SCBA AND THE WATER
The self-contained breathing apparatus is a positive-pressure system, meaning that once it is turned on, it will force air to the face mask continuously without inhalation to open a demand seal and, as we all know, the SCBA will continue to deliver air until it’s empty.
All the units we tested continued to deliver air at the surface. More often than not, we observed face mask chatter caused by the air being forced around the face seal. If you fall into the water while wearing SCBA, keep your face forward so that only the very front of the mask is submerged.
As long as air continuously is delivered to the mask, it is very difficult for water to enter the mask, making breathing at the surface possible. The large volume of air contained in the SCBA mask offers the wearer a minimal amount of buoyancy if the head is kept slightly forward.
SCBA units have several disadvantages when used in the water. Depending on the unit used, the first stage regulator may be so negatively buoyant that it will cause the entire unit to sink when immersed in water by itself. Also, the pressure (psi) in the cylinder has an inverse relationship to the amount of positive buoyancy: the greater the amount of air in the cylinder, the less positive buoyancy. An SCBA unit known to be negatively buoyant in the w’ater should be ditched immediately after the negatively buoyant helmet. Ideally, every unit used by your department should be tested in the water, both with full and near-empty cylinders; every firefighter should know whether the unit he or she is wearing will sink or float.
Once you know that your cylinder will float, you can use it as an aid by placing the bottom of it under a floating fiberglass helmet or under your upper torso. Again, don’t wait for an actual accidental immersion to find out whether your SCBA unit sinks or floats.
SCBA units were not designed for use in deep water; a pressure greater than approximately five feet or 2.5 psi may cause the unit to malfunction. Firefighters have told me that they went underwater with an SCBA unit to search for a body. Doing this is definitely asking for trouble. Besides the possibility of the regulator malfunctioning, using an SCBA unit underwater could cause a lung overexpansion injury.
Breathing underwater involves an increased pressure of approximately 0.5 psi per of water. At the surface, normal atmospheric pressure is 14.7 psi. At 10 feet below the surface, you would be breathing an additional pressure of approximately 5 psi, for a total pressure of 19 7 psi. The demand system of the regulator will try to match this new ambient pressure, resulting in the taking in of a larger amount of air than at the surface. This larger amount is translated into a greater density of air, and the denser air maintains a proper lung volume even at depth.
The denser air in the lungs does not cause a problem as long as you stay at the same or deeper depth, but as soon as you rise in the water column, that air expands. This phenomenon is described by Boyle’s Law, which states that for a gas at a constant temperature, there is an inverse relationship between volume and pressure. As pressure increases volume decreases, and as pressure decreases volume increases.
If for any reason you should rise in the water column while holding your breath, the air in your lungs will expand; the expansion can cause a perforation if it goes beyond the lungs’ elastic capability. If the lungs are full, a lung overexpansion injury can occur w ith as little as a three-foot rise. The air escaping from the lungs can enter various portions of the chest cavity, including the bloodstream, and cause a potentially fatal arterial gas embolism. Lung overexpansion injuries are among the leading five causes of death for certified divers who supposedly have been trained to avoid this problem. Just imagine what can happen to a nondiver firefighter breathing the compressed air of an SCBA unit at depth! Decompression sickness, or the “bends,” is not an issue here, since the SCBA unit won’t function at the depths and dive times causing this malady.