Abstract
The article concerns the electric techniques of moisture detection that are based on the evaluation of the apparent permittivity of the tested medium. The main goal of the research was to evaluate the non-invasive Time Domain Reflectometry (TDR) sensors’ sensitivity by measuring the span of elements and material moisture. To that aim, two non-invasive sensor designs were investigated for their sensitivity in the evaluation of the apparent permittivity value of aerated concrete. Sensors A and B were characterized by the spacing between the measuring elements equal to 30 mm and 70 mm, respectively. The tested samples differed in moisture, ranging between 0 and 0.3 cm3/cm3 volumetric water content. Within the research, it was stated that in the case of the narrower sensor (A), the range of the sensor equals about 30 mm, and in the case of the wider design (B), it equals about 50 mm. Additionally, it was stated that material moisture influences the range of sensor influence. In the case of the dry and low-saturated material, it was not possible to evaluate the range of sensor sensitivity using the adopted method, whereas the range of sensor signal influence was visible for the moist material.
Highlights
Time Domain Reflectometry (TDR) uses the measurement of the propagation time of a short electromagnetic pulse within a material sample to determine the apparent permittivity of porous materials, which is highly dependent on water content [1,2,3,4]
This paper presents the investigation results of two TDR surface sensor designs according to [36], which were developed for assessing the moisture content of rigid building materials and barriers
They are made of black polyoxymethylene (POM), which is characterized by an apparent permittivity value equal to 3.8 [45]
Summary
Time Domain Reflectometry (TDR) uses the measurement of the propagation time of a short electromagnetic pulse within a material sample to determine the apparent permittivity of porous materials, which is highly dependent on water content [1,2,3,4]. The pulse propagates in a waveguide consisting of conductive rods or wires embedded in the material to be measured [14,15,16]. The accuracy of the propagation time measurement depends on the probe construction details [7,17,18]. The probe itself forms a waveguide extending into the material under test, so the electromagnetic field propagates within the material with the velocity dependent on material apparent permittivity [5,6]. There are many TDR probe designs that have been constructed and tested for specific measurement conditions, the required accuracy, and the measured material properties [3,14,19,20,21,22]
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