The saltier the water, the heavier it is, and the faster pressure rises as you go down. Since the move to real physical pressure, MV-Plan really takes that into account: water type changes the pressure felt, therefore the nitrogen load, therefore the length of your stops.
The gap is not a detail. Between a fresh water quarry and the Red Sea, pressure at 40 m differs by 3.18 %.
The setting lives in two places, and it is the same one:
Under the selector, MV-Plan always shows what the chosen water stands for, for
instance 10300 g/l - 9.90 m/bar: the density in force, and how many metres of
water weigh one bar.
Each entry of the menu carries its density, in grams per litre, so you know what you are picking:
| Choice | Density | Weight of the water column | Metres per bar | Pressure at 40 m |
|---|---|---|---|---|
| Fresh water | 10000 g/l | 0.098100 bar/m | 10.19 m | 4.9373 bar |
| Brackish water | 10150 g/l | 0.099571 bar/m | 10.04 m | 4.9961 bar |
| Salt water | 10300 g/l | 0.101043 bar/m | 9.90 m | 5.0550 bar |
| Red Sea | 10400 g/l | 0.102024 bar/m | 9.80 m | 5.0942 bar |
Density is given in grams per litre: 10300 g/l reads as 1.03 kg/l, which is how salinity is usually written.
The last entry of the menu, Custom density, brings up a field where you type the density of your site directly, in grams per litre.
The range covers the extremes: 9500 g/l for the least mineralised altitude lakes, 12400 g/l for the Dead Sea.
Mind the unit: it really is 10300, not 1.03. A density typed in kg/l would be a thousand times too low and would fall outside the accepted range, which is exactly why it is refused.
Every measurement below was made with the settings shipped by default, at sea level, on air.
Recreational dive, 30 m / 30 min, GF 30/85:
| Water | Stops | TTS |
|---|---|---|
| Fresh water | 12/1 9/1 6/3 3/7 | 18 min |
| Brackish water | 15/1 12/1 9/1 6/2 3/9 | 21 min |
| Salt water | 15/1 12/1 9/1 6/2 3/10 | 22 min |
| Red Sea | 15/1 12/1 9/1 6/3 3/10 | 23 min |
Decompression dive, 40 m / 30 min, GF 50/80:
| Water | Stops | TTS |
|---|---|---|
| Fresh water | 15/1 12/3 9/5 6/9 3/24 | 50 min |
| Brackish water | 15/1 12/3 9/5 6/10 3/25 | 52 min |
| Salt water | 15/1 12/3 9/6 6/10 3/26 | 54 min |
| Red Sea | 15/2 12/2 9/6 6/11 3/27 | 56 min |
How to read it: 6/10 means a 10 minute stop at 6 m.
Keep the order of magnitude in mind: between fresh water and the Red Sea, count roughly 5 more minutes of decompression on a 30 m / 30 min, and 6 minutes on a 40 m / 30 min. On that same 40 m / 30 min dive, a density of 11000 g/l gives a 65 min TTS and the Dead Sea at 12400 g/l gives 91.
The direction is always the same: the saltier the water, the longer the decompression. Setting fresh water while diving at sea shortens your stops, which is not the side you want to be wrong on.
Water type and altitude add up, and a mountain quarry brings the two together. In fresh water, on air, 30 m / 30 min, GF 30/85:
| Altitude | Surface pressure | Stops | TTS |
|---|---|---|---|
| Sea level | 1.0132 bar | 12/1 9/1 6/3 3/7 | 18 min |
| 1000 m | 0.9000 bar | 15/1 12/1 9/1 6/3 3/11 | 24 min |
| 2000 m | 0.7994 bar | 15/1 12/1 9/1 6/4 3/14 | 28 min |
| 3000 m | 0.7100 bar | 15/1 12/1 9/1 6/5 3/17 | 32 min |
Altitude is set right next to water type, in the same section.
Water type and altitude are saved with the plan: a saved plan keeps the environment it was computed for, and finds it again when you reload it from your saved plans.
Plans saved before that setting existed carry no environment. They are read back as computed at sea level, in salt water, and certainly not with the environment currently on screen: without that, a wreck plan saved at sea and reopened while the application is set to an altitude lake would be recomputed wrong.
Get into the habit of checking the Environment line of the Plan page before running a calculation again, especially if you alternate between a lake and the sea. When the plan on screen was computed for another water type or another altitude than the current one, MV-Plan says so and the plan has to be computed again.
The MV-Plan salinity model is compared automatically against the Subsurface decompression engine, with the same density handed to both engines. Over the 13 salinity profiles of that comparison (brackish water, Red Sea, density 9500 g/l, Dead Sea, on open circuit as well as on a rebreather), the mean absolute gap on the runtime is 0.76 minute, MV-Plan being most often the more conservative of the two.