There are many things that can impact the accuracy of a magnetic flow meter out in the field. Incorrect installation and/or wiring can affect the accuracy of the sensor right from the beginning. Since electromagnetic sensors are capable of measuring a wide variety of material, over time, non-conductive material might deposit itself onto the electrodes. Magnetic materials can also get inside the sensor and interfere with its accuracy. In fact, many agencies, such as NIST, require annual verification of electromagnetic sensors; especially those that are involved with custody transfers.