The MAG-VIEW magnetic inductive flow meter is not suitable for use with demineralized or deionized water. 

This is because the electrical conductivity of these types of water is too low. 
For such applications, a vortex flow meter like the LIQUI-VIEW is a better choice.

MAG-VIEW meters can accurately measure liquids with an electrical conductivity greater than approximately 20 μS/cm for QA models and 50 μS/cm for PA/PN models.

Examples of conductive liquids:

• Acids
• Caustics
• Water-based slurries
• Salty solutions
• Hydrochloric acid
• Vinegar
• Honey
• Latex paint
• Lemon juice
• Sodium hydroxide

Examples of non-conductive liquids:

• Deionized water
• Ultrapure water
• Distilled water
• Boiler feed water
• Hydrocarbons
• Oils
• Fats
• Alcohols

Using the MAG-VIEW on fluids with low conductivity will result in the flow meter registering zero flow.

 You can read here more about: How does a magnetic flow meter register water - Mass Flow Online (massflow-online.com)