MicroTrap™ Operations Manual - Edition 4.4
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Care must be taken to select a good site for detonation and recording VOD of explosive samples. If possible, a permanent test site may
be constructed. A pit surrounded by an earth wall is typically sufficient for smaller detonations. Additionally, a protected shelter for the
MicroTrap™ VOD/Data Recorder and personnel can be constructed at an appropriate distance from the explosive. The distance will
depend on the amount of explosive being detonated at one time, and how the explosives are confined (be aware of steel shrapnel from
confined shots). Ensure that the area is well demarcated with the appropriate signage and that access is restricted.
If samples of explosives are to be detonated at an unprepared site, then the Operator must be careful when deciding upon what type of
ground the charges are to be placed. Avoid placement on ground with stones, rubble or anything that is likely to turn into a projectile.
The best surfaces are fines, sand or tailings.
It is always good practice to have maximum control over the time of firing of the test, therefore safety fuse initiation is not recommended.
Electronic, electric or shock tube initiation is best with the detonator either initiating the sample of explosives or the primer/booster in the
explosive sample.
4.2 The Resistance Wire Technique for Measuring VOD
The MicroTrap™ VOD/Data Recorder is capable of monitoring the continuous VOD profile of an explosive column. It can measure the
VOD of relatively short explosive samples such as cast boosters or explosive cartridges. It can also measure the VOD of explosives
loaded in multiple surface or underground blastholes. The MicroTrap™ VOD/Data Recorder provides a regulated constant excitation
signal to the probe and monitors the drop in voltage across it.
The MicroTrap™ VOD/Data Recorder uses the proven continuous resistance wire technique for monitoring VODs. An MREL-qualified probe
of known linear resistance (i.e. ohm/m or ohm/ft) is placed axially in the explosive sample or explosive column. As the detonation front
of the explosive consumes the probe, the resistance of the circuit will decrease in proportion to the reduction in length of the probe. The
MicroTrap™ VOD/Data Recorder records the resulting decrease in voltage across the probe versus time.
The DAS™ Data Acquisition Suite Software automatically converts the recorded data into a graph of distance versus time. The slope of
this graph at any position is the VOD of the explosive at that particular position. The DAS™ Data Acquisition Suite Software includes
functions that will automatically calculate and display the VOD of an explosive at any selected location in the graph. Other functions
allow the Operator to calculate and display the delay time between selected blastholes or between selected explosive decks within a
blasthole. Refer to the DAS™ Data Acquisition Suite Software Manual for more information.
4.1 Safety Considerations for Selecting an Explosive Testing Site
This Chapter provides detailed instructions on selecting a suitable site for testing
explosive samples, loading VOD PROBEROD and VOD PROBECABLE and connecting the
MicroTrap™ VOD/Data Recorder to record VODs and deck delay times.
Overview
STOP
Contact MREL for site specific recommendations for testing samples of explosives.
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