Versa Pro and Altitude

[carljess]

I wanted to understand how my compter, a Versa Pro, handles altitude dives. So I searched through the manual. What I found was fairly limited descriptions. I am hoping that someone has more information.

What the manual describes is this:

* The computer can handle altitudes from sea level to 14,000 ft.

* The computer compensates for altitude only if manually activated at that altitude.

* The depth reading are calibrated to read feet of fresh water when manually activated at altitudes above 2000 ft.

* The altitude algorithm is based on the NOAA tables.

Based on these passages I'm assuming that the computer works by adjusting the dive depth then using that adjusted depth as the basis for the nitrogen loading calculation -- which is my understanding of the NOAA altitude tables.

Does anyone know for sure how Oceanic computers handle this? Is there a better description on the Ocieanic website, I looked but could not find this.

Thanks,

Carl

 

[DandyDon]

What did you want to know, how to build one?

It's my understand that the PDC reads the Atmospheric pressure on activation - which is why you don't want to activate it before traveling to altitude, nor on a plane on the way to a landing. It reads this with a + 1,000 foot variance, so it will not be exact, but certainly close enough for bottom line needs.

I have seen times when our 3 Oceanic PDCs were activated at Santa Rosa NM around 4800 ft, and gave 3 different dive plans, in that one must have been reading 3,800 rounded to 4,000, one to 5,000, and one to 6,000 - but the approx 4% variance in each step yielded only a couple minutes difference in planned or experienced bottom time.

 

[carljess]

This was really aimed at understanding the mechanics of how the computer handles the altitude correction.

I've taken the time to understand how it computes the nitrogen loading for a sea level dive, I've checked what half time compartments are inlcuded in the calculation and tried to reproduce those calculations. I've compared the computers NDL's for a square profile with the Navy tables and with PADI's RDP. So I think I have a good idea of how the computer handles a sea level dive.

I do use the computer at altitude. I live in NM the majority of dives I've made have now been in the Blue Hole at Santa Rosa along with Seabase in SLC and Homestead Crater in Midway UT. All are altitude dives.

I don't have the same understanding of how it handles the mechanics of an altitude dive and I do have some resistance to simply accepting the computers result with no clear understanding of how that result was arrived at.

Which was the reason for my question. When I tried to look through the manual for a description of the calculation procedure for altitude what I found was fairly sketchy. So I decided to ask here, among a group that I figured would have already looked at this.

Carl

 

[DandyDon]

Ok, cool. One idea that comes to mind is to compare PDC dive planning to Chart planning...?

 

[carljess]

I've tried that. The catch is that I can't or at least I don't know how to get the computer to do altitude dive planning for any altitude other than my current altitude. And with one data point (my home altitude of 5300 ft) I still don't fully understand how it does the calculation.

I think it works at I described in the first post here. Manual activation causes the computer to read the ambient air pressure. Which it uses to calculate local altitude. If at altitude it then sets itself to read FFW rather than FSW. I believe it uses a corrected depth (Effective Depth based on an altitude correction) to modify the NDL.

The one check I can do, at my home altitude works out fairly close using the NOAA altitude correction tables and the square proflile seal level NDL's in the manual. But again that is one data point.