ESTABLISH PRESSURE DEWPOINT
Under BS4275:1997 and BS EN 12021, the requirement for moisture levels within breathing-air
systems is to establish that the pressure dewpoint is at least 5°C below the likely lowest ambient
temperature. If the ambient temperature cannot be established then the dewpoint should not exceed
–11°C.
The Safe-Air Tester has a built in thermometer to record ambient temperatures and records airline
moisture levels using a detector tube. When checking standard airline systems this moisture test is
carried out using the 50-2000 mg/m³ setting on the tester. The reading on the tube, which is
calibrated 5-200 mg/m³, should then be multiplied by a factor of 10 to establish the actual reading, ie
a tube reading of 150 mg/m³ becomes 1500 mg/m³.
To establish the pressure dewpoint reading to be entered on the results pad, us the “Pressure
Dewpoint Graph” for airline systems printed on the reverse of the results pad and apply the following
procedures:-
Take the mg/m³ reading from the bottom axis of the graph and extend until it crosses the line for the
airline pressure of the system and read the actual dewpoint from the left-hand axis of the graph.
In the case of the following example, a tube reading of 1500 mg/m³ and an airline pressure of 5 bar,
would give us a pressure dewpoint of 10°C. Therefore to ensure compliance to the requirements
then the ambient temperature in which the system can be used must be at least 5°C better than the
figure, ie 15°C. This can be compared with the ambient temperature recorded during the test to
establish a pass or fail.
Example
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