R10

Look for the Colmi rings thaf use the QRing app if accuracy doesn't matter. But buy several extras as they do sometimes arrive dead on arrival. Look on Temu or Ali for R06, R07, R09, or R10. You could even mix and match as the hardware is identical except the 9 supposedly adds a skin temp sensor.

Thank you for the comment, but clearly you haven't actually measured accuracy against other wearables including other smart rings, but, the R10 can be a good starter ring. The Colmis don't tend to have a long usable life either.

Colmi R10, achetée pour une vingtaine d'euros sur Aliexpress. Je la conseille pas, l'application est nulle à chier. Et j'ai des doutes sur sa fiabilité...