(For background, see the User Bulletin entitled "TANKTYPE" and also the sample run file entitled "BUBBLE".)

As bubble tanks are implemented in GHS, they cannot have more air pressure at a given load volume than would result in one atmosphere if the load were reduced to zero. If you give a pressure in a LOAD command such that the pressure would be greater than one atm. at zero load, it reduces the pressure. You can see this in the zero nominal load which is shown in response to the plain LOAD command. If it allowed greater pressures, it would present a problem whenever the depth decreases beyond the point where the load goes to zero and the pressure is still more than 1 atm. Since the load cannot go negative it would have to remain at zero and violate the gas constant (i.e. spill air).

There are some proposals for extending the ability to handle higher pressures:

1. Extend the Bubble type to allow a mode where air spills when the load goes to zero but is "replenished" as the depth increases to the point where the original gas constant can be supported. Since it's not a sealed tank and it's not one where the air pressure is maintained at a constant, there is a question as to whether this would model something that could happen in the real world. (If anyone has advice on this, please let us know.)

2. Provide a mode where air is allowed to spill and thereafter remain spilled. This would model something that could happen in the real world. It presents a difficulty to the solving process in that it presents an irreversible discontinuity which is hard to deal with in the solvers and in interpreting the righting arm curve.

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