halocline
Contributor
I do realize that the dive was multilevel, but the fact that I'm not trained in multilevel diving worried me. What worried me most however, was the depth.
The answer to the question "were you at risk for DCS?" depends on how aggressive your dive profile was with regards to NDL. Single "Trust me" dives are not dangerous with regards to DCS if the profile is very conservative and nowhere near NDL. In fact, I would happily do a single dive without a computer if I was diving a profile that I knew from experience (or planning) did not push NDLs at all. So, it's more like a "trust but verify" dive, to borrow a phrase from the cold war. There are plenty of profiles with a depth of 26.4 meters and 40 minutes bottom time that would fit that bill, but there are also profiles that would push or exceed NDL, so truthfully nobody can answer your question with the given information. Planning repetitive dives without accurate dive data is likely much riskier, but again, it depends on the profile.
But, certainly the best thing you could do at this point would be to learn something about basic decompression theory in recreational diving and maybe play around with some dive planning software. There's a very good chapter in the PADI encyclopedia of recreational diving on the basic deco models. Once you have an idea of how the models work, you can dive in a much more informed and confident manner.
Honestly, using your own dive computer but not having an idea of how aggressive your dive profile is based on basic deco theory is only marginally safer (in my opinion, anyway) than doing the same on someone else's computer, at least for a single dive. The computer only tracks your time, depth, and ascent rate. If you dive near NDLs, don't control your ascent rates well, etc...you're at greater risk for DCS than someone who dives conservatively, (all other factors equal) computer or not. The only way a computer adds to your dive safety is if it changes your dive behavior, based on the information it gives you.