# Comparison of Measurement Possibilities by Non-Invasive Reflectometric Sensors and Invasive Probes
## 1 Introduction  
Measurement research has been refined over the years by developing various data acquisition methods. The study of environmental factors involves many measurement aspects. One of such factors is moisture, which affects the building materials, but also the quality of life of the building users [1]. The moisture of building materials depends primarily on their absorption properties, as well as on the operating conditions and on independent external factors, which are often difficult to detect also due to the improper construction of objects, faults or the lack of anti-moisture insulation [2, 3]. Moisture tests are also carried out in order to determine all the factors that lead to the multiplication of fungi, especially molds, that threaten human health [2]. Moisture can cause physical, chemical and biological damage [4]. A moistened material is subject to faster corrosion. With increased fungal infection, it can also be a substrate for the development of other microorganisms, additionally losing its thermal insulation properties [5]. If the moisture condensed in the porous space freezes, the structure of the material is destroyed. The processes of freezing and thawing lower the strength parameters of building materials [6]. An important element when examining building structures is the assessment of moisture parameters and the identification of harmful salts that may be present in the building material in various proportions [7]. The sources of moisture are often unnoticeable and maybe hidden under the insulation or located deep in the building material, which may make their detection difficult [8; 9]. Capillary water located deep in a wall is especially harmful. This water destroys the wall from the inside and is invisible on the wall surface, unlike condensation water that appears on the surface and is the result of improper ventilation of a room [10]. For the quick detection of moisture in a wall, indirect techniques are the most useful, enabling to estimate the moisture of a partition on the basis of other physical parameters, the values of which indirectly depend on moisture. Most often, these are electrical techniques based on the measurement of electrical conductivity or dielectric permittivity [11]. An advantageous feature of the electrical methods is that most often these methods are non-invasive and allow the testing of building materials without disturbing their structure and shape [12; 13; 14]. The basic electrical method is the resistance technique, which consists in measuring the conductivity or electrical resistance of the tested materials in porous materials depending on the degree of their moisture [15]. The measures based on this method are most often calibrated to the mass moisture of the material [16] and are most often used to determine the moisture of construction wood. However, the environmental parameters of walls or partitions are commonly determined using other methods [17; 18]. These include the capacitive Frequency Domain (FD) method, which is considered to be better than the resistance technique. The FD method consists in measuring the capacitance of a properly constructed capacitor with alternating voltage. In FD methods, the conductive electrodes in the area of the test medium are treated as capacitor plates. The dielectric of this capacitor is the material to be measured. The value of the apparent permittivity of the material affects the capacity of the capacitor thus formed. The measurement of this capacity allows the assessment of the material's moisture [19; 20]. Another non-invasive method, i.e., microwaves, allows measuring the phase shift and the degree of electromagnetic wave attenuation in the tested material, which allows the further determination of the moisture content. With the help of microwave moisture measurements, it is possible to determine the degree of moisture in building partitions inside their structure and on their surface.