface usually reflects what's going on deeper in the substrate. So surface readings alone should generally not be a problem in the warmer months but condensation may be a factor during the winter: this will have to be eliminated as an interference factor during this time. Fortunately, in most cases, surface condensation is very much limited to the surface so by simply scraping away the outer 2mm of perhaps a plaster finish, the probes of a resistance type meter can be put beneath the surface for a quick check. Clearly, if condensation is a problem then a different approach will be required to examine the wall - but the condensation may be the actual problem in the first place.

The correct method for using an electrical moisture meter is to take a vertical series of readings in order to obtain a vertical pattern of readings - this is how an electrical moisture meter should be used. It is the pattern of readings which gives us guidance on the diagnosis. And this is where skill and expertise of the investigator are required in the interpretation of these patterns. Moisture meter 'profiles' with their possible interpretations have been published, and should be a part of every damp investigators background knowledge. The isolated individual readings obtained during most surveys are of little value and will certainly increase the chances of misdiagnosis! Such patterns of meter readings can be obtained very quickly and therefore a number of areas can be rapidly evaluated.


The Carbide meter:

The Carbide meter is held up by many who own them as the definitive instrument in diagnosing dampness problems. After all, they give a true quantitative result of moisture content. We are also told they are recommended in the BRE Digest 245 - more of this later.

The Carbide meter has one enormous disadvantage as a preliminary survey tool - it is a destructive method of moisture determination, i.e., it requires samples to be removed. This is hardly likely to be acceptable to a vendor as part of a pre-purchase survey!

The carbide meter



When used as an on-site instrument (as they mostly are), the investigator is usually looking for moisture contents of less than 5%. Why? Because BRE Digest 245 allegedly says that moisture contents up to 5% are acceptable. Actually, the Digest doesn't state this at all! It clearly states, "If the found moisture content (MC) is less than 5 per cent at the base, it is unlikely that the wall is severely affected by rising damp" and that "Although only a rough indicator, the five per cent threshold does represent a reasonable general guide to whether or not some kind of remedial treatment is needed. This emphasises the importance of the difference between the HMC and MC measured on samples." This clearly raises four points: (1) the guidance figure of 5% refers to the base of the wall, not 150mm or above where most samples appear to be removed! (2) these statements have nothing to do with diagnosis - they refer effectively to treatment regimes, (3) you cannot distinguish between MC and HMC on site! and (4) nowhere in the Digest does it describe or suggest the on-site use of the instrument! Unfortunately, most seem to ignore the above items, or misquote or misinterpret them.    

The Carbide meter also responds to hygroscopic salt contaminated material, i.e., hygroscopic moisture; this adds to the total moisture content of the sample. When used on site the meter cannot distinguish between a free and hygroscopic moisture, and heavily salt contaminated materials can have it moisture contents well in excess of 5% without free moisture. Indeed, there is one a classic case where rising damp had been effectively eliminated but on-site use of a Carbide meter persistently recorded moisture contents of 10-12%. After 4 damp-proof courses had been installed it was subsequently shown that all this moisture was due to hygroscopic salt contamination, not a continued failure of the damp-proof course! This is precisely why the Digest emphasises " - the importance of the difference between the HMC and MC measured on samples."

Perhaps one the most fundamental flaws in the use of the Carbide meter is the attempt to compare moisture contents of two or more totally different materials, and from this try to diagnose the origin of the dampness. Different materials have differing saturation moisture contents and so direct comparisons cannot be made between different materials. This again has led to misdiagnosis in interpreting on-site results.

Thus, as an on-site diagnostic tool the Carbide meter has a number of disadvantages, and as an on-site diagnostic tool its results are frequently misinterpreted and can lead to more errors (not treatment practices) than a surveyor competently using an electrical moisture meter. In effect the meter is subject to the same type of interpretation problems levied at the electrical moisture meter. There is also a distinct shortcoming in the education of the operator as to the limitations of the meter as well as a failure to appreciate the contents of BRE Digest 245, a document which has been so widely misinterpreted and misquoted. Furthermore, as on on-site tool it is very slow to use, a single sample taking in excess of 5 minutes to get a result. Hence the reason why most of those using such meters never take more than one or two samples and certainly not enough for use in diagnostics.

However, where the Carbide meter comes into its own is using it as described in the BRE Digest, that is to determine total moisture content (MC), hygroscopic moisture content (HMC) and hence a free moisture content in a vertical series of samples. Effectively, it is an off-site instrument taking the place of the laboratory method described in the Digest: it now becomes the true diagnostic tool as it was intended to be. Like the laboratory method also described in the Digest it is a destructive method of full moisture analysis and is thus best suited to investigative work. It requires a vertical series of samples to be removed from site and analysed for the total, hygroscopic and hence free moisture contents; the method is a fully described in the Digest.


The reality:

As general on site diagnostic tools electrical moisture meters are far more suitable provided they are used correctly with the necessary skill and expertise, together with a full understanding of their limitations. They are clean and non-destructive and will not effectively damage the property, a factor most important in a pre-purchase survey.

Carbide meters as an on-site diagnostic tool, even when used with skill and expertise, do not match the advantage of skilled use of an electrical moisture meter. Carbide meters are destructive, and it is still very difficult to interpret the results and certainly their site use has resulted in some catastrophic and expensive misdiagnosis. Whilst their ability in on-site diagnostics is certainly questionable, they can be very useful as a quick guide as to whether treatment may be required once diagnosis has been correctly made.

But the real advantage of the Carbide meter is off-site, where it can be used in place of laboratory facilities; it now becomes a very powerful diagnostic tool provided it is used fully along the guidelines as described in the BRE Digest for full moisture analyses. Unfortunately, most using the instrument do not follow these guidelines.



The two basic types of meter play totally different roles: electrical moisture meters are by far the best general survey tool for diagnosing damp problems provided the investigator is skilled in interpreting the results of the meter patterns and understands the instruments' limitations. The Carbide meter on the other hand is far more of an in-depth investigative diagnostic tool for use off-site to determine full moisture profiles which is how it was designed to be used; this is clearly identified in the BRE Digest. As an on-site diagnostic instrument the Carbide meter is surpassed by the advantages of the electrical moisture meter, although it may be helpful in giving guidance on the level of treatment required, if that is treatment is required at all.


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