DPlan - Equipment & Environment
Altitude needs no explanation. If altitude is greater than 0, freshwater will be selected automatically. Otherwise, you can select fresh or saltwater at sea level. All oceans are saltwater, very few other bodies are not freshwater. The Black Sea has an altitude of sea level, and a salinity approximately halfway between full salt and fresh, so you could use either setting. Negative altitudes are not permitted, if you want to dive the Dead Sea, you're on your own. Altitude is modeled up to 10,000 feet according to NOAA standards.
D-Plan can use either Imperial ( ft, cuft ) or metric units. Checking Metric will convert all the defaults on the control form to their approximate metric equivalents, but only if you have not already changed them yourself. Internally, all pressures are accounted in atmospheres, so the choice of linear depth unit is largely irrelevant. However, as meters are a lot coarser than feet, results between the two systems may differ slightly; all are valid.
Down at the bottom of the form by the buttons is one more 'Environment' parameter - Time-to-fly altitude. This is the cabin pressure of the aircraft in thousands of feet. FAA regulations require passenger airliners to be pressurized to 8000 feet, although they cruise in the thinner air much higher at 30-40 thousand feet. Newer aircraft are often pressurized to 6000 feet, or even less, for increased passenger comfort.
In keeping with my 'fiddle with everything' philosophy, you can use the default, or set your own up to 10000 feet, the limit of my altitude model. You can also put in the actual altitude of an unpressurized aircraft. The program uses its decompression calculations to determine a 'Stop Time' to safely ascend to what equates to a negative water depth. Conservatism options are not applied to TTF. Interestingly, the longest TTF I have been able to generate for 8000 feet is 11 hours, for any dive profile, including completely unrealistic full saturation dives. For 6000 feet, that drops to 8 hours. More typical TTFs are on the order of 1-2 hours, far short of the 12-24 hours that are recommended. Myself, I think I would go sightseeing for a day, and not be in too great a rush to get on a plane.
Four types of scuba equipment are supported:
Open Circuit, or regular scuba tanks and regs, is the default. When Open Circuit is selected, gas percentages specified in defaults, waypoints, or the Table Generator are for actual breathing gas.
CCR stands for Closed-Circuit Rebreather. When CCR is selected, gas percentages specified in defaults, waypoints, or the Table Generator are for diluent, and actual breathing mix is calculated based on the PO2 Setpoints. Physically, this models a device with separate supplies of 100% O2 and suitable diluent.
SCR stands for Semi-Closed Rebreather. There are two types of SCR: constant mass flow (cmf) and variable mass flow (vmf). When either SCR is selected, gas percentages specified in defaults, waypoints, or the Table Generator are for the main supply gas, and actual breathing mix is calculated. The cmf model fairly well approximates the old Draeger Dolphin and similar devices.
If a constant mass flow SCR goes below the minimum PO2 limit, D-Plan will attempt to correct it by increasing the flow rate. Physically, this is like hitting the 'Add Gas' button, assuming you have a PO2 warning monitor that just went off. ( The original Draeger had no O2 sensor, but many people wisely upgrade them. ) If successful, the program will print a warning in the profile; if not, the program will go to open-circuit bailout as a last resort to salvage the profile. Whether or not you think such a profile is realistic for actual diving is up to you. If you don't know the answer to this question, then you shouldn't be using a rebreather.
Warning: There is no way to accurately predict breathing gas fractions for a constant mass flow rebreather. Therefore, there is no way to calculate accurate decompression schedules for a constant mass flow rebreather, and the profiles generated here should be considered no more than very rough estimates.
There are many variations of semi-closed variable flow rebreather. Some use a single gas supply, others use two gases similar to a CCR. The model here is a single-gas semi-closed rebreather, with a simple re-arranging of the constant mass flow equations to attempt to hold a higher PO2 setpoint. Such a device has a PO2 limit determined by the gas used, which it cannot exceed.
During descents, rebreathers are modeled as running at the mix or diluent gas fractions directly. This roughly models the loop filling with gas as the pressure increases, and allows for calculation of diluent usage. During Ascents, rebreathers are modeled as constant-mix devices with the starting gas fractions. Neither of these is strictly correct, most rebreathers are capable of holding their setpoints quite well during ascents and descents, but it keeps things simple for the calculations. Mix is re-calculated once at the Ascent Rate Split Depth, since that is a convenient place in the program to do so and requires just one extra line of code.
All this may not be quite right, but it is simple, and given the time lags for the sensors and equipment, it may not be as wrong as you'd think. At any rate, given the short duration of these segments, it should have little effect on the overall profile except for very brief bounce dives, which you shouldn't be doing on a rebreather anyway! For longer decompression dives that are dominated by the bottom time, with ascents that are chopped up into Deco Stops, I think the inaccuracies in ascent/descent modeling are inconsequential, but I don't know for certain.
Below the equipment selection are various equipment parameters. SAC is the surface air consumption rate, in cubic feet per minute. SAC is used to estimate gas consumption for open-circuit. The default value is a good guess if you don't know yours. Smaller persons and women may want to use 0.6, and big people could use 0.8.
FO2 Main and FHe Main is used to specify your main gas mix. This is your bottom mix for open circuit, your SCR gas, or your diluent for CCR. This value can be over-ridden in waypoints, for travel gases, odd deco mixes, etc, however, doing so will force a rebreather to open circuit mode for that segment.
O2 Rate is the user's O2 consumption rate for all rebreathers, also in liters per minute. The default value is a good guess if you don't know yours. These values are used to estimate steady-state mix percentages and gas consumption.
Loop Vol is the volume of the rebreather breathing loop, including your own lungs. It is used to estimate gas used to fill the loop during descents, for all rebreathers. The default is a good guess, use a more accurate number if you have one, but it makes little difference.
SCR Flow is the mass flow rate of the constant mass flow rebreather orifice, in liters per minute. It is also used as the base flow rate for the variable mass flow rebreather.
The PO2 Boost setting controls how far a variable-flow rebreather will go in boosting the PO2. A setting of 0 is equivalent to a cmf rebreather, while a setting of 1.0 would be open circuit. A limit of 0.9 is imposed, resulting in approximately half the open-circuit gas consumption.
The next two parameters are PO2 setpoints for CCR. The shallow setpoint is used at depths shallower than the split depth, while the deep setpoint is used at depths greater than the split depth.
For a CCR on open circuit bailout, if 'use O2 for OCB' is checked, a hidden fourth deco mix of 100% O2 is enabled, simulating use of the CCR O2 bottle. Bailout will begin on diluent, and ( if set ) switch to O2 as dictated by the max PO2 decompression setting ( not the CCR PO2 setting. ) Any other available deco mixes will also be used, including 'off-board' O2, which will be preferred to 'on-board' O2 if available. SCR bailout is on the main gas and any available deco gases.
An SCR may not violate its gas mix MOD, and a CCR may not violate its diluent's MOD, even though in reality you might get away with it. To 'pre-breathe' a rebreather, use a depth of 1 foot. If a decompression gas is selected with a rebreather, open-circuit decompression will be used when possible, so be aware of this setting.
