Quote: (Originally Posted by
divetheworld)
Sorry, you have me confused there, and its not just because of the Karl-like length of the post.
The documents relate to moisture content of breathing gases of all pressures including >200bar pressure. It even gives moisture content of 25mg/m3 for 300bar.
Perhaps there is a mis-understanding here in what you are specifically talking about.
Ah when it comes to discussing dew point and moisture monitoring the devil is in the details. Just look at that first HSE report #427 you posted which took 26 pages to only examine what the moisture specification should be for the compressor systems less than 40 bar. I thought my post was quite succinct relative to those long winded Brits.
It is interesting to note that although this report only deals with the moisture specification for compressor systems below 40 bar the executive summary strongly suggests that for air pressures up to 200 bar the maximum moisture specification be brought down further to 20 mg/m3 or about 28 ppm moisture as the current moisture contents are "likely to result in free water condensing within a compressed gas system at the expected ambient conditions of use." For "expected ambient conditions" they are refering to your cold water at this time of year.
Have a look at the attachment below and you can see the HSE's very valid concerns here. With the current 35 mg/m3 (45 ppm H20) moisture spec a tank at 3000 psi (207 bar) yields a pressure dew point above 0 C. When your ambient temperature drops below this pressure dew point and is sub zero, as I suspect the water temperatures can be in the North Sea this time of year, you will have the moist air within some regulators depending on material construction freeze and lead to a free flow. For higher tank pressures you will require an even tighter moisture specification for winter diving in order to prevent free flows. By dropping the moisture spec to 20 mg/m3 at 207 bar your pressure dew point is much colder (drier air) and becomes -10 C thereby greatly reducing the risk of free flow.
Remember it is the pressure dew point we are truly interested in, but in many cases this is converted to atmospheric dew point on the air quality reports as that is what the analytical chemists have traditionally used. The atmospheric dew point will look ridiculously cold, but when converted to a pressure dew point the extent of the potential for free flows with wet air becomes clear.
For ice diving we will check our gas prior to diving with a Drager tube to ensure it is in the 0 to 5 ppm (0 to 4 mg/m3) range for moisture which gives a much colder pressure dew point and reduces the risk of free flow substantially.
So despite all this stuff looking quite theoretical there are very practical reasons for close realtime moisture monitoring especially for diving in cold water. The EN 12021 committee is meeting this quarter so I suspect in the next revision you are going to see that moisture level spec for the high presssure systems drop further
