My Cobra, NDLs and Ceilings

Hi everyone,
I just downloaded our vacation dives from the Cobras to the PC Dive Manager. When using the software to look through the dive profiles, I noticed something that bothered me.
At about 44 feet and 46 minutes of the second dive of the series (first dive day of the vaca) I see that my NDL expired and it showed a ceiling of 10 feet with an ascent time of 2-4 minutes...which then turned into my usual 3 minute safety stop since the dive was ending anyway. The ascent time was not blinking and there were no arrows telling me that I had to ascend, I think this was a mandatory safety stop?
Since I am constantly monitoring my computer throughout the dive, it really freaks me out that I didn't notice something so important. I watch my NDLs adamantly, (especially this trip since we were going to be diving multiple days), and I just KNOW that I would've noticed a message that my computer displayed for 2 minutes!
Because of my unwillingness to believe that I can do anything wrong, I decided to do a little more research. According to the Suunto manual, page 2 of the computer's logbook will show the 'ASC TIME' label if the dive was a decompression stop dive...and when I page through my Cobra, I do not see that.

What's up with this? Anyone with similar experiences or explanations??

SDM on your PC will often show slightly different things than you saw during the dive. This is because during the dive, the computer is constantly monitoring your current depth and updating its calculations several times a second. However, the PC software only get your depth measurements once every 20 seconds. So it has to assume what you were doing in the 19 seconds between measurements. While it may not seem like a very long time, this 20 seconds of uncertainty, combined with rounding errors in how the profile is recorded, can produce NDLs that are several minutes different than what you saw during the dive. If you are near your NDL, it can be the difference between getting a Deco warning or not.

-Mark

My SUUNTO computer sometimes displays readings that are conderably different than what shows up on my PC, when I download my dives. The most notable example I have was a dive that I made with several dive buddies. (Naturally we paired off for "real buddies", but there were seven pairs of divers.) We'd found some nurse sharks on the dive which we found interesting enough that we all push our computers toward their NDL limits... or in the case of the SUUNTO computers, well past. When I eventually decided to begin my deco stop, my computer had me ready for a 20 minute decompression stop. Two other divers in our group had SUUNTO computers, and had been at very similar dive profiles. We floated a safety sausage and did out hang until all 3 computers said we were safe to ascend. (All the other divers had been back on the boat for 10 - 15 minutes.) Although my computer was the first of our 3 SUUNTO computers to "clear" me, I stayed down another 2 minutes until we were all clear. (3 divers comparing the same brand of computer... not much chance we weren't seeing them right.) ...and yet, my dive log shows that I got out of the water with 9 minutes left to decompress.... I find that a bit disturbing.

Off by a tad here or there, not a big deal. Off by 9 minutes, makes me concerned. True, I shouldn't have been doing a decompression profile while only carrying an AOW certificate... but my point is that dive computers do not always match the PC software designed for the download... You'll have to decide for yourself how much you want to trust your computer. (...and no, I don't think the other divers' profiles were any safer, just because their computers didn't say they needed to hang for as long. I use my computer to help me decide what risk is associated with some particular profile... and clearly, a person should limit how much they trust a computer.)