I recently did a series of dives while on holiday. It was two mostly two divers per morning, with an hour to hour and a half surface interval between each. Everything went fine during the holiday, some of the profiles got a bit close to entering deco and I had to keep a close eye on the computer to try to avoid entering deco. I remember entering deco once during the week and immediately started to ascend, by the time I reached the safety stop at the end of the dive, the computer showed there were no deco penalties left to complete, just the 3 minute safety stop.
Imagine my suprise and horror when I downloaded the dives onto my pc using Sunnto Dive Manager, and the graphs revealed multiple deco dives and missed stops!!
Before anyone thinks I've misread the computer underwater, I can assure you I didn't! I've completed over 200 dives with this computer, probably half of those have been deco dives, so I'm very familiar with this computer and I can confidently make the following statements:
- I only noted the computer entering deco once during the week.
- On no dives did I go to the surface before the display showed that all stops (including safety stops) had been completed.
Going through the graphs, I have noted that only one dive shows the yellow triangular marker, which indicates the computer has entered deco. (does anyone know if this is generated by the computer or the diver manager software?) Despite having appeared to miss deco stops on 4 dives during the week, missed stops ranging from 3 to 6 mins, at no time did the computer lock out. I've searched through the Dive Manager index and finally found this explanation:
Dive Manager sometimes over or under estimates the decompression required for a dive. Use the Profile Graph window to study dives transferred into your logbook through a PC Interface. You'll notice that some dives show tissue saturation over 100% on the bar graph, meaning that decompression was required. But during the dive your dive computer remained in no-decompression mode the entire dive. Dive Manager sometimes reports that you surfaced too soon, when you know you did not. Here's why: Your dive computer breaks the dive into time equal segments called samples. It records dive information in its memory once during each sample. The information recorded is valid for one instant of time, not necessarily for the interval as a whole. The longer the interval, the less likely that this accurately represents the whole sample.
Dive Manager can only know the information for one instant during each sample. It must guess what happened during the remainder of the time. For example, your dive computer records the maximum depth reached during each sample. If you change depth, that number misrepresents the whole of the interval. Dive Manager can only assume you spent the entire interval at the recorded depth During a playback, Dive Manager reconstructs the dive computer's math model using information for each sample. It cannot download the exact status of the dive computer's decompression model because the dive computer does not keep that information in its memory. Dive Manager estimates what the calculations must have been based on information recorded for each sample. The closer it represents the interval as a whole, the closer Dive Manager reconstructs the dive computer's math model.
Correct estimates are more likely if your dive computer uses a short sampling rate. Set the sampling rate to a shorter duration, but take care to avoid running out of dive computer memory. Set the sampling rate in Transfer, Settings.
So that explains that then! I spent the week after my hols frightened to lift my toothbrush incase I got a bend!!!