I'm after a stopwatch for interval training (running).
What features should I look for when buying one?
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For me, the gold standard is whether I can use the device to time the Tabata protocol. That protocol is 8x(20s exercise, 10s rest). The most common exercise is sprinting. This is a difficult protocol to time. Your device must:
The other difficulty with timing devices is accidental activation of the device. This happens frequently with watches if you are doing HIIT with barbell work or exercises with wrist hyperextension such as pushups or handstand pushups. Then your left hand frequently presses against the right side of the watch where the timing control buttons usually are. I have never found a watch that conveniently times my crossfit wods, and smartphones are too large and fragile for me to consider wearing one while in the middle of a high intensity workout.
(Q & an A, moved to answer for space.)
Are you specifically seeking a "stopwatch", a single-purpose tool, a watch with interval capabilities, or a smartphone app (my recommendation)? Are you looking to track intervals, in which case anything with lap counting will do, or be notified at interval changes? Do you have simple, or complex, interval plans?
I current using Impetus for Android. It can set up complex interval programs easily through its UI, and if you can generate your own XML, you can improve over its UI. I currently get my XML from my stealth startup's DB based on a variety of training criteria.
The MotoACTV can do intervals, although not with the complexity of Impetus. It's a great dedicated device, though, modulo some dumb decisions on Moto's part.
I love my Timex Data Link USB, because it is a 200 lap stop watch. I can lap and lap and lap as needed, review it save it, download it to my computer as an Excel spreadsheet.
Accuracy is as fast as you can push the button. Alas this watch is no longer manufactured.