F. Shot Instant Detection

In the survey we used plain No. 6 blasting caps and safety fuse. Because of the uncertainty in burning time of the safety fuse, (over a 24cm length of fuse the burning time ranged from 29 seconds to 36 seconds) a more accurate method of predicting the explosion time was needed so the recorder could be started in time without wasting too much paper.

To give a 6 second warning we taped an ordinary silicon diode to the safety fuse, 8cm from the detonator - as the fuse burned past this diode the temperature was raised, changing the forward voltage drop over the diode. This change in voltage triggered an oscillator tone which was transmitted to the recorder.

When the detonating cord fired, a piece of wire taped to the cord was broken, terminating the tone. This termination was recorded and gave us our shot instant on the records.

The operation was satisfactory, although there were problems with feedback when using the radio on high power but this could be easily remedied with better screening. Also if the wire broken by the explosion was engulfed in sea water thrown up by the explosion, the tone returned, sometimes in a few tens of milliseconds. Uncertainties in shot instant times are probably less than 0 milliseconds.