Earlier this year I built a custom heart rate monitor to wear when diving. It's based around a sealed Polar chest strap and a waterproofed Arduino unit with relevant sensors and data logging. Yes, I know such devices do already exist (ironically a few weeks after I finished my device I picked up a Garmin Descent watch), but I wanted something that I could tinker with and which would provide complete detail with no rounding or smoothing of the data. ​ Out of many thousands o