TESTING ABSORBENTS AND GAS MASKS
One of the first necessities in the development of absorbents and gas
masks was a method of testing them and comparing their deficiencies.
While the ultimate test of the value of an absorbent, canister or
facepiece is, of course, the actual man test of the complete mask, the
time consumed in these tests is so great that more rapid tests were
devised for the control of these factors and the man test used as a
check of the purely mechanical methods.
TESTING OF ABSORBENTS[31]
Absorbents should be tested for moisture, hardness, uniformity of
sample and efficiency against various gases.
[Footnote 31: See Fieldner and others, _J. Ind. Eng. Chem._, =11=, 519
(1919).]
_Moisture_ is simply determined by drying for two hours at 150°. The
loss in weight is called moisture.
The _hardness_ or _resistance to abrasion_ is determined by shaking
a 50-gram sample with steel ball bearings for 30 minutes on a Ro-tap
shaking machine. The material is then screened and the hardness number
is determined by multiplying the weight of absorbent remaining on the
screen by two.
The _efficiency_ of an absorbent against various gases depends upon a
variety of factors. Because of this, it is necessary to select standard
conditions for the test. These were chosen as follows:
The absorbent under test is filled into a sample tube of specified
diameter (2 cm.) to a depth of 10 cm. by the standard method for
filling tubes, and a standard concentration (usually 1,000 or 10,000
p.p.m. by volume) of the gas in air of definite (50 per cent) humidity
is passed through the absorbent at a rate of 500 cc. per sq. cm. per
min. The concentration of the entering gas is determined by analysis.
The length of time is noted from the instant the gas-air mixture is
started through the absorbent to the time the gas or some toxic or
irritating reaction product of the gas begins to come through the
absorbent, as determined by some qualitative test. Quantitative samples
of the outflowing gas are then taken at known intervals and from the
amount of gas found in the sample the per cent efficiency of the
absorbent at the corresponding time is calculated.
Per cent efficiency =
p.p.m. entering gas - p.p.m. effluent gas
----------------------------------------- × 100.
p.p.m. entering gas
These efficiencies are plotted against the minutes elapsed from
the beginning of the test to the middle of the sampling period
corresponding to that efficiency point. A smooth curve is drawn through
these points and the efficiency of the absorbent is reported as so many
minutes to the 100, 99, 95, 90, 80, etc., per cent efficiency points.
The apparatus used in carrying out this test is shown in Fig. 74.
Descriptive details may be found in the article by Fieldner in _The
Journal of Industrial and Engineering Chemistry_ for June, 1919.
With modifications for high and low boiling materials, the apparatus
is adapted to such a variety of gases as chlorine, phosgene,
carbon dioxide, sulfur dioxide, hydrocyanic acid, benzyl bromide,
chloropicrin, superpalite, etc.
As the quality of the charcoal increased, the so-called standard test
required so long a period that an accelerated test was devised. In this
the rate was increased to 1,000 cc. per minute, the relative humidity
of the gas-air mixture was decreased to zero, and the concentration
was about 7,000 p.p.m. The rate is obtained by using a tube with an
internal diameter of 1.41 cm. instead of 2.0 cm.
CANISTERS
After an absorbent has been developed to a given point, and is
considered of sufficient value to be used in a canister, the materials
are assembled as described in Chapter XII. While the final test is
the actual use of the canister, machine tests have been devised which
give valuable information regarding the value of the absorbent in the
canister and the method of filling.
[Illustration: FIG. 74.—Standard Two-tube Apparatus for Testing
Absorbents, Showing Arrangement for Gases Stored in Cylinders.]
The first test must be that for _leakage_. The canister must show no
signs of leaking when submitted to an air pressure of 15 inches of
mercury (about half of the normal atmospheric pressure).
The second factor tested is the _resistance to air flow_. This is
determined at a flow of 85 liters per minute and should not exceed 3
inches. The latest canister design has a much lower resistance (from 2
to 2½ inches).
The third test is the _efficiency_ of the canister against various
gases. For routine work, phosgene, chloropicrin and hydrocyanic acid
are used against the standard mixture of charcoal and soda-lime:
Chloropicrin is usually used against straight charcoal fillings, while
phosgene and hydrocyanic acid are used against soda-lime.
