Exactly how does the Apple Watch calculate pulse?

Thanks, but I had seen that site. It does not answer my question., It does not tell me whether my pulse measurement takes for example, 1 minute, or 5 seconds, or whether it counts pulse and divides by time.

So pulse = beats/time.

Does the watch count for a given time, measuring pulse - if so How Long?

or does the watch count say 10 beats, and measure the time taken? - If so How many beats?

I want to know which of these it uses, and what are the parameters - either time taken for measurement or beats counted. This is not an idle question, it relates to factors in heart arrhythmia.

Thanks Ferd 2. It seems as though only Apple knows the answer to what is really a straight forward question - what maths does the Apple Watch use to calculate pulse rate? And what parameters? There are plenty of studies that show in general the Apple Watch is pretty reliable means of measuring pulse rate, and my comments were not meant to be interpreted as using the watch for checking medical conditions. Rather, for those with irregular heart beats, can we be sure that the watch is truly measuring pulse rate? - if the tests are over a short period of time, the results would be quite variable for such. If measured over a minute, then the results would seem pretty indicative, but if measure over say five seconds, a one off double pulse in that time for example would result in a much higher pulse rate that would be calculated if the measurement was over say 30 seconds or more. I'm still hopeful that someone knows the answer!

Thanks iMuse ***, You are right on. Some experimentation with the Breathe and Heart complications, show that the measurement seems to time out at close to 5 seconds. The only point now is whether within this period it measures time taken for complete pulses (and gets rate by dividing the number of full pulses by the time taken*60), or whether it measures total pulses (full and incomplete over the 5 seconds, and measures pulse rate by dividing the total pulse rate (including part pulses), and divides by 5 *60). The latter would require a pretty good algorithm, but who knows. Either way it is measured over a very short period, so that any irregular beat in that time would have a significant effect on stated pulse rate.