I’ll just throw in a personal experience with the Wahoo app and chest strap as a caveat (in case that’s the software you’re using). I believe this would only apply to this software, and even then maybe they’ve fixed it by now. My experience was from 2 years ago.
I have the Wahoo TICKR-X heartrate strap, and before I got a watch, I used the Wahoo app to record heartrate data. This combo did NOT work for the Pa:HR feature in Training Peaks if you are trying to do the outdoor test. (I suspect it would have been OK for a treadmill test but I didn’t have access to a treadmill).
For the outdoor test done on a track, I’d get crazy Pa:Hr numbers like -50% or +20%. It took me a while to figure out it was because this hardware and software combo sampled data excessively often, leading to noise in the speed channel. So even if I Was running at a steady slow pace (around 11:30 minute miles if I remember right!) it would sample me at 20 minute mile, then a 5 minute mile, then everything in between, all within a second. It would average out correctly, but for whatever reason the Training Peaks Pa:Hr algorithm would not handle it well. You could even see in the Training Peaks data that the pace and heartrate data was full of major spikes.
Once I got a watch and recorded through that, my data looked way more normal and the Pa:HR numbers made sense. I haven’t used the Wahoo app much since, but perhaps they’ve added features to allow users to smooth out the sampling rate.