[PSUBS-MAILIST] Acceptable cabin pressure swing

[Sean T. Stevenson via Personal_Submersibles]

The rules pay particular attention to design so as to avoid an increase of cabin pressure of one atmosphere in the event of a piping or pressure vessel failure which releases gas into the cabin. For that reason, I would be inclined to design windows and hatches to withstand that amount of internal pressure when secured. As for what pressure to alarm at, I would look at dive tables as a guide - i.e. what is the maximum permissible depth that still provides for an unlimited NDL time? This is, varyingly, 15 - 20 fsw, which would be the absolute maximum, but of course, you want to alarm before that. The other thing you could look at is the typical range of atmospheric pressures prior to closing your hatch. One standard atmosphere is defined at 101.325 kPa, which is the value I would use for automatic control, but if the atmosphere only can vary a few millibars, I might set alarms at the lowest delta that can't possibly be real. As Hank has previously mentioned, a warm sub dropped into
cold water can show a pressure drop from the temperature change alone, which you don't necessarily want to alarm on, so you need to set them wide enough to behave. Better yet, measure and consciously monitor continuously.

Sean


On February 24, 2016 12:12:45 PM MST, Jon Wallace via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>
>I think this is somewhat subjective but what is general consensus on 
>maximum cabin pressure swing (higher/lower) relative to hatch closing
>at 
>the start of a dive before setting off alarms?  My thought is that an 
>alarm should sound well before any point of emergency.  I'm considering
>
>sounding a warning at 2psi +/- and an alarm at 4psi +/-  but I'm not 
>sure if this is too strict.
>
>Jon
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