You can skip this discussion, but it does have a bearing on understanding the multiplicity of SCBA types out there. If you are a captain or mate taking over on a ship, and are faced with a number of mismatched SCBAs from obviously different eras, you might want to continue reading.

At the end of World War 2, with the invention of the Cousteau-Gagnan regulator, made famous as the "Aqua-Lung", breathing from bottles of compressed air became feasible for a variety of industries. A big user of the earliest compressed air breathing devices was the mining industry, which used them for rescuing trapped miners. That’s why you find SCBAs made by a company called Mine Safety Appliances (MSA) aboard some ships.

Obviously, firefighters quickly made use of the new technology. At first there was little or no difference between an SCBA sold to a mining company and that sold to a fire department. Relatively low pressure air was used (1800 PSI), because that was all that was practical given the early technology.

As in other areas of technology, geography played a part in SCBA development. The SCBAs in use in North America were of a different design than those used in Europe, and the components were not compatible. This is still the case today.

High temperature synthetics such as Nomex came on the market and were incorporated into SCBA design, to meet the needs of firefighters. But because these materials were not needed in mining applications and for escape equipment in industrial plants, different lines of SCBAs were developed for different industries, with the firefighting-SCBAs being more expensive than rest.

Regulator design became more complex, though for the operator activating the SCBA became much simpler. The old "two-knob design"--one for main regulator activation, the other to activate the bypass--was reduced to a single, redundant, bypass control. The new SCBAs started up automatically, on detecting the operator inhale. If the regulator failed, an automatic bypass cut in without operator intervention.

In 1981 the National Fire Protection Association (NFPA), a non-governmental organization that researches and publishes fire safety guidelines, published industry guidelines for SCBA design for firefighting use. If you see a label marking an SCBA as being 1981 NFPA compliant you can be sure it is safe for firefighting, as long as it is complete and in good repair.

Bottles evolved in more ways than just pressure. Today you can purchase new or used bottles in steel, aluminum, or composite construction, ranging from 2015 PSI to 4500 PSI. In the fleet I administer we even had a venerable 1800 PSI bottle, which first saw use when Eisenhower was president.

As SCBAs evolved the older units were not eliminated from use, except by special jurisdictions, such as fire departments. For example, land-bound fire departments phased out demand-only regulators years ago, based on the 1981 NFPA guidelines. But "older" style SCBAs, such as the Survivair Mark 2 are still in production and work perfectly well.

Aboard ship it is common to find a number of different types of SCBAs stored away for firefighting use. You need to find out what you have, how each different unit works, which can swap bottles between them and which can't.

I am smart enough to make no claims about the superiority of one SCBA brand over the other. Off the top of my head, I can think of six different SCBA manufacturers in the U.S. This is meant to be a practical article, however, and I would be remiss if I did not mention the most common brands. If you are going to purchase SCBAs you want to have them serviced by someone nearby, if at all possible, and get parts quickly. This can be difficult for the unusual brands.

Scott Aviation, Survivair, and MSA are the three brands commonly found on U.S. vessels, with Racal and Draeger coming up behind. They are all reputable brands. As with any expensive piece of equipment, you should weigh several factors before buying a new or used SCBA. One of these factors is the service issue. For example, I prefer Survivair SCBAs, not because they are better than the other types, but because, where my fleet is headquartered (Seattle), we happen to have an excellent Survivair-authorized service technician nearby. I cannot in good conscience say this about the other brands. I can get Survivairs fixed quickly. My company does have non-Survivair SCBAs too. When those units need service it takes weeks for the simplest repair.

I am a big proponent of using surplus, rebuilt equipment on ships, if only to make use of the surplus supply of old steel bottles out there. If you choose to go this route, check with your supplier to see what kind of bottles are most prevalent in the surplus market before your buy.

Generally speaking, parts between SCBAs are not interchangeable, with some exceptions, such as the high-pressure coupling O-ring. When it comes to SCBA bottles, any manufacturer will say that only their bottles can be used with their SCBAs.

Part of this is litigation protection, part is actual design considerations. In truth, you can often use Brand-X's bottles with Brand-Y's SCBAs. But the matter is complicated.

I am not advocating mixing and matching, nor am I warning against. I am suggesting, as strongly as possible, that you test all components of an SCBA system before you put it on your ship.

Positive Pressure or Pressure/Demand - With the Positive Pressure or Pressure/Demand regulator air is constantly supplied to the mask, with extra air supplied with each breath. It almost eliminates the chance of toxic vapor getting in the mask.

The term "Pressure/Demand" has also come to refer to a type of regulator that can be used in either the Pressure or Demand modes, by flipping a switch. A prime example of this type is the old Scott Air-Pak II, of which there are plenty still out there. They are allowable for use on ships, but should be used only in the Positive Pressure mode (see USCG NVIC 6-93).

Demand Only- A demand regulator supplies air only when the wearer inhales. This is the oldest type. It’s draw back is that toxic vapors can possibly get under the mask seal. These can no longer be used aboard ship.

The SCBAs found on ships use "low pressure" bottles. "Low pressure" means 3000 psi or less. The bottles must be filled with special compressors. (A ship’s air compressors typically produces less than 500 psi, and the air contains oil injurious to the lungs).

2015 PSI, round-bottomed, steel bottles are still common on ships. The more modern low-pressure SCBAs use a 2216 PSI bottle.

Oddly, if a steel 2015 PSI bottle has a plus (+) stamped in it after a hydrotest date it can recharged to a pressure 10% greater than the stamped pressure: 2216 PSI to be exact.

As a practical matter, there is a high, if not perfect, degree of interchangeability between bottles for the older model SCBAs, including Survivair Mark 2's, Scot Air-Paks, or MSA Ultralites. You can usually use a Survivair 2015 PSI bottle on a Scott Air-Pak II, for example. You have to check beforehand because of subtle differences in design. MSA composite bottles, for example, do not fit in a Survivair Mark 2 frame.

What can never be substituted is a 2015 PSI bottle in a new generation SCBA, such as the Scott Presur-Pak 2.2 or Survivair Panther.

3000 psi low bottles are also made. These are not commonly found aboard ship. You should check with an expert before using them with a SCBA designed for 2216 psi or less.

High pressure bottles exist that carry 4500 psi for roughly a hour’s worth of air. However they are not commonly found aboard ships and should not be used with low pressure SCBAs.

Made of a thin aluminum liner wound with an exterior fiberglass shell. Most of the strength is in the fiberglass. Lighter than metal tanks, composites are preferred by professional fire fighters. But the weight difference is little, the bottles are comparatively delicate, and composites have a life span of only 15 years.