forum.heinrichsweikamp.com

Hi,

as we can read in howOStech, page 35, an isobaric counter diffusion occurs when the N2 content in a tissue increase while the He content decrease.

I was wondering until what % increase of N2, with respect to % decrease in He, considers the howOStech to be acceptable and from which the IBCD warning appears on the display.

BR

Hi mobu,

the OSTC Tech firmware doesn't use the Burton approach. Instead, it creates an IBCD warning if following conditions are met:

- N2 goes into a tissue while He leaves it OR vice versa

- the net tissue pressure is increasing,

- this all happens in the leading tissue, i.e. the tissue being closest to it's Bühlman limit.

There is a very very tiny minimum threshold on the net pressure increase for triggering the warning just for purpose of suppressing toggeling effects (same is done for the tissue graphics in the custom view).

In most cases an IBCD stops by itself after a short time, as the changing tissue pressure in turn changes the N2/He in/out rates, bringing things back into stability. But that is not predictable, or more accurately said: the OSTC doesn't try to compute a forecast on this. Instead, the current GF is shown so the diver can observe if IBCD caused rising of the GF slowes down over time or if the GF is going straight up through the ceiling aka 100%.

By the way: IBCD is also checked for in the deco calculator and when it detects an IBCD condition anywhere during the dive, it gives a cue on the summery page.

best regards,
Ralph

Hi Ralph,

yes, Steve Burton says that since the solubility of N2 is 4.5 times more soluble tan He, N2 percentage should never increase more tan 1/5 the reduction of the He percentage to keep the aiming point for the disolved gases the same.

This is a simple rule whe can take in count whe we plan our deco gases.

Anyway, I was wondering what we should do if during a dive we see the IBCD warning appears on the display.

BR

Hi,

yeah, Burton gives a rule of thump on how IBCD shouldn't occur. The OSTC calculates IBCD in real-life taking into account actual tissue loading conditions, all within the Bühlman framework.

When you get an IBCD warning, you should look at the GF value. If it's rate of rising is slowing down and doesn't seem to come near your desired GF-high limit - your deco is just suboptimal. If the rising of the GF does not stop and steadily approches the 100%, you should instantly switch back to your previous gas. What to do next is hard to say because it heavily depends on the circumstances. You probably should extend your current stop on the old gas and even do the next stop on it in an attempt to unload the tissues some more such that there will be no out-of-control IBCD when you finally switch again.

BR
Ralph