Seconded , those analog voltage sensors like LM36 can be up to +-2 degrees C out when used with the arduino and ATMega328 I have found . Put one side of a multimeter to earth and the other to the heat sink of the voltage regulator on th arduino to get the actual voltage supplied to your chip. Mine was 4.8 V odd when plugged into the computer but it does vary depending on what is fed in.