It is noted that the existing methods of magnetic monitoring of iron impurities in petroleum products need refinement, including validation of the specimen flow rate limits in the analyzer. Based on a simplified condition for the competition between magnetic and hydrodynamic forces, an expression has been derived to determine the limiting specimen flow rate in the analyzer. It is shown that the rate depends parabolically on the ratio of the size of the iron particle to the distance to the source of the field of the magnetic system in the analyzer.