This is my go-to heart rate monitor while training. It is small, light and comfortable. I usually get around six months out of the battery at a rough estimate. To change the battery, you have to unscrew four screws, which is a little bit painful. It has a memory so you can take it in the pool, although Garmin recommends you avoid using it regularly in the pool (it is fine for occasional use). For heavy swim use, see the HRM-Swim.
The main drawback of the HRM-Tri is that it is ANT+ only - no Bluetooth. Bluetooth heart rate monitors are larger, though.
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