mounted on the canister guard with an elbow support
bracket. The elbow adapts the breathing bag outlet hose
The standard canister has been discontinued by
to the coupling end of the inhalation breathing tube.
MSA. However, there are standard canisters in
(5) Plunger valve assembly. A plunger valve
stock which should be used until the supply is
assembly is mounted on the canister guard.
expended. At that time, three quickstarting
assembly has a connection at the top for the exhalation-
canisters will be supplied with the M20.
breathing-tube coupling and a connection at the side for
(1) Standard canister The standard canister
the breathing-bag inlet hose. The bottom of the assembly
contains chemicals (oxides of sodium and potassium) that
is shaped to receive the neck of the canister. A plunger
release oxygen when exposed to moisture. A foil seal
pierces the copper-foil seal that covers the canister neck.
covers the neck opening to protect the chemical filling
(6) Timer and timer bracket. A timer is mounted
from moisture until the canister is installed for use. A disk
on the plunger valve assembly by a timer bracket. The
and a tearoff cap protect the foil seal. They are removed
timer knob, when turned clockwise, simultaneously winds
before the canister is installed. A flange and gasket on
and sets the timer (up to 60 minutes). (A safe limitation
the canister neck seal its junction with the plunger valve
of 45 minutes has been placed on the life of the canister.)
assembly when the canister is installed in the guard. A
The timer dial is marked in increments of 1 minute. Every
plunger in the valve assembly pierces the canister's foil
5 minutes is marked with a heavy dark line; every 10
seal and opens the neck. Inside the canister a copper
minutes is marked with a number. An alarm bell rings at
tube extends from the center of the neck opening to the
40 minutes and again at the end of the 45 minutes (the
bottom. Exhaled breath enters the canister through the
set time). Some timers have a stop nut that limits the
tube, passes upward through the tube and through the
setting to 45 minutes.
chemical filling. Moisture and carbon dioxide in the
d. Harness Assembly. The harness assembly (fig.
exhaled breath react with the chemicals to produce
oxygen which passes out the canister neck from around
B-3) has two shoulder straps, two chest straps, and two
waist straps that support the backplate assembly on the
(2) Quick-starting canister.
operator's back. During assemblies fasten the shoulder
and chest straps to the top of the backplate assembly.
canister is similar to the standard canister, except that a
Snaphooks on the waist straps fasten to waist strap eyes
chlorate candle is installed in its base. The chlorate
at the bottom of the backplate assembly. An eyelet on
candle supplies oxygen until the chemical reaction begins
one chest strap and a hook on the other chest strap fasten
in the canister. The candle also produces heat which
the harness around the operator. A tie strap holds the
accelerates the rate of action between the filling and the
shoulder straps in place on the shoulders. Two breathing
exhaled breath. A cover protects the starting mechanism
tube straps hold the breathing tubes in position. Slides on
of the chlorate candle A lanyard connects the cotter pin to
the shoulder, chest, and waist straps adjust the strap
the cover. Withdrawing the cotter pin releases the
length for proper fit.
starting mechanism that ignites the chlorate candle. Early
e. Oxygen-Generating Canisters. Either a standard
models of the quick-starting canister have two chlorate
cnadles installed in the base of the canister.
canister (gray, requires outside air to activate) (fig. 1-1)
or a quick-starting canister (green, self-activating) can be
used with the M20 breathing apparatus. The standard
(1) Carrying case. The luggage-type case (fig.
canister is used when the ambient temperature is above
of composition board and metal.
50 F.; the quick-starting canister is used when the
leather-strap handle. The case is compartmented to hold
ambient temperature is below 50F. If desired, the quick-
the canisters, breathing apparatus, and accessories. Two
starting canister may be used at all temperatures. (Its
luggage clamps secure the hinged lid.
starting time is the same in all temperatures). The
standard canister can be used when the temperature is
is supplied with the apparatus. The wrench is used to
below 50 ., but it is slow to start (requires as along as
tighten or loosen the breathing tube couplings.
20 minutes). If a standard canister is to be used in
(3) Antifogging kit. The antifogging kit furnished
temperatures below 50 ., it should be kept warm before
with the breathing apparatus is a
use for a faster start. User instructions are printed on the
outside of each canister.