Appendix E3

E3.1 Atmospheric Dew Point (Table 4)

Table 4, is provided to assist the user in determining the atmospheric dew point required to protect cylinders and receivers against the problems associated with excessive moisture content eg. regulator freeze-up in cold weather operations, and internal corrosion in compressed breathing air cylinders. To determine the atmospheric dew-point temperature required to achieve this protection proceed as follows:

1. Select from Table 4 the pressure dew point temperature column representing the pressure of the user's SCBA;
2. Determine the lowest temperature to which the SCBA may be exposed;
3. Locate the temperature closest to this in the column headed "Pressure dew point temperature" for the appropriate cylinder pressure;
4. Select the temperature 5ºC (9ºF) below this temperature, as a safety factor; and
5. The temperature directly opposite this in column 1 with the heading "Atmospheric dew-point temperature" is the required dew-point expressed in °C or °F. The corresponding water vapour content in mL/m³ (ppm) is shown in the column "Water vapour content at atmospheric dew point temperature and pressure".

A firefighter using a 15.3 MPa (2216 psig) air supply is working in a geographical location where the lowest temperature to which the SCBA is exposed is -25ºC (-13ºF):

1. Select Table 4 [(15.3 MPa) (2216 psig)]
2. The lowest temperature to which the SCBA is exposed is -25ºC(-13ºF)
3. Locatethe temperature closest to this in the column headed "Pressure dew point temperature" for the appropriate cylinder pressure: -25ºC(-13ºF);
4. Select a temperature 5ºC(9ºF) below this temperature: -30ºC(-22ºF)
5. The dew-point opposite in Column 1, "Atmospheric dew-point", is -65ºC(-85ºF) and corresponds to a water vapour content of 5 mL/m³ (ppm).