Yes, that was the big problem. I could never be entirely sure I wasn't compensating for above or below average fin performance in order to maintain my usual speed. The only way of judging this was how tired I felt after the dives. I couldn't tell any difference, but then the difference after only 30m would have been slight anyway. Any thoughts how I could control effort input? One way might be to cover all exposed skin so that I couldn't feel how fast I was going and close my eyes or dive with my back to the line. I never count kicks but that might also be a good idea when doing these tests. Increasing the depth would probably help - it would be more difficult to compensate on a 50m dive without feeling the effects somewhat. However I'd like to include more fins and 20 back-to-back 50m bifin dives would make me very fatigued, which would skew the results. I'm not even sure I could complete a test like that. It would also be asking for a DCS hit!
Another way might be to do a monofin dive after each bifin dive, to 'reset' my perception of speed.
It was interesting that the two shortest, stiffest fins in the test were also the slowest. I really got the impression I was going quite fast with the C4 30s, but the graphs showed otherwise. It kind of demonstrates that the 'feeling' of a fin can be very misleading with regards to its actual performance.
This is how the graph looks when you take away those two C4s (the 80s are still in the mix and their acceleration is just as good as the other fins'). Absolutely nothing in it:
