# Comparison of Measurement Possibilities by Non-Invasive Reflectometric Sensors and Invasive Probes
## 1 Introduction
Microwave radiation is absorbed by matter through ionic conductivity or through the phenomenon of dielectric losses resulting from dipole polarization [21; 22; 23; 24]. Water is a dipole which, while appearing in the structure of another material, still maintains the asymmetric nature of its molecules [25]. A separate group of measurement methods are the direct methods that enable to measure the presence of water. These are mostly invasive methods that interfere with the structure or shape of the tested element. The invasive measurement method which is most frequently used to determine the moisture in walls is the gravimetric method [26]. The basic instrument used in this method is a moisture analyzer, which can be used to test the material taken from a wall. The test material is weighed and then dried and reweighed in order to determine the weight differences [27]. It is a method that requires the destruction of the material structure. Destructive methods also include the carbide method (CM) [28]. This is an indirect method that also involves sampling and testing with a CM hygrometer using the chemical process of calcium carbide decomposition by water. During decomposition, acetylene gas is released, which causes an increase in pressure in the device [29]. Other invasive methods supporting the moisture measurement process also include measurements with a Peltier probe, which determines a sample's suction potential in a given moisture state. Instead of the material moisture, the psychometric Peltier probe measures the water potential, which is an indicator of the ability of a porous material to bind water [30]. Thus, the relative moisture of a porous material can be calculated [31]. In order to conduct invasive psychrometric measurements, the probe must be placed in the tested material by drilling a hole, which, if the material is loose, can be done by pressing the probe [31]. It is a standard form of measurement, additionally used to determine parameters that can support the process of measuring the moisture content of building materials. A measuring technique that can combine the advantages of destructive and non-invasive techniques is Time Domain Reflectometry (TDR), which uses the measurement of the propagation time of a short electromagnetic pulse in a material sample to determine the apparent permittivity of porous materials, which is strongly dependent on the water presence, i.e., material moisture [21; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41]. The apparent permittivity of the medium determines the velocity of signal propagation along the waveguide. The TDR sensor can also be used for the non-invasive monitoring of changes in moisture content in rigid porous materials [42]. The design of a TDR sensor determines the shape of the reflected signal, and the accuracy of the propagation time measurement depends on its design details [43]. Common applications of the TDR technique are based on the use of typical invasive probes introduced into the tested porous medium; this technique is most often used in soil science. With reference to building materials, its use involves the destruction of samples or walls [44]. Therefore, in the case of hard and rigid media, which include building materials, surface sensors are an alternative to traditional probes [42; 45; 46]. In the measurement of the basic environmental factors of building partitions, which include moisture, the TDR technique should be treated as a method at the stage of implementation for the purpose of testing walls. In order to improve this measurement technology, the obtained test results should be properly analyzed using typical invasive and non-invasive sensors and compared to the reference values obtained by direct testing [11]. The aim of this article was to determine the possibility of measuring moisture in building materials with non-invasive TDR sensors and to compare the measuring potential of this method to that of the TDR method using traditional invasive sensors.