Differences in software or did I screw up

[benjammin17]

I am giving subsurface a test and in comparing it to multideco I noticed that there is a huge difference between the two for what should be the same dive plan. The stops and runtime are very different. Did I miss a setting or something along the way? Any thoughts on why there is that much difference?
I put a dive into both for 120ft using 30% and 100% and a time of 60 min. Both have last stop at 20ft, ZHL16-C at 40/70, fresh water, 30ft/m ascent, 60ft/m descent.

Multideco:
Decompression model: ZHL16-C + GF
DIVE PLAN
Surface interval = 5 day 0 hr 0 min.
Elevation = 0ft
Conservatism = GF 40/70
Dec to 120ft (2) Nitrox 30 60ft/min descent.
Level 120ft 58:00 (60) Nitrox 30 1.36 ppO2, 102ft ead
Asc to 50ft (62) Nitrox 30 -30ft/min ascent.
Stop at 50ft 1:40 (64) Nitrox 30 0.74 ppO2, 40ft ead
Stop at 40ft 6:00 (70) Nitrox 30 0.65 ppO2, 32ft ead
Stop at 30ft 12:00 (82) Nitrox 30 0.57 ppO2, 23ft ead
Stop at 20ft 25:00 (107) Oxygen 1.59 ppO2, 0ft ead
Surface (107) Oxygen -30ft/min ascent.
OTU's this dive: 149
CNS Total: 77.1%
Gas density: 5.5g/l
218.6 cu ft Nitrox 30
24.0 cu ft Oxygen
242.6 cu ft TOTAL

Subsurface:
Subsurface (4.9.6.0) dive plan (surface interval 1:00) created on 9/1/20
Runtime: 75min
depth duration runtime gas
➘ 120ft 60min 60min EAN30
➚ 30ft 3min 63min
- 30ft 2min 65min
➚ 20ft 0min 65min
- 20ft 9min 74min oxygen
➚ 0ft 1min 75min
CNS: 36%
OTU: 65
Deco model: Bühlmann ZHL-16C with GFLow = 40% and GFHigh = 70%
ATM pressure: 1,013mbar (0ft)
Gas consumption (based on SAC 0.70|0.60cuft/min):
127cuft/2,097psi of EAN30 (9cuft/142psi in planned ascent)
— Minimum gas (based on 3.0xSAC/+4min@120ft): 66cuft/

 

[atdotde]

Hmm, when I plan this on Subsurface, I get something pretty close to Multideco. Could you check that you have all the settings the same and that you are planning from an empty log (although existing dives should make deco longer, not shorter).
Maybe you could post a screenshot as well?

 

[benjammin17]

Thanks for the reply. I forgot to put in an additional planner point
Here is a screenshot and it is much closer now. It doesn't match the values you have though. I also noticed the MOD it calculates is off. It should be 121-124ft depending on the value of ft = 1atm you use.

 

[atdotde]

Unfortunately the resolution of the screen shots is so low that all text is unreadable. I am sure there is some subtle difference in some of the settings, the dive planning code has not been touched in a long time so same input should produce the same output.

Regarding MOD: You found a bug. When initialising the planner, the water type (fresh vs salt water) is not properly initialised. In the computation, it uses sea water but the user interface shows fresh water. I submitted a patch that should be included in the next release. You can work around this by manually changing the water type to sea water and then back to fresh water. Then it computes the bottom MOD to 122ft.

 

[benjammin17]

Link to a full res screenshot Here. Thanks for the info on the MOD.

 

[atdotde]

In the top of your runtime table it says "surface interval: 1:00". This indicates that there is another dive in your log that ends an hour before this planned dive starts. Dives within the previous 48h potentially affect the deco schedule due to nitrogen still in your body as repetitive dives. If you don't want to plan this dive as a repetitive dive, use the date chooser to place it further in the future.

 

[benjammin17]

I checked and there isn't another dive. The last dive recorded was 8/25. I closed out of the software and loaded it again and now it seems to be calculating correctly and doesn't show a surface interval. Thanks for your help. I will continue to test it out. The software is pretty cool.