Canisters
During the development of the facepiece, as discussed above, the American canister underwent changes in design which have been designated as A to L. These changes were noted by the different colored paints applied to the exterior of the canister.
Type A canister was exactly like the British model then in use, except that it was made one inch longer because it was realized that the early absorbents were of poor quality. The canister was made of beaded tin plate and was 18 cm. high. The area of the flattened oval section was 65 sq. cm. In the bottom was a fine wire dome 3.4 cm. high. The valve in the bottom was integral with the bottom of the container, there being no removable plug for the insertion of the check valve. The absorbents were held in place by a heavy wire screen on top and by two rectangular springs.
Fig. 56.—American Canister, Type A.
Inhaled air entered through the circular valve at the bottom of the canister, passed through the absorbents and through a small nipple at the top.
The filling consisted of 60 per cent by volume of wood charcoal, developed by the National Carbon Co., and 40 per cent of green soda lime, developed and manufactured by the General Chemical Company, Easton, Pa. The entire volume amounted to 660 cc. The early experiments with this volume of absorbent showed that â…– soda-lime was the minimum amount that could be used and still furnish adequate protection against the then known war gases. It was, therefore, decided to use â…– soda-lime and â…— charcoal by volume and this proportion has been adhered to in all of the later types of canisters. It is interesting to note that these figures have been fully substantiated by the later experimental work on canister filling.
The charcoal and soda-lime were not mixed but arranged in five layers of equal volume, each layer, therefore, containing 20 per cent of the total volume. The layers were separated by screens of crinoline. At the top was inserted a layer of terry cloth, a layer of gray flannel, and two steel wire screens. The cloth kept the fine particles of chemicals from being drawn into the throat of the person wearing the mask.
This canister furnished very good protection against chlorine and hydrocyanic acid and was fairly efficient against phosgene, but it was useless against chloropicrin. These canisters were never used at the front, but served a very useful purpose as experimental canisters and in training troops.
It was soon found that better protection was obtained if the absorbents were mixed before packing in the canister. This procedure also simplified the method of packing and was used in canister B and following types. Among other changes introduced in later types were: The integral valve was replaced by a removable check valve plug which enabled the men in the field to adjust the valve in case it did not function properly. The mixture of charcoal and soda-lime was divided into three separate layers and these separated by cotton pads. The pads offered protection against stannic chloride smokes but not against smokes of the type of sneezing gas. The green soda-lime was replaced by the pink granules. In April, 1918, the mesh of the absorbent was changed to 8 to 14 in place of 6 to 14.
About July 1, 1918, the authorities were convinced by the field forces of the Chemical Warfare Service that the length of life of the chemical protection of the standard H canister (the type then in use) was excessive and that the resistance was much too high. Type J was therefore adopted, July 27, 1918. In this the volume of the absorbent was reduced from 450 cc. to 300 cc. It was packed in two layers, â…” in the bottom and â…“ in the top. One pad was placed between the layers and one on top. This change gave a lowering of the resistance of 27 per cent (to 2.5 inches) at a sacrifice of 50 per cent of the length of life of the canister, but not of protection during the shortened life. Type L differed from this only in having 325 cc. of absorbent, a change made to decrease leakage about the top cotton pad.
Fig. 57.—U. S. Army Canister, Type J.
The following table shows the relative efficiency of various canisters:
| p. p. m. | U. S., Type H | British, S. B. R. | French, A. R. S. | German | |
|---|---|---|---|---|---|
| Chloropicrin | 1000 |  770 | 17 |  2 |  43 |
| Phosgene | 2500 |   85 | 54 |  5 |  16 |
| Hydrocyanic acid |  500 |   70 | 90 | — |  10 |
| Mustard gas |  100 | 1800 | — | 35 | 195 |
The figures represent time in minutes till the first traces of gas begin to come through.
Fig. 58.—Type J Canister and Contents.