A curl is easy to recognize and surprisingly easy to change while doing it. Stance, arm position, and momentum can all drift as a set becomes demanding. Before worrying about the dumbbell number, choose a version you can repeat comfortably. A stable starting point makes the next session easier to compare with this one.
Set your feet and arrange enough room around you. Select resistance that lets you control the movement without throwing your torso backward. Keep the chosen range comfortable rather than forcing a position to resemble someone else's demonstration. If you are unsure about the exercise, a qualified coach can help you establish an approach appropriate for your experience.
Notice what happens near the end of the set. If repetitions begin to look different, record that context rather than counting them as identical work. Changing the load is only one possible adjustment; improving control or repeating the setup more reliably may be the more useful task. Pain is a reason to stop, not a cue to swing harder.
Name the exact variation in your log and note the equipment used. A cable curl, a machine curl, and a dumbbell curl are not interchangeable numerical tests. Give the selected version enough consistent practice to learn from it. The aim is purposeful arm training that fits your routine, not a new variation every time the rack is busy.