Decompression Calculations
Posted by: robmason ()
Date: April 30, 2019 02:46PM

Hi All - I've just been using my OSTC2 for a couple of months and I'm preparing for my first serious deco dive. I'm not confident with the deco calculations given by the OSTC2.

I have planned a 42m dive for 20 minutes on Air only (just for comparative purposes). I use ZHL16+GF 30/85. Ascent speed is 10m/' - last deco is 6m - gas change +1'

This calclulates a TTS of 55mins, with the last decompression stop of 35mins at 6m. The *equivalent* calculation using MultiDeco gives the last decompression stop at 6m of 22mins.

This seems a huge difference - the OSTC is giving me an additional 13 minutes. Is this correct???

Re: Decompression Calculations
Posted by: edhowarth ()
Date: April 30, 2019 05:25PM

When you say TTS of 55 minutes, I assume you mean a total run time of 55 mins. I've just done a plan on MultiDeco and got a run time of 51 mins so pretty close.

Re: Decompression Calculations
Posted by: Ralph ()
Date: April 30, 2019 07:14PM

There are two factors the OSTC always figures in, whereas in Multideco you have to explicitly set them:

1) Residual saturation from previous dive(s) - propably not the dominating factor in your case.

2) Surface pressure. Default in Multideco is to calculate for sea level. The OSTC takes the ambient pressure that is shown in the surface mode screen. What pressure do you have / had when yielding the above results? When i set my Multideco to 750 m height, it calculates an even longer TTS than your OSTC did, so i assume you may be sitting somewhere at 500 m above sea level and/or in an area with low(er) air pressure due to weather.

As air is a pretty ineffective deco gas and 6 meters is quite deep for final decompression on air (so ineffective in it's own, too), these two factors amplify the effect of the surface pressure on the duration of the last stop.

Last but not least, the OSTC puts in a final ascent from the last stop to the surface with 1 meter / minute. So a last stop at 6 m will add 6 minutes to the TTS. This is done because the diver should do it because it's healthy to do and also to incorporate a little safety margin in the gas needs calculation.

Re: Decompression Calculations
Posted by: robmason ()
Date: May 07, 2019 09:48AM

Thanks for your replies guys. This has helped clarify my understanding of the OSTC calcs.

I'm not suggesting the additional conservatism is a bad thing (it's not). My concern was around the whether the calculations should be trusted.

My sample calculations wer:

~ First dive (no residual saturation)
~ all at seal level (1013Mb?)
~ Last deco is 6m - this is a mandatory safety requirement for my diving (typically in the North Sea or Atlantic with wave action/swell).

The 1 meter/minute is interesting and I wasn't aware of that. It seesm that teh OSTC correctly favours safety over minimising deco time.

I think it is corretc to assume that OSTC2 users are using it to plan run times?


Re: Decompression Calculations
Posted by: Ralph ()
Date: May 08, 2019 06:32PM


as one of the programmers i would like others to answer, but personally i trust in the calculations the OSTC does and rate it as a sufficient planning tool in it's own. But the real power comes when planning with another tool and then cross-check by running the planning on the OSTC, too. This will double-check not only the planning results by themselves, but also verify if all deco and gas settings have been properly set.


Edited 1 time(s). Last edit at 05/08/2019 06:33PM by Ralph.

Re: Decompression Calculations
Posted by: sharpw ()
Date: May 12, 2019 07:45PM

Way too fiddly for me to use the OSTC to plan run times... but I do trust it.

I normally run the plans on Subsurface as an initial check; then on PastoDeco for a detailed plan and What-If iterations. Everything then gets written on a slate. If I use the OSTC for planning, it is to make sure there will not be any massive deviations between the prepared plan and the actual dive, but I cannot say it is my main planning tool.

I can say that, from experience, the OSTC provides a reliable match to the plan - if you are honest with yourself about ascent/descent rates and you stick to your runtimes.

Edited 2 time(s). Last edit at 05/12/2019 07:46PM by sharpw.

Re: Decompression Calculations
Posted by: robmason ()
Date: June 11, 2019 05:10PM

Thanks for the replies guys - firm advice. I'll stick with cross-checking between calculators.

Re: Decompression Calculations
Posted by: jb2cool ()
Date: August 12, 2019 03:34PM

I was seeing similar behaviour and that led me to find this post, using the advice I found in this thread (,20642) I found that my conservatism being set on 0 was not what I wanted, I actually wanted this set to conservatism = GF. This makes the new OSTS Plus with the sport firmware behave much more like the older OSTC Mk.2 that I have.

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