Driving home to altitude

[rsingler]

Hmmm. From the Perdix Nitrox Mode manual, I see this:
"GF99
The gradient factor as a percentage (i.e. super-saturation
percent gradient).
0% means the leading tissue super-saturation is equal to
ambient pressure. Displays “On Gas” when tissue tension is
less than the inspired inert gas pressure.
100% means the leading tissue super-saturation is equal to
the original M-Value limit in the Bühlmann ZHL-16C model."

In the pic above, the inspired inert gas pressure is the bold black line. In Air, this line is 79% of the way from the left to the ambient pressure.
The ambient pressure is the Green/Yellow boundary.

So, since the inert gas pressure in the leading compartment is still less than ambient pressure, GF99 is displayed as zero.
At least, that's how I'd interpret it until @Shearwater chimes in...

I think you have a great testbed there! Perhaps on your next dive, if you can pump it down after a shorter surface interval, we might see a compartment pop above ambient.
Or another thing to try would be to compare pics of the tissue graph before and after pumping it down. Hopefully the green/yellow boundary will shift, or the compartment lines will shift, or both.
All this is easy for me to say, with you doing the work, lol!
Hey, thanks for going to all this effort!

 

[ATJ]

I just stuck the Perdix in again showing the tissues graph. I sucked out -30kPa and the graph didn't change at all.

 

[rsingler]

I'm out of ideas.
Thank you again for working this issue!
Can't wait to hear from @Shearwater !

 

[rsingler]

@Shearwater , as asked in another thread, does the tissue bar graph "offgas" over time as compared with the barometric pressure noted at the start of the dive, or does the tissue graph offgas against repeatedly re-measured barometric pressure as the surface interval progresses?

 

[broncobowsher]

OK, pressure pot test idea. Set the end dive as late as possible in the Shearwater. I know I have mine bumped out to 5 minutes and I think it will go 10 minutes, maybe more. Put the SW in and add a little pressure, start a dive. Surface and start a little vacuum. Does that pull the GF99 and/or tissue chart into an off-gassing situation while still in a dive? As opposed to surface mode.

 

[rsingler]

That is a GREAT idea!
I'll try that as soon as I get home next week. Need to get a small suction pump.

Does anyone know if this is rated for negative pressure? I'd hate to pop the window off the rim with too much suck. There's no negative scale on that gauge.

If it's not suitable, I'll try to glue up a connection to the clear water filter pressure pot in post #5 above. That's o-ring sealed, so it should do okay.

 

[Mike1967]

@ATJ don't forget to change your battery

 

[broncobowsher]

Negative pressure is no more than 14.7 PSI. That pressure pot look like it will take it. I can't tell what the scale goes to, hopfully at least 14.7 PSI (or 33' depth if that is what the scale is in). Besides you are not going to be putting it in a full vacuum. Just a little vacuum for altitude. The only thing I see is the seals are set up for one direction only? They will just leak.

I have seen my slower tissues below the tissue loading line while on the surface before. Spend a lot of time on very high O2 levels will do it. I have done it in a swimming pool keeping the PPO2 up around .7 for a couple hours (rebreather training) and coming in after a couple hours hanging on deco. It is strange to think that you just surfaced from a multi hour long decompression dive only to be on-gassing at the surface because the faster tissues are undersaturated.
That is just the fast to medium tissues, the medium to slow were leading the deco, and the really slow tissues never loaded enough.

 

[Shearwater]

We have to be very clear that neither our computers nor the Buhlmann GF are designed to help you manage the risk of DCS related to altitude increments incurred after the diver has surfaced and the dive is ended. Before the dive has begun, the difference between mBar Now and mBar Surface is that mBar Surface will establish surface pressure as the second lowest barometric pressure recorded in the last 10 minutes, whereas mBar now just reports current barometric pressure. The surface pressure is set when the computer is turned on. If the Altitude setting is set to SeaLvl, then surface pressure is always 1013 millibars.

@Shearwater , as asked in another thread, does the tissue bar graph "offgas" over time as compared with the barometric pressure noted at the start of the dive, or does the tissue graph offgas against repeatedly re-measured barometric pressure as the surface interval progresses?

If you didn't turn off your computer after you finished the dive, it is measured against the mBar Surf established before the dive. Otherwise a new mBar Surf will be established once you turn on the computer again.

 

[rsingler]

If you didn't turn off your computer after you finished the dive, it is measured against the mBar Surf established before the dive. Otherwise a new mBar Surf will be established once you turn on the computer again.

Soooo....
If I finish my dive and turn off my computer, quickly drive up 2000 feet and turn on my computer, will my tissue graph look different than it would have if I turned it off and turned it back on after a similar interval, but stayed at sea level? (I'm trying to not box you in to a place you don't want to go. I'm just trying to understand my Perdix)