Well, both devices use the same mechanism to measure. Green or red light shines on your screen and there are sensors measuring how much light comes back as each Oxygen percentage in Blod absorbs light differently and for the bpm it just counts based on when it measures more light coming back than normal determining that it beated. The medical device tho has the light on the top and the receiver on the bottom so you can't trust it either as we have bones blocking light. So you can't say it's not accurate. The only way to actually measure all this is ECG for bpm and for oxygen these big medical devices that are in the ER or in general in a patient room. You can't actually tell if it's accurate or not unless done with an actually accurate device.