810 
=qe@e 
The air trapped underneath the telcothene, the thickness and 
composition of the telcothene as well as that of the thick rubber tape all 
contributed to distortion of the recorded pressure pulse from an underwater 
explosion. 
The thick steel outer electrodes have been replaced by layers of 
silver sintered onto the crystal faces, and fine annealed copper tags are 
spot soldered to these to enable connections to be made to the cable braid. 
The central steel electrodes has also been replaced by a thin silver foil 
fused between the negative faces of the two orystals. 
Thick metal eleotrodes have been shown to produce serious oscillations 
superimposed on the true pressure-time signature. These not only distort 
the pulse but give false peak pressures, and modify the time of rise of the 
‘steep wave front associated with the detonation wave from an underwater 
explosion. 
Although the pressure pulses recorded from identical gauges when fine 
annealed copper electrodes "gelvaed" onto the crystal are not much, if any, 
inferior to those recorded with sintered silver electrodes, nevertheless 
mechanically the adherence of annealed copper electrodes is not sufficiently 
robust to record peak pressures of thousands of pounds per square inch, ise. 
with charges close up to the gauge. 
The total thickness of the complete two-ply miniature gauge is now 
" and it consists of two tourmaline crystals %" in diameter. The 
hickness/diameter ratio of the gauge is, in part at least, defined by the 
strength and quality of the tourmaline discs. The gap between the copper 
tags at the base of the gauge adjacent to the central conductors and braid 
of the cable is filled with a molten rubberoid wax composition, and great 
care is taken to prevent air being trapped. As thin a film of natural 
latex as possible is then cured onto the gauge unit. 
Experiment has proved that the more nearly the tourmaline gauge 
resembles the bare twin arystals, the more faithful the reproduction of the 
pressure-time pulse of the detonation wave. 
For frithful reproduction, the gauge has to be directed edge-on to 
the pressure front with the cable leading away from the explosion wave, 
the faces of the orystals constituting the tourmaline gauge being at right 
angles to the water surface. For a 1% 1b. charge, the sharp time of rise 
of the steep-fronted wave is modified to about 8 microseconds which is the 
period taken by the wave to traverse the gauge diameter: this causes a 
peak pressure reduction of about 3% for this particular charge, but for a 
charge of about 30 lbs., the reduction is not greater than 1%. When the 
pressure pulse is travelling parallel to the plane of the gauge, it will 
be undisturbed by it so that the pressure on the plane will be equal to 
that in the pulse. The time of rise of the wave front as registered with 
the gauge face-on is of the order of 1 microsecond or less but a false 
peak pressure (described as an overshoot in the body of the report), is 
recorded, particularly with a thick gauge rigidly held towards the pressure 
wave. This peak pressure has been proved to lie somewhere between the 
true value of the incident wave pressure and twice this value when the 
reflected wave is in phase. 
The amplifier developed for piezo-electric recording at this 
Establishment is capable of recording faithfully a steer fronted wave 
having a duration of only 1 or 2 mioroseconds, and this is about the limit 
of film resolution and writing speed with the present drum camera, 
Experimental gauges having 4" and 1" diameters respectively have 
also been used satisfactorily in the tests carried out and described 
in this report. 
It is believed that except for the natural frequency oscillations 
set up in the tourmaline discs themselves, that the miniature gauge will 
NOW cecce 
