So I am on a cardio machine at the gym - cycle, running, stairs, whatever - wearing a telemetry strap around my chest to pick up my heart beat and transmit it. I am also wearing a watch that was sold with the telemetry strap, that picks up the transmitted signal and gives a readout of my heart rate.
Meanwhile the machine itself is picking up the same signal and showing my heart rate. (I am not touching the sensors, so it is not reading my pulse directly; it has detected the telemetry strap.)
The two heart rate displays regularly differ, normally by 2-10 bpm.
Is there a general rule about which one I should believe?
The watch is the same brand as the telemetry strap, and is better tuned to it. Trust the watch over the cheap electronics they put in gym machines.
The machine has a much larger antenna, and is plugged into mains so it can be more sensitive. Trust the machine over the low-power processing they put in watches.
One of them is missing beats. Trust whichever reads higher.
Like uncalibrated scales, they all have their biases, so use your watch because at least the bias is always consistent, no matter which machine you are on.