Growth of the Self-Contained Mask

Growth of the Self-Contained Mask

The first closed-circuit rebreathing-type apparatus is shown in the drawing at the top. In the center is an example of the present demand-type mask. One concept of breathing apparatus of the future is shown at the bottom. It features complete body protection, from head to foot, with total sealing against any leak.

From closed-circuit rebreathers to the compressed air tank

Although few details are available, historians tell us that miners first used respiratory devices in Europe during the period from 23 to 70 A.D. Then, in 1795, Dr. Alexander Von Humboldt developed one of the first self-contained breathing masks. This early but unsuccessful attempt to provide complete respiratory protection consisted of an air storage bag, a bellows for filling it, and a partial facepiece.

Dr. Theodore Schwann introduced a new concept in the middle 1800’s when he designed a closed-circuit rebreathing-type apparatus which used compressed oxygen and a CO2 scrubbing chemical. This was followed in 1850 by a self-generating device which used an oxygen-producing material. However, a really practical device was not produced until the late 1930’s.

Compressed air for masks

The present concept of compressed air breathing apparatus originated in the United States in the 1930’s, but it was not until 1945 that manufacturers produced mask assemblies for industrial and fire service use. During the past 20 years there have been many improvements in original designs, in addition to the development of some completely new models. Significant developments during this period include the low cylinder pressure alarm, antifog nose cup assemblies, and voice amplification equipment.

The Bureau of Mines, which has done a great deal of research and testing on respiratory equipment, developed a standard work rate test for establishing the period of time that masks can be worn. Mask assemblies are approved for 1/2-hour use when trained operators can perform certain tasks within this period of time while wearing the mask.

However, fire fighters complained that these masks would only supply air for approximately 20 minutes or less when used at fires. Such complaints were verified when later tests proved that fire fighters used air at an abnormal rate while under fireground conditions.

There were still other factors that had to be considered. Abnormal air consumption might occur if a man was in poor physical health, emotionally disturbed, had been exposed to carbon monoxide gas, or just normally required unusually large quantities of air.

Furthermore, a low duration rate might result from improperly serviced equipment. (Mechanical defects seldom occur in modern assemblies.) A major cause of air leakage, for instance, was found to have been bad hose connections and defective packing.

How to repack valves

The servicing facility should stock packing and glands. Their replacement is not a difficult job. After the cylinder pressure has been released, the valve stem is removed and the packing or glands can then be renewed. Oil or grease must never be used in repacking the valve. The packing usually needs to be renewed more often than the glands.

When recharging a cvlinder, it is a good practice to listen after shutting off the compressed air supply and closing the breathing apparatus cylinder valve. If the sound of escaping air is heard, often only a quarter turn or so of the valve stem packing nut is needed to eliminate the leak. The packing nut will usually accept several adjustments before it is necessary to renew the packing. In most cases, the glands can be used again with the new packing.

Filling cylinders

The most common reason, however, for masks lasting less than ‘A hour was that improper methods were used in filling compressed gas cylinders. As a rule, the Bureau of Mines based approval for ½-hour breathing apparatus on the charging of a cylinder to 1980 psig or 2216 psig. This pressure requirement was a must. A misconception arose that a cylinder was fully charged when filled to 1800 or 2015 psig, and that the 10 percent overfill allowance was a reserve.

The half-hour duration was granted each apparatus on a given cylinder pressure. The question then arose that if the cylinder pressure was increased, why was the duration not extended? The answer to this question was that the half flow rate demand regulators which were used in some of the newdesign apparatus allowed the wearer to obtain more air per inhalationexhalation cycle. This merely provided a greater air supply to meet the halfhour duration requirements. This seemed to be a very minor point, but there were design limits that had to be met to insure proper operation. And an accumulation of many of these small problems would adversely affect the performance of any apparatus, regardless of the type or the manufacturer.

