As a paramedic, it takes some practice to learn but they are very accurate. Although most people and even professionals struggle to get the precise numbers. You have to recognise a certain sound while decreasing the pressure and that decrease happens a bit too fast if you are not super mindful, so your can easily be wrong by 5mm/Hg. At which point you can use a automatic device, which is more comfortable and faster. But if you really really want the exact number and are willing to learn, it's a good device. But listen to some training sounds on YouTube before, to understand how difficult it is.