OSTC4 Vs Shearwater Perdix 2 Real world diferences

[Divert]

Hello I recently got my OSTC4 and a new Perdix 2. I have done 3 Deco dives with them, I am wearing the OSTC in one wrist and the SW on the other.
Configured both with GF 30/80.

Here are my observations.
1.- The NDL was very similar on both computers.
2.- Both computers display the same depth within 10cm (pretty accurate I would say)
2.- The OSTC being Aluminum reacts much quicker to temperature changes on depth.
3.- The amount of deco is significantly different.

This is what is bothering me. At one point I had 1 minute @ 6m on the OSTC where the SW was asking 4 minutes @ 6m!!!

Trying to figure this out, I know that Shearwater uses the ZH-L16C coefficients and all the documentation I have for the OSTC only mention ZH-L16 without specifying A, B, or C, maybe that is the source. ¿Does the OSTC uses the C coefficients?

Additionally I have noticed my OSTC starting my dive at sea level reading 1040mb which puts my altitude around -100m when the actual pressure was 1013mb ¿Could this be the issue? I checked with Subsurface and all the adjusment I can do is only 20mb, I have not adjusted the computer's sensor.

@matthias please advice.

I tried uploading one of the dives in Subsurface with the differences mentioned. but the forum will not accept ssrf files.

[heinrichsweikamp]

Hi,

We also use ZH-L16C. What was the deco requirement on 3m and the total deco time for this dive? What gases did you use?

You can find the used a and b constants for N2 and He here: https://code.heinrichsweikamp.com/publi ... rc/decom.c

Regards,
Matthias

[Divert]

Thank you so much Matthias for the information you are awesome!

There was no obligation at 3m as I had both configured to do a last deco stop at 6m, at 46:20 I had 7min deco and 9 min deco respectively I was breathing Nx22 (I know, there was no O2 in my filling station and refilled with air)

Here you can download the dive in Subsurface format with both dive computers in it in case it is of interest to you.
https://nube.buceoluegoexisto.com/s/saLAs2KWndA9iEZ

I am starting to believe that the differences may be because of the different sample rates, being the HW at 2 seconds and the SW at 10. Next dive both will be set at 2 seconds.

[Ralph]

Your GF setting of 30/80 will also amplify the impact of minor differences in the computed tissue pressures on the stops, as a GF low of 30% keeps you long in having very little pressure differences / a slow off-gassing / more time for minor differences to stack up.

[Divert]

Interesting Ralph, I don't think that my GF Low had any role in this dive as there was only one stop (at 6m depth). There were not any other stops. Am I missing something? What do you suggest is a good pair of GF values?

[Thorsten]

Hi,
I had a look at the download link but could find only one divelog (the SW). Could you add the the OSTC dive in a separate file?

Cheers,
Thorsten

[Ralph]

Hi Divert,

> I don't think that my GF Low had any role in this dive as there was only one stop (at 6m depth)

Well, a short recap on how deco calculation works: The computer cyclically calculates an ascent with 10 m/min, and when this calculation ends up with the leading tissue supersaturation beeing Gf-high or less, it'll declare being in NDL (no deco obligation). Once this calculation ends up with a supersaturation in any tissue of > GF-high, the computer switches into deco mode, In deco mode, it does basically the same calculation, but will demand a stop once the leading tissue reaches a supersaturation of GF-low. This might e.g. happen at 2 meters of depth. This depth will be rounded down to the next multiple of 3 meters, revealing an indicated stop at 3 meters in this example. Now the 'last deco' setting kicks in and moves the stop further down to 6 meters. Given that GF-low is 30%, you will have your leading tissue at 2 meters at 30%, at 3 meters it may be at 20% (just a guess, just to show the tendency), and at 6 meters it may be at 5% only. This gives a very slow offgassing rate, with minimal pressure deltas that get subtracted from the calculated tissue pressures. And exactly here kicks in the fact that a dive computer is just a computer, and computers have limits in their numerical accuracy. Esspecially adding / subtracing very small numbers (tissue pressuer changes) to/from relatively much bigger numbers (total tissue pressure) is likely to loose digits from the small number, i.e. they get lost in the add/sub operation. And that's exactly what may keep you long on stops that are too deep to produce reasonably large pressure deltas for the offgassing calculation. In this situation, different computers using basically the same algorithm, may produce different results depending on how the calculations are actually implemented in computer code. By the way: that's why i implemented a floating point arithmetic with dynamic mantissa size in the 2's OSTC firmware for their PIC micoprocessors a few years ago.

My GF recommendation? It depends on the type of dive. If you belive in Bühlmann and think your body is in line with the statistics of all the test divers that worked for Bühlmann, you could switch GF off or set it to 100/100. Or put some safety margin in, say 80/80. Changing the slope, i.e. setting GF-low < GF-high, has two origins: people trying to imitate the VPM deco schedules with the Bühlmann model (indicator: very low GF-lows), and diving on helium / trimix, as there is a feeling that the Bühlmann b-coefficients for He do not fit as good as those for nitrogen do (Bühlmann's test dives were all on air, the a & b coefficents were calibrated on those test dives, but the helium coefficents are not based on test dives but were computed from the nitrogen ones by applying factors for the differences in the physical properties of the two gasses). For trimix dives, i am opting towards a 50/80 setting.

Hope that helped,
Ralph

[Divert]

Hi,
I had a look at the download link but could find only one divelog (the SW). Could you add the the OSTC dive in a separate file?

Cheers,
Thorsten

They are both there, I checked before sending the file, and just re checked now. When you have the dive open, just use your left or right arrow to switch between HW and SW. profiles and data.

Humberto.

[Divert]

Hi Divert,
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Hope that helped,
Ralph

Thank you so much for such an elaborate answer, yes this satisfies my question. Thanks again.

[heinrichsweikamp]

There was no obligation at 3m as I had both configured to do a last deco stop at 6m, at 46:20 I had 7min deco and 9 min deco respectively I was breathing Nx22 (I know, there was no O2 in my filling station and refilled with air)

I'd make the last stop at 3m if you are not using oxygen, you get better desaturation because the pressure differences are higher. The "last" deco stop only starts at 0m anyway...

Greetings,
Matthias

[Divert]

Thanks Matthias. The reason I had configured a last stop at 6m is because we get some big waves here , and sometimes it would really be difficult to stop at 3m and try to maintain a good steady depth, but I guess you are right.

Greetings.
Humberto.