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The uncertainty of the measurements depends on several factors. The application presented here only requires comparable data for an assessment of the attained consolidation effect. Therefore, consistent sets of relative values are adequate, and the absolute values of the US velocities are not necessary.

235When using the rail arrangement for measurement of probe distances above those possible with the standard scissors double-probe device, it is necessary to slightly adjust the oscilloscope parameters, which in turn impacts the determination of US velocities. This effect, however, does not hinder the reliable determination of relative material characteristics.

Another important consideration is that measurement is sensitive to faulty contacts between the tested material and the transmitting and receiving probes. This is crucial when drilling holes for the probes. Sandstone is heterogeneous – it does not have a uniform density throughout its volume – and parts of the stone with higher densities respond differently to drilling than those of lower density. The resulting hole may therefore be irregular in shape, i. e. neither perfectly cylindrical nor perpendicular to the surface.

Variations in the determined US velocities for Hořice quartz sandstone are presented in Table 1. The uncertainty characteristics were calculated from 20 sets of 10 measurements done on an ashlar block, with probe distances of around 50 mm for the scissors and 170 mm for the rail arrangement.

Table 1: Statistics of the wave propagation velocities.

cond./type of device	average velocity [ms-1]	st. dev. [ms-1]	co. of var. [%]	aver. rep.dif [ms-1]
dry stone scissors	2,790	27	1	20
satur. stone scissors	2,903	8	0.27	6
dry stone rail	2,509	42	1.7	35
satur. stone rail	2,541	10	0.4	8

Furthermore, the presence of water in the pores of the stone affects the propagation velocity of the sound wave. The wetting of stone causes either an increase or a decrease in the speed of sound wave propagation, up to a point when saturation of the stone reaches its maximum value. The character of the change in sound propagation due to water saturation is dependent on the properties of the stone as well as the wave propagation direction with respect to the sedimentation layers of the sandstone. Typical values for the Hořice quartz sandstone are shown in Table 2.

Table 2: Change of wave propagation velocities in m/s.

Moisture condition	Direction of wave propagation
parallel	perpendic.	longitudin.
dry	2,690	2,662	2,742
saturated	2,588	2,775	2,189

The data in Table 2 was acquired on specimens of dimensions 300 mm × 50 mm × 50 mm. The axis of the beam corresponds to the longitudinal direction of the wave propagation.

Graphical representation of the impact of water, as well as reproducibility of acquired data, is shown in Figure 3.

It is apparent that differences arising from repeated measurements are sufficiently lower than the difference resulting from water saturation.

The velocity of the sound wave propagation in homogeneous materials generally depends on their modulus of elasticity in tension, density, and porosity. In heterogeneous materials, the propagation speed varies, resulting in different values when measured over various distances. The impact of probe distance was checked in the laboratory as well as on a stone wall.

The results presented in Figure 3 and Figure 5 indicate that on a practical scale the measured velocities are influenced less by distance than by other factors, namely the contact between the probe and the stone surface. Also, in real situations, differences are more the result of moisture content than variations in material quality. The surface layers of the stones in the masonry wall may be dryer or wetter than the inner layers of the wall, which can cause apparent variations in US velocity along the depth. However, the difference is only of the order of 3,2 % to 4,3 % in the tested Hořice sandstone wall.