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Water Vapour Content & Dew Point:
The "Dew Point" is the temperature at which
water
vapour will start to condense
from air. This value depends upon water content & pressure. SCBA air is required to be dry enough to prevent malfunctions (i.e. air
flow blockage) due to internal condensation or "icing" of the regulators.
Elevated water levels can also cause tank corrosion, bacterial growth, and inhibits catalysts that convert carbon
monoxide into carbon dioxide.
Standard CAN3-Z180.1-00 states that compressed breathing air equal to or greater
than 2216 psig shall have an atmospheric dew point not exceeding -53°C or a
water vapour concentration not exceeding 27 ppm. The pressure dew point should
be at least 5°C below the lowest temperature to which the cylinder and piping
may be exposed at any season of the year.
The measured Dew Point is the quantity of water vapour found in the air
sample at ATMOSPHERIC pressure. To determine the temperature at which water vapour
may condense and freeze in the pressure reducing regulator, it is
necessary to compare the pressure dew point to the ambient temperature
at which the SCBA is used. Conversion tables are included in CAN3-Z180.1-00.
For example: The proposed maximum (atmospheric) dew point of
-53°C or 27 ppm of water, corresponds to a pressure dew point in the
regulator of -12°C or 2148 ppm of water at a line pressure of 2216
psig. However the CSA standard requires that the (pressure) Dew Point not
exceed 5°C below the minimum temperature to which any part of the breathing
air system is exposed at any season of the year. Thus a compressed breathing
air sample with a (atmospheric) Dew Point of -53°C could not be used
at a line pressure of 2216 psig in any areas where the temperatures are
less than -7°C.
The standard also states under no circumstances shall the Dew Point (atmospheric)
exceed -53°C for systems at or above 15.3Mpa (2216 psig).
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