Moisture testing units

CM measurement

Screeds are used almost everywhere in the building industry: in residential and commercial buildings, in industry and commerce, in underground car parks as well as balconies and roof terraces. Depending on where they are used, they are subject to special requirements and stresses. Screeds mixed with water require drying times of up to 8 weeks, depending on air humidity or heating heat. Before the screed can be covered with a floor covering, the residual moisture must be checked. This is done by a so-called CM measurement. The values are determined with a calcium carbide (CM) measuring device. The values for the permissible residual moisture are, for example, for cement screeds < 2 CM-% and for calcium sulphate screeds or anhydrite screeds < 0.5 CM-%. The CM measurement is currently the only approved and generally recognized method for screeds, which is also suitable for the craftsman. Even if opinions differ there. The CM-measurement provides reproducible results and can be determined quickly and reliably when used correctly. Of course, the values are only reliable if the samples are taken carefully and the measurements are carried out according to a uniform procedure. This requires a calibrated CM measuring instrument, which should be checked for leakage at regular intervals. CM measuring instruments work according to the calcium carbide method. Here, the calcium carbide reacts with water or moisture of the material to be measured. This reaction creates a gas pressure in the form of an acetylene gas-air mixture. The moisture content in CM% can be read off directly on the manometer or via a table in the operating instructions. Depending on the type of screed and covering, it can be decided after the measurement whether the screed is ready for covering or whether additional measures are required before the screed can be covered with tiles, parquet, elastic or textile coverings. Of course there are other methods available, but unfortunately these do not provide comprehensible results. The determination of moisture by means of electrical measuring instruments can serve as a pre-measurement. However, electrical measurements do not yet replace the professional CM measurement.

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