Scott 2.2/ 3.5/ 4.5 Air-Pak SCBA

The Scott Air-Pak SCBA is an open circuit, self-contained breathing apparatus designed to meet the National Fire Protection Association (NFPA)Standard 1981. All components, excluding the air cylinder, have been designed and manufactured by Scott Health and Safety, a division of Tyco International, Ltd.

Cylinder

The cylinder and valve assembly is used for storing breathable air under pressure.
*Dimensions:
a. Cylinder = 19.6” X 5”
b. Water Capacity = 285 cubic inches
c. Air Capacity = 45 cubic feet for a 30 minute air supply [ [http://www.scotthealthsafety.com/Americas/en/Products/Accessories/cylinders.aspx Scott Health & Safety - Cylinders ] ] (1274 liters)
d. Weight = 11 Lbs.
e. Weight with AP-50 = 23.7 pounds

*Cylinder Construction:
Cylinders are constructed of spun aluminum allow, are manufactured in a cold extrusion chamber, and rammed at 7 million psi in order to form a strong, seamless cylinder. There are various types of "wraps" which the aluminum alloy is then wrapped with; the most popular being Carbon Composite and Kevlar Wraps. Carbon Composite wrapped cylinders "shall be manufactured in accordance with DOT specifications and have a working pressure of 2216, 3000, or 4500 psig."; http://www.southportland.org/vertical/Sites/%7B7A5A2430-7EB6-4AF7-AAA3-59DBDCFA30F2%7D/uploads/%7B1E27576C-6577-4B1A-B326-31B57B6EA2F8%7D.PDF] the are metallic-grey in appearance, are wrapped with 6 strands of carbon fibers called "toes". Each toe is .007 ml wide and 596 ft. in length, totaling 3,576 ft of wrap. They weigh 9.6 lbs. Kevlar wrapped cylinders are wrapped with 1,650 miles of Kevlar Composite monofilament. Each strand of the monofilament is approximately 1/3 the diameter of human hair. [luxfercylinders.com]

*Testing/ Burst Hazards:Cylinders are tested and rated according to NFPA 1981: Standard on Open-Circuit Self-Contained Breathing Apparatus (SCBA) for Emergency Services [ [http://www.nfpa.org/aboutthecodes/AboutTheCodes.asp?DocNum=1981&cookie%5Ftest=1 NFPA 1981: Standard on Open-Circuit Self-Contained Breathing Apparatus (SCBA) for Emergency Services ] ] The burst pressure for cylinders used in the Scott Air-Pak 4.5 is rated at 12,000 PSI. Without the aforementioned wraps, the aluminum alloy alone would would withstand a maximum rating of 6,500 PSI. Cylinders are required to be hydrostatically tested every 5 years [http://en.wikipedia.org/wiki/Hydrostatic_test] , and have a maximum life of 15 years. They are tested to 5/3 of the normal operating pressure.
ex.: The 4,500 psi rated cylinder is hydrostatically tested to 7,500 psi.

Valve Assembly

The Valve assembly assembly is the appliance which will make the connection between the cylinder and the hose (which will send the air to a pressure reducer). It is constructed of forged aluminum alloy, and is angled to reduce stress of the connection made between to the high pressure female coupling of the high pressure hose. Newer cylinders contain a "tri lobe, ergonomically designed hand wheel."This hand wheel is a one way, ratcheting type knob which is a push to close style knob. This protects the user from unwanted accidental shut off during use.
There is also an "an upstream connected frangible disc safety relief device" located on the valve assembly which is also known as the "tri-dimensional burst disk". This burst disk protects the user against situation where the pressure in the bottle would increases with exposure to heat during a fire; it prevents unwanted failures somewhere else in the bottle. It is set to release at 12,000 psi, or 400°F. It is "tri-dimensional" in that, if it is activated, it would disperse the air from the bottle in three directions so as not to throw the user off his or her balance.
The high pressure male coupling, included in high pressure bottle connections, is set to met the Compressed Gas Association Standards #346 and #347.
Also included in the assembly are the dual sided pressure gauge (which reads the cylinder pressure directly from the cylinder), elastomeric bumper (a protective, rubber like stop), and hangar plate (for positive locking attachment of the assembly to the backframe).

AP-50 Backframe and Harness Assembly

The AP-50 is now made of one piece of stamped aluminum- used for its high strength and heat tolerance. The straps are made of Kevlar- the shoulder straps being in the parachute style, and waist straps being in a seat belt style. Both shoulder and waist straps contain alligator style clips. The hose used in the AP-50 is steel woven wrapped with neoprene rubber and is connected to the cylinder valve assembly by the high pressure female coupling (aka. "knurled coupling", due to the knurled ridges around the assembly). It contains an "o-ring" rubber gasket, which is vital to the operation of the SCBA and should be checked for prior to use.
The air from the cylinder is sent from the high pressure hose (at 4,500 psi) to the "2-Stage pressure Reducer". The primary purpose of the Pressure reducer is to reduce the pressure coming out of the cylinder, to a more manageable psi. The primary stage will reduce the pressure to between 85 and 110 psi. The Secondary stage is activated when the cylinder reaches 1/4 left of air (or if there is a failure in the primary stage) and will increase the pressure to between 145-165 psi. The extra increase in pressure will activate the "Vibralert" in the mask mounted regulator (discussed below) .

1. RIC/ UAC Connectiona. High-pressure hose. b. Must be available within 4” of CGA #346 Connection c. The connection is integrated into the high pressure hose between the cylinder valve and first stage pressure reducer. d. Contains a self setting relief valve.

Effect of temperature on pressure gauge readings

The pressure gauge's indicated gas pressure changes with ambient temperature. As temperature decreases, the pressure inside the cylinder decreases. The relationship between the temperature and the pressure of a gas is determined by using the formula PV = nRT. [http://en.wikipedia.org/wiki/Universal_gas_constant] What is particularly important to understand from the formula is that the temperature is in degrees Kelvin, not degrees Fahrenheit. Consider the freezing point of water at 32 degrees Fahrenheit (0 Celsius) and compare it to 96 degrees Fahrenheit (35.6 Celsius; normal human body temperature is 38 Celsius). While 96 is arithmetically three times 32, the difference in temperature from a scientific point of view is not threefold. Instead of comparing 32 to 96, Kelvin temperatures of 273.15 and 308.71 should be compared. [ [http://www.onlineconversion.com/temperature.htm Online Conversion - Temperature Conversion ] ] The actual scientifically valid change in temperature from 32 to 96 degrees Fahrenheit is by a factor of 1.13 (308.71 / 273.15), not 3. If an air cylinder is pressurized to 4,500 psi at 96 degrees Fahrenheit and later the temperature drops to 32 degrees Fahrenheit, the pressure gauge will indicate 3,981 psi (4,500 / 1.13). Stated differently, a drop in temperature of 10 degrees Fahrenheit causes a pressure decrease of about 82 psi. Failure to accurately account for the effect of temperature on pressure readings can result in underfilled air bottles, which in turn could lead to a firefighter running out of air prematurely.

References