Electronic moisture meters offer a means of testing for moisture that is none invasive, quick, and easily repeatable. In expert hands, they are very reliable and a useful aid in the diagnosis of dampness; however, electronic moisture meters are calibrated for use on timber and readings obtained on masonry and plasters do not represent the actual moisture content of the material. In addition, materials contaminated with salts, particularly those found in rising damp, will produce exaggerated readings when an electronic moisture meter is used.
Of course, it can be argued that if salts are present that they must have come from somewhere. If the salts found in a wall are chlorides or nitrates, this will often have been subject to rising damp as those minerals are present in soil. Nevertheless, it is useful to know precisely the moisture content of a brick, stone, concrete, or plaster sample. Clearly, an electronic moisture meter cannot be used for this purpose. Instead, it is necessary to employ a calcium carbide meter or gravimetric (oven drying) methods. BRE Digest 245 'Rising Damp in Walls: Diagnosis and Treatment' advises that only these two techniques can provide quantitative moisture values of masonry samples.
A carbide meter is a sealable metal flask with a pressure gauge attached to the base. A sample of the plaster, wall or floor material to be tested is obtained by drilling. The sample is carefully weighed, placed in the flask, and mixed with a small amount of calcium carbide. The carbide reacts with moisture in the sample and produces acetylene gas, which increases the pressure within the flask. This pressure is converted into a percentage moisture content that can be read directly from the gauge on the base of the meter. The reading obtained is proportional to the amount of moisture in the sample: the wetter the sample, the higher the reading.
The gauge on the base of the meter pictured during a test. The moisture content of this sample is 5.3%. |
Gravimetric testing involves the use of laboratory scales and a drying oven. A sample of material is obtained by drilling; weighed, dried in the oven, and weighed again. The moisture content is determined by a simple calculation.
Britannia Preservation uses these laboratory scales apparatus to accurately measure the sample material. The drying oven is an important piece of equipment and a key part of the process. |
Samples are labelled and placed inside the drying oven and then measured again to ascertain how much moisture has been removed. |
Electronic moisture meters are often criticised for being inaccurate and misleading – after all, they only measure electrical resistance and not moisture. In contrast, carbide meters and oven drying methods are considered by many to be the gold standard in moisture measurement. While electronic moisture meters are extremely limited in their ability to accurately determine the moisture content of materials other than timber; they do provide invaluable information and in the hands of a skilled surveyor they are an indispensable tool.
As previously discussed, one drawback to the use of electronic moisture meters is the exaggerated readings when used on a wall contaminated with hygroscopic salts. These salts, typically chlorides or nitrates, are symptomatic of rising damp and immediately raises the question of how the salts got into the plaster if there is no rising damp!
The reason hygroscopic salts produce high readings on electronic moisture meters is because they conduct electricity. The hygroscopic nature of the material also has to be accounted for when undertaking carbide meter or oven drying testing. For this reason, samples should always be placed in a humidity chamber to determine their hygroscopic moisture content, and this value should be deducted from the total moisture content of the sample to determine the capillary moisture content (the unwanted moisture). Without considering the hygroscopic potential of the material, carbide meter and oven drying tests can be very misleading.
Moisture profiling is a complex process and one that is crucial for the correct diagnosis of dampness problems. Britannia Preservation has the expertise, experience, and equipment to undertake moisture evaluation and this means that our clients receive the correct diagnosis first time; avoiding costly and unnecessary remedial works.
We also provide an off-site diagnostic service: simply remove a small sample of masonry using a slow speed drill and send it to us in an air-tight container. We will carry out laboratory analysis and advise you of the precise amount of moisture present in the sample. Contact us for further details.
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