Short desat times

[JochenH]

Hi all,

I am new to this forum. I hope you can answer a question that came up when I started to use the OSTC.

I am a relatively new user of the OSTC MK II and love it. I used to plan and dive my trimix dives on tables. Now that I started carrying the OSTC with me I noted one strange thing : it gives me extremely short desat and no fly times after dives with a significant exposure to inert gases.

For Example a dive on Monday :
45ms for 35mins on TMX 21/35, almost rectangular profile, deco gases NX50@21ms and O2 @6ms. Total dive time 95mins. L16-GF with 20/75 ( conservative setting, I´ve got a PFO ). Desat time directly after the dive : 4hs plus something, no-fly something like 2,5hs.Same profile again yesterday : desat time 9hs plus something, no fly time about 5hs ( according to the 60% setting in the CFs ).

I would have expected a desat time of 16 to 18hs, maybe even 20. As I never had a desat calculation for this kind of diving before : is this due to the Helium ( goes fast in, goes fast out ) ? Hard to believe, because the TMX21/35 still contains 44% N2, and the exposure was quite heavy.


best regards

Jochen

OSTC MKII 8192
2.90

[Bardass]

i simulate your dive
desat time and no fly are different from yours

[JochenH]

Thanks Bardass, your table looks pretty much like mine ( there are small differences like descent/ascent speeds and so on ) . I am sure this dive gave mit 4h30 desat time. I didn´t play around with the deco settings. Does anybody have an idea ?

Cheers
Jochen

TAUCHPLAN
Oberflächenpause = 5 Tag(e) 0 hr 0 min.
Höhe über N. N. = 0m
Konservatismus = GF 20/75

Abst. 45m (3) Triox 21/35 15m/min Abstieg
Tiefe 45m 32:00 (35) Triox 21/35 1,12 ppO2, 20m ead, 26m end
Aufst. 27m (36) Triox 21/35 -10m/min Aufstieg
Stop 27m 0:12 (37) Triox 21/35 0,76 ppO2, 10m ead, 14m end
Stop 24m 1:00 (38) Triox 21/35 0,70 ppO2, 9m ead, 12m end
Stop 21m 3:00 (41) Nitrox 50 1,52 ppO2, 9m ead
Stop 18m 1:00 (42) Nitrox 50 1,37 ppO2, 8m ead
Stop 15m 1:00 (43) Nitrox 50 1,23 ppO2, 6m ead
Stop 12m 4:00 (47) Nitrox 50 1,08 ppO2, 4m ead
Stop 9m 5:00 (52) Nitrox 50 0,94 ppO2, 2m ead
Stop 6m 21:00 (73) Oxygen 1,58 ppO2, 0m ead
Oberfl. (79) Oxygen -1m/min Aufstieg

[heinrichsweikamp]

Hi,

can you download the dive with the "OSTC tools" and post the .dive file here?

Regards,
Matthias

[JochenH]

Hello Matthias,

nice to meet you. Before you look at the file : I just got aware that I messed a little bit when selecting the bottom gas. I thing there is for a very short time an NX32 active that I didn´t use. I don´t think that has something to do with the issue.

cheers

Jochen

[kaheksa]

You did about 12 minutes of extra decompression in the water. I think that may be your answer

[JochenH]

I did a dive with a typical sport profile ( bounce dive; 4mins BT ) to 43ms yesterday on air and my OSTC gave me the exspected 19hs of desat time.
I learned from your comments that oxygen rich deco gases not only shorten deco times but also reduce desat times to a few hours. Doesn´t sound like a surprise but I didn´t know. Thanks everybody for sharing your thoughts.
Jochen

[heinrichsweikamp]

Hi,

The Desat. routine we're using hasn't been touched for quite some time now. I'm sure it's working fine. There will always be differences between different computer models. Your profile is very conservative (GF_hi 75%) and you did a lot of extra deco on Oxygen at 6m.

[img]file.php?2,file=589,filename=profile.PNG[/img]

4:55hours is well within the expected range.

Regards,
Matthias