[Illustration: FIG. 75.—Apparatus for Testing Canisters Against
Chloropicrin.]
Different types of apparatus are required for these gases. They are
very complicated, as may be seen from the sketch in Fig. 75, and
yet a man very quickly learns the procedure necessary to carry out
a test of this kind. The gas is passed through the canister under
given conditions, until at the end of the apparatus a test paper or
solution indicates that the gas is no longer absorbed but is passing
through unchanged. This point is called the “break point,” and the time
required to reach this point is known as the life of the canister. This
time is also the time to 100 per cent efficiency. Other points, such
as 99, 95, 90 and 80 per cent efficiency are determined. These are
used in comparing canisters.
The canister tests were of two general classes: continuous and
intermittent. In the first the air-gas mixture was drawn through
continuously until the break point was reached. The results obtained
in this way, however, did not give the time measure of the value of
a canister in actual use. The intermittent test differs only in that
the flow of air-gas mixture is intermittent, corresponding to regular
breathing. Special valves were adapted to this work.
Canisters must also be tested as to the protection they offer against
smoke. These methods are discussed in Chapter XVIII.
MAN TESTS
The final test of the canister is always carried out by means of the
so-called “man test.” Special man-test laboratories were built at
Washington, Philadelphia and Long Island. These are so constructed
that, if necessary, a man may enter the chamber containing the gas and
thus test the efficiency of the completed gas mask. In most cases,
however, the canister is placed inside or outside the gas-chamber and
the men breathe through the canister, detecting the break point by
throat and lung irritation.
The following brief description of the man test laboratory at the
American University will give a good idea of the plan and procedure.[32]
[Footnote 32: Taken from Fieldner’s article mentioned above.]
The man test laboratory is a one-story building, 56 ft. in length and
25 ft. in width. The main part is occupied by three gas chambers,
laboratory tables, and various devices for putting up and controlling
gas concentrations in the chambers. A small part at one end is used as
an office and storeroom.
Good ventilation is of great importance in a laboratory of this nature.
This is secured by means of a 6 ft. fan connected to suitable ducts.
The fan is mounted on a heavy framework outside and at one end of the
building. The fan is driven at a speed of about 250 r.p.m. by a 10 h.p.
motor. The main duct is 33 in. square, extending to all parts of the
building. A connection is also made to a small hood used when making
chemical analyses.
The gases, fumes, etc., drawn out by the fan, are forced up and out of
a stack 30 in. in diameter, extending upward 55 ft. above the ground
level.
The main features of each of the three gas chambers are identical.
Auxiliary pieces of apparatus are used with each chamber, the type of
apparatus being determined by the characteristics of the gas employed.
[Illustration: FIG. 76.—Man Test Laboratory, American University.]
Each chamber is 10 ft. long, 8 ft. wide and 8½ ft. high, having,
therefore, a capacity of 680 cu. ft. or 19,257 liters. The floor is
concrete, and the walls and ceiling are constructed on a framework
of 2 × 4 in. scantling, finished on the outside with wainscoting and
on the inside with two layers of Upson board (laid with the joints
lapped) covered with a ½ in. layer of special cement plaster laid upon
expanded metal lath. The interior finish is completed by two coats of
acid-proof white paint. The single entrance to the chamber is from
outside the laboratory, and is closed by two doors, with a 36 × 40 in.
lock between them. These doors are solid, of 3-ply construction, 2½ in.
thick, with refrigerator handles, which may be operated from either
inside or outside the chamber. The door jambs are lined with ³/₁₆ in.
heavy rubber tubing to secure a tight seal.
At the end of the chamber opposite the doors, a pane of ¼ in. wire
plate glass, 36 × 48 in., is set into the wall, and additional
illumination may be secured by 2 headlights, 12 in. square, set into
the ceiling of the chamber and of the air-lock, respectively, and
provided with 200 watt Mazda lamps and Holophane reflectors. Openings
into the chamber, five in number, are spaced across this end beneath
the window and 9 in. above the table top.
