The properties of mercury are well known and have not changed over thousands of years. A good, vintage mercury thermometer is 100% accurate, 100% of the time. I have two that I purchased off ebay in the "Antiques" category - one was from a drug store in the 1960s and had never been opened! No batteries, no variation in sensor accuracy. Of course, there would be no way to "connect" it to a digital device... so the temperature would need to be manually entered in a health app. My 2¢