A History of Research and a Review of Recent Developments

Experimental pressure measurement 121
Figure 6.2 Gauge by Baker and Ewing to be flush-mounted in the surfaces of blast
loaded aerofoils (from Baker and Ewing, ref. 6.3).
The measurement of reflected pressure at normal incidence to a plane surface
is easier than the recording of side-on pressure, and most of the American
experimental data on reflected blast waves discussed earlier was obtained
from gauges designed by Hoffman and Mills [6.4]. These used piezo-electric
elements mounted in a cavity in the front face of a heavy metal housing, and
could measure peak pressures up to 1500psi. An improved design by Granath
and Coulter [6.5] could deal with peak pressures of 4500 psi, and according
to Baker has been sold commercially.
An alternative method of pressure measurement is to sense the deflection of
a ferromagnetic diaphragm under pressure by measuring the change in inductance
of coils. By this means very tiny gauges can be made, having a thickness of 0.1
in thick and a diameter less than 0.2 in. The deflection of a diaphragm can also
be measured by the interception of flux lines from a magnetic coil as the deflections
occur. As more flux lines are intercepted the eddy current losses increase, thus
changing the coil impedance. Pressure can also be sensed by supporting
diaphragms on tiny tubular columns fitted with semiconductor strain gauges,
and measured from the output of the gauges as the diaphragm deflects under
pressure. This type of strain gauge tends to have a low sensitivity output but a
high-frequency response characteristic. The diaphragms are made from many
types of material, ranging from stainless steel, nickel, and stainless steel covered
with Teflon or coated with flame-sprayed aluminium oxide. Baker points out
that for high blast pressures near the detonation point of underground nuclear
explosions the diaphragms must be protected by a heatshield baffle.
As well as the measurement of peak instantaneous pressures and the decay
of pressure with time, the time of arrival of the blast wave is often required. A
number of types of gauge are now available commercially which generate a
large signal on arrival of the shock front but do not need to be sensitive enough