New developments

While the shoulder and back harness type had been in service for many years, later developments in design saw the return to the shoulder and hip, or alpine-type, back pack. (This harness had been used by several manufacturers many years before in their first breathing apparatus.) In an effort to meet customer demand, several manufacturers supplied their apparatus with either type of harness. The ability to remove and service the harness assemblies was a much needed improvement and was adopted by several of the manufacturers. This simple change made it possible to service the harness in the field without taking the apparatus out of service to send it to the factory for harness repairs.

The use of quick-adjusting harness buckles reduced the critical time used in putting on and taking off masks. The design of a multi-diameter cylinder clamp, which allowed the use of different size cylinders, was another innovation—a practical idea, but not currently approved under the present Bureau of Mines schedule 13D.

The adoption of standard cylinder valve threads by all manufacturers but one, eliminated cylinder charging problems. (The one manufacturer who uses a nonstandard cylinder valve thread can supply adaptors for their cylinder valves.)

Another major complaint concerning apparatus assemblies was the total weight. Models weighed anywhere from 25 to 35 pounds, depending on the manufacturer and the accessories used. The problem of weight was not easy to solve. Unfortunately, cylinder volume and weight were directly related. Any increase in cylinder volume meant an increase in weight.

Cylinders were available which afforded lighter weight for a given volume of gas, but they were expensive when compared with standard cylinders, and the total weight reduction was very slight. New designs of liquid air and oxygen apparatus extended the use time considerably, but there was little or no weight reduction. However, the minutes of use per pound ratio was lower.

Re-breather systems improved

The introduction of improved designs of re-breather systems which used compressed oxygen and a highly efficient chemical agent also increased the use time, but the apparatus weight was still in the 25 to 35-pound range.

The weight problem, common to all manufacturers, is currently under study.

Present-day apparatus is functional, well-designed and efficient in operation. And these units, although heavier and offering less time of usable air, do afford a high degree of life safety. The promise of longer-duration, lighter-weight apparatus will require a great deal of research, design and evaluation before the improved designs of the future become a reality.

The fitting of all shapes of faces with one facepiece is considered impractical; however, a facepiece designed to fit about 95 percent of possible users is possible. As of this moment, however, no one model facepiece will fit all wearers of breathing apparatus. For each facepiece has its own special design features.

Proper facepiece fit is extremely important. The practice of storing breathing apparatus facepieces outside their protective covers has caused rapid deterioration of the rubber compounds and has become somewhat of a problem in certain areas of the United States. Improved facepiece rubber compounds have been introduced over the past few years but the problem still exists.

Future design

Breathing apparatus of the future will have to be of the total concept design if we are to provide complete life-support protection for the wearers. One aspect of such a total concept design consists of complete body protection, head to foot. And to give complete respiratory protection, it will be necessary to provide a positive pressure demand system for breathing, as well as total sealing against any leakage. This concept, once considered too extreme, is being developed at this time for use in the near future.

A parallel effort in this area can be found in the work being done on personal life support systems for men to wear on the moon and beyond. And there are environments on earth as hostile to human life as those on the moon. These can be found in many of today’s chemical and high-toxicity fires.

A system of radio and voice communication will also be mandatory, taking into account recent developments in micro-circuitry and transistor power supplies. And the use of special techniques to eliminate visor fogging is possible and will be required.

Internal harness design will reduce the weight of the breathing apparatus portion of the system as will a modular design of breathing apparatus components. In addition, multifunction controls will simplify operations and the use of high-pressure compressed gas cylinders of lightweight design will be a standard feature. And finally, the use of liquid air and oxygen systems, as well as re-breather systems, are all practical methods for supplying breathable atmosphere to the wearer. It will also be possible to include additional equipment in such a design and yet reduce breathing apparatus components to about half their present number and half their present weight.

The total concept design, as envisioned for the men of the fire service, will be based on review of the past, evaluation of the present, and planning for the future.

No posts to display