Fans are installed for keeping the concentration uniform.
[Illustration: FIG. 77.—Details of Canister Holder.]
Various devices have been installed for attaching the canister to be
tested (Fig. 77). This arrangement allows the canister to be changed at
will without any necessity for disturbing the concentration of gas by
entering the chamber.
Arrangements for removing the gas from the chamber consist of a small
“bleeder” which allows a continuous escape of small amounts and a large
blower for rapidly exhausting the entire contents of the chamber.
Other general features of the equipment deal with the determination
of the physical condition surrounding the tests, often a matter of
considerable importance. The temperature of the gas inside the chamber
is easily ascertained by means of a thermometer suspended inside the
window in such a position as to be read from the outside. The relative
humidity of the mixture of air and gas in the chamber is determined by
means of a somewhat modified Regnault dew point apparatus mounted on
the built-in table.
PRESSURE DROP AND LEAK DETECTING APPARATUS
Another piece of apparatus consists of a combined pressure drop machine
and leak tester (Fig. 78) for measuring the resistance of canisters
and testing them for faulty construction. This is mounted on a small
table, with the motor and air pump installed on a shelf underneath.
The resistance, or pressure drop, of canisters is measured by the
flow meter _A_ and the water manometer _B_. Air is drawn through the
canister and the flow meter _A_ at the rate of 85 liters per min.,
the flow being adjusted by the needle valve. The pressure drop across
the canister is read on the water manometer _B_, one end of which is
connected to the suction line, the other open to the air. The reading
is generally made in inches, correction being made for the resistance
of the connecting hose and the apparatus itself.
Canisters are tested for leaks by the apparatus shown at _D_ in Fig.
78. The canister is clamped down tightly by wing nuts against a piece
of heavy ¼-in. sheet rubber large enough to cover completely the bottom
of the canister and prevent any inflow of air through the valve.
Suction is then applied, and a leak is indicated by a steady flow of
air bubbles through the liquid in the gas-washing cylinder _E_. A
second gas-washing cylinder, empty, is inserted in the line between _E_
and the canister as a trap for any liquid drawn back when the suction
is shut off. If a leak is shown, it can be located by applying air
pressure to the canister and then immersing it in water.
[Illustration: FIG. 78.—Apparatus for Determining Pressure Drop and for
Detecting Leaks in Canisters.]
METHODS OF CONDUCTING TESTS
Three general methods of conducting man tests are followed:
(1) Canisters are placed in the brackets outside the chamber or
fastened to the wall tubes within the chamber. The subjects of the
test remain outside the chamber, and the facepieces of the masks are
connected directly to the canisters, in the first case, and to the
wall tubes connecting with the canisters, in the second case. The
concentration is established and the time noted. Then the men put on
the masks and breathe until they can detect the gas coming through
the canisters. Reading matter is provided for the men during the test
period. When gas is detected, the time is again noted and the time
required for the gas to penetrate the canister is reported as the “time
to break down” or “service time” of the canister. Ten canisters are
tested at one time, and the average of the results for the 10 canisters
is taken for that type of canister. Much less accurate results are
obtained when the final figure is based on a small number of canisters.
This is largely due to the various breathing rates and sensitiveness of
different men.
(2) The canisters are placed as in (1), but it is only necessary
to know if they will give perfect protection for a given length of
time. The procedure is the same as in (1), except that the test is
arbitrarily stopped at the end of the indicated time, and the number of
canisters and the service times of the same noted.
(3) When the canisters are of such a type that they cannot be properly
tested as in (1), or when it is desired to test the penetrability of
the facepiece, the men wear the complete mask and enter the chamber.
They remain until gas penetrates the canister or the facepiece, as
the case may be, or until it is determined that the desired degree of
protection is afforded. The service time is computed as in (1).
(4) Maximum-breathing-rate tests are made either by men in the chamber
or by the men outside, in which they do vigorous work on a bicycle
ergometer. In this test the average man will run his breathing rate up
to 60 or 70 liters per min.
The concentration of the gas is followed throughout the test by
aspirating samples and analyzing them.
