OSTC 2c strange behaviour when using a travel gas...

[wood634]

Hello,

I was recently on a trip to Malin Head in Ireland and we were diving on the HMS Audacious at 65m. I had brought a mix with me and it was a bit too lean to breathe at the surface so I configured my computer to use a travel gas as follows -

G1 - 15/42, change at 30m, Active gas, First gas - No.
G2 - Ean70, Change at 11m, Active gas, First Gas - No.
G3 - Ean30, Change at 42m, Active gas, First Gas - Yes.

So I jumped in using Ean30, descended to 30m where I gas switched to 15/42, dived as normal for the entire bottom time, and then ascended. Everything was normal until I reached 30m on the ascent, at which point I suspect the OSTC threw a complete bender giving me multiple minutes for every 1m of ascent. I resorting to using those pesky tables on my slate that we use to plan our dives - and was grateful for having them along.

So my question here is does the OSTC handle a travel gas gracefully, or did I simply program my OSTC incorrectly.

I suspect in hind sight I probably should have left the 15/42 as the main active gas and simply travelled on the Ean30 and manually just switched gases at 30m. The difference in deco penalty is marginal and it keeps it simple. Incidentally that is exactly what I did for the rest of the holiday...

Upon surfacing the OSTC was not bent and in fact whilst completing my table stop penalties the OSTC did in fact clear (I had still been doing the gas switches on it during my ascent). However when entering the menu structure to review the logbook the computer white screened and was only recoverable by using the trusty magnetic reset. Thankfully we lost the following days diving due to weather, so the loss of existing deco state was not an issue.

Any opinions on the things I saw?

Its a new OSTC 2C running 2.7 software.

After the reset it behaved faultlessly - making me think it was definitely my actions that screwed it up...

[Bardass]

I think your probleme is due to Gaz3 : when first gaz, you do no set depth change. Set at 0 only.
but you need to use your Ean30 for deco, so you need to set depth change to activate this gaz

so, i think you should set 4 gaz :

G1 : Ean30, first gaz, active, depth change 0
G2 : Tx15/42, change at 30m, active
G3 : Ean30, change at 42m, active
G4 : Ean70, change at 11m, active

otherwise, as you wrote, you set Tx15/42 at first gaz and you use Ean30 as travel descending for 1 or 2 minutes.

[wood634]

Hi Bardass,

I did consider putting the gas in twice, but in all honesty I think the change at value is an ASCENT depth, not a descent depth. As such I dont think the OSTC is really set up to do travel gases - unless you are manually switching the gases during descent without being prompted.

I regularly dive with deco gases and my back gas is NEVER the optimum gas to be breathing during descent - yet the OSTC does not prompt to gas change during descent.

I suspect if you dive with a change at depth configured on your bottom gas as per your example it simply expects you to change back onto it as you pass the change at depth (that you set to use on the descent) on your ASCENT - its not smart enough to tell the travel gas from the back gas.

As we both observe this is not a big deal, more of a nice to know.

Lessons were learned - the advantage of carrying a slate with backup run times on it, and not exceeding your planned run times at depth (so as to negate you backup slate plan).

Everyone got out of the water safe, and thats what counts.

A section on using travel gas in the manual may not go amiss though?

[jamesboyle]

Very strange.. Last week, on the same wreck (!!) I had a similar problem. I've passed the information to Matthias already, but might be useful for others so I'll post here also.

Gases descent and beginning of dive on ali80, 17/46 (set up as G1 change depth 0m), then switching to backgas 17/40 (set up as G2, change depth 60m). Other gases irrelevant, but included EAN50 and 100% O2). Gradient factors 15/85.

I swiched from my ali80 (G1) to backgas (G2) at 57m. OSTC decided to give me a ceiling of 55m for 10 mins This is at RT 13 mins. I was close to getting on my slate and getting out of there as I knew that the new decoplan was a bit crazy, but my slate and gas plan allowed for another 10 mins anyways, so... New decoplan following the 10 mins at 55m specified a stop at 27m, which was closer to what it should have been, so I decided to continue dive, and keep checking if computer deviated much from planned profile on my slate. After 10 mins, decoplan on computer did correspond roughly with what I expected so I used it to get to 6m and then padded out the remainder to coincide with my slate.

I think the theory is that the change depth associated with G2 (backgas) confused the OSTC deco algorithm as it tried to come up with some kind of profile using trimix as a decompression gas (assumption made by computer is that a change depth means it is a decompression gas). I'm not sure about how the software differentiates between bottom and decompression phases of the dive, but it would appear that the computer managed to confuse the two in this case?

This is just the impression I got from my converstation with Matthias - it could very well be innaccurate, please correct if it is!

J