RepOne measures rep data by first identifying what it believes is a 'rep'. A simplified explanation is it detects transitions from eccentric to concentric (down to up), and then concentric to eccentric (up to down). Generally this is a high reliability method of rep identification because systems with mass that are in motion have significant momentum and it's difficult to impact non-meaningful motion into that system.
A problem arises when the lift begins with the concentric phase, and therefore it begins with low momentum, and it becomes much easier to impart non-meaningful motion into the barbell. That's the case with the deadlift, and it can lead to RepOne Sensor 'beginning' a lift when it detects some non-meaningful movement due to bracing, rolling, etc. This is a slightly more common issue in RepOne Sensor than in previous OpenBarbell devices because of the significantly higher accuracy. This accuracy allows us to detect transitions in up to down and down to up very accurately, but it also means we can detect non-meaningful motion very accurately.
Here are a couple workarounds for you to use.
First, place your RepOne Sensor on the outside of the barbell. Placing it in the center when deadlifting allows it to measure bar bend, which can happen well before a lift initiates.
Another good tip is to either not roll the bar before your deadlift, or if you must, roll the bar *toward* your RepOne Sensor (or place the sensor slightly behind the bar). Since RepOne Sensor by default measures the concentric phase only, rolling the bar toward your device will be ignored by our rep calc algorithms.
In firmware version 0.6.X we're solving this by including a 'sensitivity' setting that you can modify from the RepOne Personal app and StrengthOS. This setting reduces how sensitive RepOne Sensor is to non-meaningful movement at the beginning of a 'concentric-first' lift. It continues to do so for the remainder of the set. The sensitivity setting will initially be set to ignore the duration of the first 10mm of motion, but this can be increased or decreased, or even turned off if set to zero.