# Comparison of Measurement Possibilities by Non-Invasive Reflectometric Sensors and Invasive Probes
## 5 Conclusions  
The use of TDR surface sensors (noninvasive S1 and S2 type) showed that the Time Domain Reflectometry technique can be successfully used for non-invasive research to determine the moisture content of rigid porous materials used in construction. The TDR sensors provide very good responses similar to the measurements made with traditional invasive sensors.  
Since, in contrast to the invasive probes, the electrodes of the TDR sensors do not require to be immersed into the tested material, the measurements are significantly simplified, and potential damage to both the sensor and the tested object is avoided.  
When switching to the use of TDR sensors, however, one should be aware that they require individual calibration, which may be technically difficult to perform, especially in the case of partitions of objects, the characteristics of which are not known before starting the test.  
It was also noticed that the RMSE of the tested sensor was higher for the almost dry and almost saturated states of the measured material. The obtained test results allow concluding that more accurate measurement results are achieved with the use of invasive sensors. The extended measurement uncertainty U(w) for measurements with an invasive probe (middle range of material moisture \(w\)) was equal to 2.955 [-] for the cellular concrete 400, while, for the cellular concrete 600, it was 2.897 [-]. In the case of the non-invasive sensors S1 it was for the 400 c.c. sample, 3.114 [-], and for the 600 c.c. sample, 2.889 [-]. In the case of S2 and the cellular concrete 400, it was 1.956 [-], and for the cellular concrete 600, it was 2.893 [-]. Finally, it should be stated that the expanded uncertainty for the FP/mts probe was 94.1% of that for S1 and 151% of that for S2 (cellular concrete 400); 99.72% of that for S1 and 99.86% of that for S2 (cellular concrete 600).  
This does not change the fact that the surface sensors, despite the slightly different results compared to those obtained with the invasive probe, have a number of advantages. The following can be mentioned among them: (1) a simple and fast implementation allowing for more measurements per unit of time; (2) the time needed to prepare the device and the test object itself is much shorter than in the case of measurements with invasive probes; (3) the surface TDR sensors allow measuring the moisture of the tested objects without damaging them. The lack of need to introduce elements deep into the tested object allows maintaining the continuity of the structure of the object. Therefore, it is possible to measure fragile objects, e.g., objects with historic value or objects whose structure must not be destroyed.  
Z.S.: Conceptualization, methodology, data curation. M.P.-L.: software, investigation, writing--original draft preparation. M.W.-C.: visualization, funding acquisition. J.Z.: formal analysis, writing--review and editing. All authors have read and agreed to the published version of the manuscript.  
This research was funded by Lublin University of Technology grant's number's M/KIRP/FD-20/IM-5/142/2022 and FD-20/IS-6/025.  
Not applicable.  
The authors declare no conflict of interest.