Here is one for the Apple watch 1: https://journals.lww.com/acsm-msse/Fulltext/2017/08000/Variable_Accuracy_of_Wearable_Heart_Rate_Monitors.22.aspx Here is a paper for the Apple watch 3: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6732081/ Here is a paper for the Polar OH1: https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0217288 But we do not know of sellers of devices who sell products for scientific studies who picked those up in their store. Furthermore all of these devices …