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ETL 1110-8-11(FR)
15 Jul 91
Table 2
Airblast Damage Criteria (Johnson 1971)
Overpressure (psi)
Degree of Estimated Damage
0.029
Possible window damage, particularly to large store windows
0.044
Some damage to large plate glass windows can be expected
0.065
Some damage to average size windows can be expected
0.19
Extensive damage to windows; probably damage to average wooden doors
0.58
Most small casement windows smashed
>0.58
Structural damage possible
measurements range from 1/8 to 2 in. in diameter .
b. Tourmaline crystal water shock gages can be
The diameter of the crystal selected is a compromise
manufactured, purchased commercially (e.g. PCB
between choosing one that is physically small (gage
Piezotronics, Inc.), or sometimes acquired via govern-
transit time small relative to shock duration) but large
ment sources (e.g. Naval Surface Weapons Center,
enough to provide adequate electrical output for the
US Army Engineer Waterways Experiment Station).
recording system. A rule of thumb to estimate the
In all cases, the tourmaline must be encapsulated in
error in the peak measured pressure due to the gage
an insulating material to prevent the generated charge
transit time Td, for θ/Td ratios > 5 is given by (from
from bleeding into the surrounding water. The
encapsulating material should have an acoustic
Tussing 1982):
impedance (sound speed times density) as close as
possible to water to minimize distortion of the pres-
0.5Td
(5)
% peak error
100%
sure measurements. It is extremely important that the
θ
encapsulating material have a high dielectric constant
and not generate charge when stressed. This would
cause errors in the measurements. One method of
d. The equation applies to cases in which the
construction which satisfies these requirements is to
amplitude of the direct wave is not affected by sur-
use a tygon or PVC boot filled with silicone oil (Fig-
face-relief or bottom-reflected waves during the
ure 6) to encapsulate the tourmaline crystal (Tussing
engulfment time of the gage. The effect of the error
1982).
is to cause the measured peak pressures to be below
the actual peak pressures. The engulfment time, Td,
of the gage is the gage diameter divided by the wave
velocity in water (5,000 ft/s). "Theta" is given by
Equation 3. For gages using the silicone fluid as the
encapsulating material around the gage, the calculated
transit time should be increased by 50 percent
(Tussing 1982).
e. Tourmaline crystals are often stacked together
Figure 6. Four-stack tourmaline crystal water
and connected in a parallel network to increase the
shock pressure gage employing tygon boot filled
sensitivity of the gage without increasing the diameter
with silicone oil
of the gage (Figure 6). Typical gage sensitivities
range from 2.5 picocoulombs per psi for a 1/4-in.-
c. The amount of charge developed by the gage,
diam doublet crystal gage, to 90 picocoulombs per psi
the gage sensitivity, is directly proportional to the
for an eight-stack pile of 1-1/8-in.-diam crystals (Cole
area of the tourmaline crystal being loaded.
1948).
Tourmaline crystals used for water shock
6
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