=Type of Masks Used.= In the future the 1919 model will be used for
all tests. In general, during the War, the following procedure held,
although variations occurred in special cases:
When men entered a gas-chamber, the full facepiece was, of course,
required. The type of facepiece was determined by the nature of the
gas. If the gas was most easily detected by odor or eye irritation, a
modified Tissot mask was used. If it was most easily detected by throat
irritation, a mouth-breathing mask was employed.
When men were outside the chamber, the choice was made in the
same manner, except in the case of detection of the gas by throat
irritation. In this case the mouthpiece was attached to two or three
lengths of breathing tubes and a separate noseclip was used. The
facepiece was not needed and the men were much more comfortable without
it.
=Disinfection of Masks.= Mouthpieces are disinfected after use by
first holding them under a stream of running water and brushing out
thoroughly with a test tube brush; then the latter is dipped into a 2
per cent solution of lysol, and the inner parts of the mouthpiece are
brushed out well; finally the mouthpiece and exhaling valve are dipped
bodily into the lysol solution and allowed to dry without rinsing.
Tissot masks are wiped out with a cloth moistened in alcohol, followed
by another cloth moistened in 2 per cent lysol solution. The flexible
tubes are given periodic rinsings with 95 per cent alcohol.
=Applicability of Man Tests.= Man tests are applicable to all gases
which can be detected by the subject of the test before he breathes a
dangerous amount.
The man test laboratory described above provides facilities for
obtaining information concerning the efficiency of canisters,
facepieces, etc., within very short periods of time, without waiting
for the construction of special apparatus required for machine tests.
To get satisfactory results from machine tests, a delicate qualitative
chemical test for the gas is essential. Man tests can be made when such
a qualitative test is not known. Further, man tests can be made with
higher concentrations of some gases than is practicable with machines.
Evolution of excessive amounts of moisture when high concentrations of
some gases are used causes much more trouble with machine tests than
with man tests.
On the other hand, man tests are adversely affected by the varying
sensitiveness and lung capacities of the men, and the humidity of the
air-gas mixture is not subject to as exact control as is the case with
machine tests.
FIELD TESTS
It will be observed that all of the above tests are concerned only
with the efficiency of the absorbent and its packing in the canister.
No attempt was made to determine the comfort and general “feel” of the
mask. For this purpose field tests were devised, covering periods from
two to five hours. The first test was a five-hour continuous wearing
test. It was assumed that any mask which could be worn for five hours
without developing any marked features of discomfort could, if the
occasion demanded it, be worn for a much longer period of time. A
typical test follows:
8:00 to 8:30 Instruction and adjustment of gas mask.
Gas-chamber tests
8:30 to 9:30 Games involving mental and physical activity
9:30 to 11:30 Cross-country hike with suitable periods of rest
11:30 to 12:00 Tests of vision
12:00 to 12:30 Games to test mental condition of subjects
12:30 to 1:00 Gas-chamber fit test
[Illustration: FIG. 79.—Hemispherical Vision Chart.]
Vision was tested by means of a hemispherical chart (Fig. 79). This
chart was 6 ft. in diameter and was constructed of heavy paper laid
over a wire frame. A hinged head rest was provided for holding the
subject’s head firmly in position with the center directly between the
eyes. The subject wearing the mask took up his position, and with one
eye closed at a time, indicated how far along the meridian of longitude
he could see with the other eye. The observer sketched in the limit of
vision by outlining the perimeter of the roughly circular field allowed
by each eyepiece. The intersection of the two fields gave the extent of
binocular vision possible with the mask.
Various other tests were also used, in order that the extent and nature
of the vision could be accurately determined.
Aside from the problems of comfort, protection, vision and other
important features of gas mask efficiency, the question arose as to
whether certain designs of masks or canisters were mechanically able
to withstand the rough treatment they were certain to receive in
actual field service. A test was, therefore, developed to simulate
such service as transportation of masks from base depots to the front,
carrying of supplies and munitions by men wearing masks in the “alert”
position, exposure to rain and mud, hasty adjustment of masks during
gas alarms and typical mistreatment of masks by the soldiers.
All these tests were of great value in the development of a good gas
mask.