(I’m going to move your question to a new thread as it is not quite the same discussion.)
On an emonPi, the ds18b20 is connecting to the Arduino-like emonpi add-on board not directly to the RPi bit.
New sensors are only recognized in setup() in the emonpi firmware, so you must restart the emonpi add-on board, this is not done by rebooting the RPi based emonPi device (confusing, I know). Try power cycling the emonPi or using the tiny reset button on the end of the emonPi case.