Moisture in SCUBA & SCBA Air
There are 3 reasons for controlling and monitoring moisture in compressed breathing air: (1) When the moisture content is wetter than a dew point of -50º, that means that the main filters are no longer active. When the charcoal becomes damp, the trapped volatile hydrocarbons start to leave the charcoal and enter the air stream. When the Hopcalite becomes damp, it can no longer convert Carbon Monoxide into Carbon Dioxide. If Carbon Monoxide is present, it will pass directly into the storage flasks and SCUBA flasks. At high levels, the CO becomes lethal. (2) High moisture in cold environments can build up the regulators and cause free flow. (3) Is simply rust.
Understanding Dew Point & ppm moisture
This topic is very complex; we will discuss just the basic parts. The term Dew Point is the temperature at which the gas can no longer hold the moisture in the evaporated state, but causes it to spontaneously condense onto surfaces or remains suspended as micro droplets. Moisture is expressed in three ways: Dew point (in degrees Fahrenheit or Celsius), parts per million by volume (ppmv) and mg/cubic liter.
If you have air that has a dew point of -65ºF at one atmosphere pressure, the water content will be 24ppmv, and the air is considered dry. If you then charge a SCUBA tank to 3100psig with that same air, the moisture content will still be 24 ppmv, but because the water and air molecules are now compressed together, it is easier for the water molecules to "stick" together and condense onto the cylinder walls. If you measure the dew point of the air in that cylinder at the 3100 psi pressure, you would discover that the dew point inside the cylinder is now +28 ºF (that dew point is called the Pressure Dew Point). That means that if the outside temperature becomes colder than +28ºF, the compressed air in the cylinder will condense onto the SCUBA flasks interior walls.
Reasons to measure and control moisture
1. It is generally understood that wet air is a problem in cold climates because it can super-cool and condense in expansion areas of regulators and valves. Wet air can result in frozen regulators.
2. In long term storage, steel tanks can become rusty, and rust can clog tiny pathways in the first stage of air regulators. Even in fairly dry systems, the rust process continues over time. We often receive our kits back wherein the analytical filter is laden with very fine rust.
3. Rust particles do not adhere to surfaces, and have been shown to cause oxygen ignition when air is blended with oxygen during Nitrox preparation.
4. Rust formation in high pressure storage flasks can weaken the walls of these tanks, which can invite an explosion. In 1999, a large storage flask exploded spontaneously in a commercial dive center, killing one person and seriously wounding two others.
5. In order to avoid the extreme cost of frequent hydrostatic checking of large storage flasks of high pressure air, the US Navy monitors the moisture level in them. If the air is dry (dew point no warmer than -65ºF), the flasks can continue to be used for storing high pressure air. We perform tests on local Navy storage flasks, and find that the air typically has a dew point of colder than -70 ºF.
6. Another moisture problem that can occur is mold formation when damp air remains for a long time in tanks (especially in warm climates). The odor of mold can be misinterpreted as oil or some other compressor induced problem which leads the investigator away from the proper corrective action.
7. Not being a doctor, I do not know the effects of mold spores on the human body but, I do recall that the deadly legionnaire’s disease is caused by pathogens that thrive in damp air conditioning systems. Mold spores can also cause serious allergenic reactions in some people.
8. A much more common effect of moisture is that it builds up in the Molecular Sieve filters which are present in nearly all filter designs. As the water builds up in the molecular sieve packing, it displaces the previously trapped carbon dioxide. This buildup plus multiple pressure swings (compressive heating) will result in a large release of carbon dioxide into the SCUBA and SCBA tanks. We have seen levels in excess of 2000 ppmv caused by this phenomenon.
9. A high level of moisture in your compressed air will deactivate Hopcalite, thereby reducing the ability of the catalyst to destroy carbon monoxide.
10. Moisture interferes with certain devices that prepare Nitrox (especially pressure swing devices).
Fire from Water
A long time ago, moisture was removed from compressed air by passing it through silica gel filters. Nowadays, filters use molecular sieves to dry the air instead of silica gel. Molecular sieves are modified clay with pores that trap molecules based on size, and are very effective in removing moisture, carbon dioxide and certain small molecules. However, the absorption coefficient of water in molecular sieves is very high. If you have a problem with your water dump valve which allows the excess liquid water to travel into the filters, the molecular sieves will become very warm. If the filters are new or very fresh, and the liquid water enters the sieves, they become very hot. There are reported incidents where the filter cartridge becomes so hot that the plastic melts and a fire started!
In one case involving a brand new system, rainwater entered the air intake piping and was pumped directly onto filter beds of molecular sieves. This caused the filter cartridges to get so hot that the plastic melted. The system pressure pushed the molten plastic into the piping; the piping had to be replaced at significant cost.
Acceptable Levels of Moisture
Fire departments generally follow the moisture requirements of the NFPA specifications which require that the breathing air in SCBA flasks have a dew point be no warmer than -65 ºF. For Divers, the rules are more vague because there are more gas possibilities (normal air, Nitrox, and Tri-mix), and there are more associations that have their own ideas (AAUS, ANDI, IANTD, and the CGA). The most common specification in use is CGA Grade E for diving, but this specification does not require routine moisture determinations; it only suggests them.
The paragraph below is a direct quote from CGA G-7.1 - 2011.
The water content of compressed air required for any particular QVL can vary with the intended use from saturated to very dry. For breathing air used in conjunction with a self-contained breathing apparatus in extreme cold where moisture can condense and freeze causing the breathing apparatus to malfunction, a dew point not to exceed -65 ºF (24ppm v/v) or 10 ºF lower than the coldest temperature expected in the area is required. If a specific water limit is required, it should be specified as a limiting concentration in ppm (v/v) or dew point. Dew point is expressed in ºF at 1 atm abs (101 kPa, abs).