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Looks like N-m if using Ant+. https://www.powertap.com/...tes-to-offset-values
If I'm reading that correctly each increment is 1 Nm (for both ANT+ and BLE, you just have a decimal point in the reporting value when using BLE). That's extremely blunt at ~10W for each step at 95rpm. Unless your powermeter is quite unstable, better not touch the zero offset at all if that is correct!
Just did 10 zero tests with my PT G3 with the dual ANT+ and BLE pod. On a Garmin I got 7 times the value -1 and 3 times -2. With the powertap app I got 7 times -0.12 Nm and 3 times -0.22 Nm displayed. Thus, I guess the ANT+ increment is 0.1 Nm i.e. -1 means -0.1 Nm.
With over 10 years experience with PT hubs, the zero setting always has been and still is a problem. Using auto zero in a TT is like gambling. In out and back courses sometimes I did 20 W more in the first leg, sometimes 15W more in the second leg, although it felt the same effortwise.
PT hubs have some plus points in aero testing but be careful with the zero setting, check it once and than donâ€™t touch it again, the increment is just too large, and donâ€˜t use auto zero!
VE is sort of like a microscope, or maybe an amplifier. It can help you "see" whether something changed. That change can be a change in slope, or a change in wind, or a change in the zero calibration offset. It can't tell you exactly which changed but it can tell you something changed. In your case, you suspect that the zero offset changed at the turnaround. VE could (perhaps) help you identify that since a change in zero offset should often be distinguishable from a change in slope, or a change in wind.
Not sure that is a good idea. IME a changing temperature will change the offset significantly. In hundreds of TTs I've never experienced the autozero failing. There was one where the readings were oddly low, but they were low in both directions and I'm pretty sure I coasted enough at the turn for it to reset. I really wish there was a way to record the offset in a file.
.1 N-m is about the same resolution as 1 in-lb. Still much too crude. And I still don't understand how RChung is getting .1 in-lb resolution on his.