In most cases, moisture meters are used when it is already moldy. So that the result is correct, you should pay attention to a few things. The technique that best measures can be found in a post that will be published here in mid-May.
Before the measurement results of a moisture meter can be classified, some questions have to be answered in advance, especially in case of mold attack:
- When did the damage occur for the first time?
- Which external influences (weather conditions, showers/bathing) control how the damage picture?
- In which chronological order do the different symptoms of damage appear?
- Is the damage picture continuous or recurring?
How Does Moisture Meter Work
In general, the more accurate the accuracy of the moisture measurement, the more detailed the circumstances of the damage are documented.
A proper moisture meter measures not only the moisture content but also the air temperature and humidity; these parameters are essential for the damage analysis. The simplest type is the so-called thermo-hygrometers, which score with fast reaction time and high accuracy. Since a moisture meter is mainly used for mold, it often offers a dew point temperature calculation. If this temperature is fixed, this is already a first indication of whether wrong ventilation behavior or structural defects favor the mold.
Many moisture meters only show a comparison value depending on the density and composition of the measured building material (“digits”). Due to this blurring of the measurements, the handsets are often only suitable for a preliminary test for the actual and much more accurate Darren or CM measurement. The different measuring methods will be presented below.
CM-measurement
In the Calcium Carbide Method (CM), a sample (10-50 g) is weighed and placed in a pressure vessel containing steel balls and a calcium carbide glass ampule. Everything is crushed in the closed container and mixed. The water contained in the sample reacts with the calcium carbide to form acetylene.
This reaction leads to a pressure increase, which is measured by a manometer. From the rise in pressure, the amount of sample and a table, the available water can be accurately quantified to +/- 1 – 3%.
The CM measurement is the type of analysis approved for the test requirement for the readiness for screeds on screeds. Possible errors are too small a sample size or too large particle sizes (max 2 – 5 mm) as well as premature evaporation of the water during the comminution of the sample in the mortar.
Advantage: The CM measuring procedure is easy to handle; no laboratory examination is necessary.
Disadvantages: With the CM measuring method, the component is slightly destroyed, the sample preparation is expensive.
Resistance moisture measurement
In resistance moisture measurement, two electrodes of the moisture meter are knocked or drilled into the component. First of all, the electrical resistance is correlated with electrical conductivity—the lower the strength, the lower the power of the element. In addition to the determined value, the result must also include the type of building material – a conversion table is therefore also required for the measurement. While this method was only possible with wood for a long time, mineral building materials can now also be measured.
Possible errors in the measurement are due to an uneven moisture distribution, temperature, homogeneity, or density of the component. Even a poor contact of the electrodes to the material often leads to incorrect measurements.
Advantage: The resistance measurement is a simple procedure without extensive investigations.
Disadvantage: The results are often not sufficiently accurate; only near-surface humidity is recorded.
Capacitive measurement
In the capacitive analysis (also called dielectric water content measurement method), electromagnetic waves are used to measure the moisture. High-frequency waves are emitted via applied measuring bars; the regular size is the capacitance of the capacitor in the measuring device. Calculated from the data is a readable percentage. It is essential to know the density of the building material. Capacitive measurement works with wood and mineral building materials.
Measurement errors can occur on an uneven surface if the moisture measuring gauge is not fully seated. Series measurements are recommended to minimize the mistakes.
Advantage: a completely non-destructive measuring technique.
Disadvantage: only near-surface measurement possible as pre-measurement for the actual CM measurement.
Microwave moisture measurement:
In microwave moisture measurement, electromagnetic waves are reflected by the test material. Since water has a higher permittivity (permeability to electric fields) than solid constituents, the material composition and thus the water content can be calculated from reflection, refraction and scattering of the waves. In most materials, such as wood or masonry, the water content can be determined down to a depth of 30 cm. The microwave process is a fast but also expensive process. To select the exact measuring frequency (0.3 – 20 GHz), the salt content and temperature of the component must be known.
Darr measurement (optional):
As with the CM measurement method, a sample must also be taken from the component/building material in the Darr measurement (even gravimetric moisture measurement). The sample is weighed and then dried at a specific building-site-defined temperature (usually about 105 ° C). Specific to building materials means that only the free, but not the bound, water can escape through drying, and this point is considerably lower for some building materials (e.g., anhydrite screed). After re-weighing, the loss of mass and thus the water content can be determined. If you then water the sample and weigh it, you get the saturation moisture. Therefore, one can simultaneously determine the maximum water absorption capacity of the building material.
One possible source of error is too long a waiting period between sampling and drying; since then, an adaptation to the ambient humidity can take place.
Advantage: The Darr method is a very accurate moisture measurement for wood and mineral components.
Disadvantages: The measurement is only very selective; it is also not feasible on-site and takes a relatively long time (24 h drying time). At least one CM measuring process is a contractual ancillary service for the contractor of the flooring works. Ideally, he decides after consultation with the client or architect about the time of measurement. Of course, the number of measures must be adapted to the size of the object.
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