The MAG-VIEW will not work with demineralized water. As the electrical conductivity is not enough, a vortex flowmeter is a good choice, such as the LIQUI-VIEW.

The MAG-VIEW  measures the velocity of conductive liquids in the instrument, such as (waste)water, acids, AdBlue and slurries. 

MAG-VIEW meters can measure properly when the electrical conductivity of the liquid is greater than approximately 20μS/cm for the QA models and 50μS/cm for the PA/PN models.

Using the MAG-VIEW on fluids with low conductivity, such as deionized water, boiler feed water, or hydrocarbons, will cause the flow meter to measure zero flow.