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Dumb Question - resetting MoD

Posted: Wednesday 9. June 2021, 16:11
by robmason
Hi All - apologies for a dumb question. I did my first decompression diving for the first time since lockdown at Scapa Flow last week. I was confused by the reasoning behind having to reset the MoD when I set my O2 mix? This caught me out on one dive where my Nitrox mix was set at 26%, but the MoD was still set to my prior dive mix of 30% - hence I got depth alarms.

Why does the MoD need to be manually reset??


Re: Dumb Question - resetting MoD

Posted: Wednesday 9. June 2021, 18:45
by Ralph

"Reset to MOD" is just a convenience function to adjust the change depth to the MOD (max operating depth), which is computed by the O2 in the mix and the type of gas (work vs. deco). The key parameter is the change depth, and this is not automatically slaved to a calculated MOD as change depths (and hence when to go off or on a specific gas) depends on the collection of gases carried during the dive, personal choice of N2 narcosis limits, etc. So for example one wouldn't (well, shouldn't) set the change depth of air to 57 meters just because this will give an accepted ppO2 of 1.4 bar.

Anyhow, a "depth alarm" is caused by a set overall depth limit. Change depth settings will have an effect on the calculated deco stops and will trigger the gas change advices. If the mix is 26% O2 but the change depth is set for a richer gas (30%), this works to the save side. Or did you get a blinking depth along with a gas change advice? Then the selected gas at the current depth exceeded your set ppO2 limit and the OSTC tells you you have 2 options to resolve the situation: change to another gas or change your depth. The idications are made to be very intuitive in showing what's the problem and what can be done against it.


Re: Dumb Question - resetting MoD

Posted: Thursday 10. June 2021, 09:41
by robmason
Thanks Ralph. I didn't get a change gas option - my only other gas was my 60% deco, which would only kick in at 15m. I can't recall whether it was blinking (I think not) - I recall it was just red text. My ppO2 is 1.4 for work gas and 1.6 for deco.

Just so that I understand, one does not have to reset the MoD if the gas is set correctly? e.g. MoD set at 34m and work gas is Nx26. This should be the cause of the red text at 40m?

I'll recheck my settings (depth limit) - I suspect this is likely to be user error due to not undertaking any decompression diving since before lockdown.

Re: Dumb Question - resetting MoD

Posted: Thursday 10. June 2021, 13:08
by Ralph
You don't set the MOD of a gas! You set the CHANGE DEPTH (with the +/- menu items)!!

The CHANGE DEPTH controls which gases are deployed during the deco ascent calculation.

The "reset to MOD" is only a one-step-adjustment aid to set the change depth.

Best to run a demo dive in the simulator mode and see how the OSTC behaves & reacts to your settings :-)