Читать книгу Monument Future - Siegfried Siegesmund - Страница 385

Before using the method at real scale, the relevance of the calibration law has been tested for several scenarios. One of them consisted in the desaturation survey of the limestone sample. IR thermography technic is carried out simultaneously with an electric technic and sample mass measurements.

Tests are carried out on cylindrical samples of 2 cm diameter and 2 to 4 cm height previously saturated using the 48 h porosity protocol. The electrical and infrared measurements are done inside a black chamber specially designed to attenuate the environmental infrared noise sources. During the electric and infrared measurements, it has been simultaneously measured the mass loss with a 1 mg accuracy. Multiple sensors are placed inside the black chamber allowing to record the chamber temperature, atmospheric pressure and relative humidity variations.

The monitoring of the desaturation process by the electrical method is done by analyzing the electrical resistivity variation using the two-point probes method. The electrical resistivity is calculated using the Ohms law. The voltage is measured between the two extremities of the sample for a fixed current.

The signal injected at the top of the sample is a square waveform of 12 V of amplitude and 137 Hz frequency. This signal is compared to the signal received at the bottom of the sample in order to calculate de variation of the voltage during the desaturation process. An Average Waveform Generator incorporated to a digital oscilloscope (TiePie Handyscope HS5) is used. The same oscilloscope is used to record the evolution of the signal with a resolution of 12 bits and a sampling frequency of 10 kHz.

The thermal scene has been recorded with the same camera previously presented. IR images of the thermal scene are recorded at a frequency of 0.01 Hz. From each IR Image a surface area has been selected to highlight the temperature evolution of the limestone sample and the reference (crumpled aluminum-foil).

Finally, a mass balance allows to measure the mass loss during the experience.