Some Information
Bob,I copy and pasted this from Filter Manufacturer's Council Website. There is also a link below that describes particle sizes.Q: What is the difference between a primary and secondary diesel fuel filter?A: A primary diesel fuel filter is the first filter in a two filter system. It is located before the secondary filter, and usually before any transfer pump in the system. A primary filter is used to remove the bulk of free water and contaminants and thus protect the transfer pump (if present) and secondary filter. Typical primary filter efficiency is 95% to 98% @ =10 to 50 microns.A secondary filter is the second filter in a two filter system. It is located between the primary filter and the fuel injection system, often after a transfer pump. Typical secondary filter efficiency is 95% to 98% @ = 3 to 10 microns. Newer high pressure fuel injection systems require removal of the smallest measurable hard particles.The main advantage of a two filter system over a single filter system is that much of the dissolved asphaltenes present in diesel fuel are caught by the primary filter, leaving the secondary filter relatively clean and ready to remove free water and fine particles. A properly sized two filter system will often outlast and outperform a similarly rated single fuel filter system.Mike D