1. Field of the Invention
The invention relates to gas mask filters. In one of its aspects, the invention relates to a gas mask with removable filtration cartridges. In another of its aspects, the invention relates to multi-stage filtration cartridges with an optional TIM filter. In another of its aspects, the invention relates to a gas mask with twist and lock removable filtration cartridges.
2. Description of the Related Art
It is traditional for combination filters such as those used in industry or by the military to have two filter media in sequence: firstly, a particulate filter to remove solid or liquid aerosols, droplets and particulate matter such as dusts, smokes, bacteria and viruses; and secondly an adsorbent layer, usually an activated charcoal to remove gases and vapors. A wide variety of charcoals with or without impregnants are available for particular filtration applications. The charcoal adsorbent may have more than one type of charcoal in intermixed or filled as separate layers into the filter body. See, for example, U.S. Pat. No. 5,660,173, issued Aug. 26, 1997 to Newton.
Military canisters typically have two types of media, particulate and charcoal. The charcoal is impregnated with such metallic salts of Cu, Cr, Ag, Zn, Mo and triethylenediamine in order to broaden the scope of chemical filtration by including both physical adsorption and chemical interaction with the impregnants to remove those chemicals that are poorly adsorbed and retained by physical adsorption alone. See, for example, the Grove et al. U.S. Pat. No. 6,176,239, issued Jan. 23, 2001, which incorporates by reference the U.S. patents to Braun et al. U.S. Pat. Nos. 5,033,465 and 5,078,132.
Attempts have heretofore been made to develop a filter medium that has the capability to remove both particulate matter and to adsorb gases. See, for example, British Specification No. 516,268, published Dec. 28, 1939. These filters are often referred to as “intimate mix” filters. One very good example of this type of filter was the “Cheekpad” design of filters used in the U.S. Military M17 Mask. However, it was found that the filtration efficiency of such media was compromised for both chemicals and for particulates. As a result, these types of filters are not in use today.
Each filter has a lifespan that relates to the amount and type of filter media. When any of the filter types have been saturated, the filter canister must be replaced. Thus, the life of any canister is only as long as the weakest filter medium. It is possible to construct a filter canister with sufficient amounts of each of the filter media to give a long life for all types of gases. However, the cost, size and weight of the canister must be taken into account in selecting the amounts of filter media that is to be incorporated into each canister. In addition, breathing resistance increases as the amount of the filter material increases. For military purposes, the canisters must be relatively small and light in weight. Yet, the canisters must be able protect the soldier from the exotic as well as the ordinary gases to which the average combatant might reasonably be subjected. Ordinarily, military personnel rarely face industrial gases and the addition of filter material to remove significant amounts of industrial gases is for the most part unnecessary. However, these gases must be filtered when they are encountered in the field, however infrequently. Thus, a balance must be struck between maximum protection against all types of gasses, weight, breathing resistance and bulk. These compromises have been made with smaller canisters that are replaceable when spent. The canister must be easily and quickly replaced so that a spent canister can be discarded and a new one added.
U.S. Pat. No. 4,850,346 to Michel et al. discloses a bayonet-type respirator fitting for a respirator port in a gas mask. The inhalation port includes an inhalation valve formed of a resilient membrane or flap, and mounts a chemical cartridge by a bayonet-type mount. The chemical cartridge can further mount a filter retainer housing a mechanical filter such as a felted fibrous disk.
British Specification No. 516,268 discloses a gas mask cartridge in which the air flows through a felted filtering mass comprising homogeneous mixture of a fibrous material adapted for mechanical filtration and substances capable of absorbtive and adsorbtive removal of noxious components in an air stream passing though the cartridge. The cartridge is made of layers of filter material that are axially stacked with radial passages from a central conduit for parallel axial flow through the filter media.