R. Meyer J. Köhler A. Homburg Explosives

247 Permissibles; Permitted Explosives
a methane-air mixture (containing, e.g., 9.5% CH4 to give the most
dangerous composition), and the charge is fired. Whether or not
ignition of the gas occurs is observed from a safe position.
Amongst the known types of mortars is the borehole cannon, as
shown in Fig. 18. A steel cylinder about 1.5 m (5ft) long and about
35 cm (1–1/8ft) in diameter has in it a borehole of 55 mm (2–11/64
in.) diameter and 1.20 m (47 in.) length. The explosive to be tested is
placed in the borehole, unstemmed or stemmed by a clay plug, and
the detonator is introduced last in the hole (direct initiation). If the
detonator is inserted first, followed by the train of cartridges, initiation
is “inverse”. The required test conditions can be severe; ignition of the
gas mixture is more probable to occur using unstemmed charges and
inverse initiation than with stemmed charges and direct initiation. The
different mortars are designed to simulate different underground conditions.
The borehole cannon in the testing gallery illustrates the action
of a single shot in the roadway of gassy mines. The British break test
and the slotted mortar in Poland imitate the exposure of a charge and,
consequently, the more extended contact between the firing charge
and the firedamp atmosphere where breaks in the strata intersect a
shothole:
Fig. 19. Break test.
Two steel plates are held at a given distance by means of a closing
angle and a plug. The lower plate has a groove for the cartridge train.
The plate arrangement is covered with a polythene sheet laid upon
two steel side walls; the gas-tight room is filled with the methane-air
mixture after charging. The break test conditions are varied; permitted
explosives which meet the most stringent test conditions belong to the
British safety class P4.
The slotted mortar allows similar test procedures.