A device for monitoring heat transfer systems. A non-heated temperature sensor is used to determine the approximate temperature of a flowing fluid being monitored. A heated temperature sensor is exposed to and is thereby cooled by the flowing fluid being monitored. A flow measurement is made using a flow sensor and the heat transfer from the heated temperature sensor to the flowing fluid is determined. An electrolytic means is provided for chemically altering the environment of the heat transfer surface of the heated temperature sensor to determine its use or effects for testing, operation and maintenance of the heat transfer system and also the device. A hot tap insertion flow probe configuration is presented which includes a cost effective miniature magnetic flow sensor using permanent magnets.