Gini, Good description ! I will add to yours.
There are two filters used in the Tug/Cut fuel systems with Cummins, Yanmar and Volvo.
The Racor is the Primary filter. The primary filter is normally coarser filtering 10 micron. It filters debris and water that is picked up from the tank. It is a pre-filter for the fuel system and and also protects the lift pump, electric or mechanical from damage due to contaminated fuel. The Racor is on the suction side of the lift pump.
The secondary filter is the filter mounted on the engine. It is normally engine manufacture specific. The filter used at the secondary on most common rail Diesels ( Volvo and Yanmar ) is a finer filter usually rated at 2 micron. This filter is on the discharge side of the Lift pump (fuel pump) and it's purpose is to remove any residual water that passed through the primary and to remove extremely small particles that could damage the high pressure injector pump and the injectors.
Primary = suction side
Secondary = discharge side of the pump
Racor filter without a gauge relies on the captain to determine the condition of the filter element. If the fuel tank and fuel system are clean and all fuel that is pumped into the fuel tank is clean. Filter inspections are done as preventative on a manufactures recommended interval. If contamination gets into the tank this could create an unknown event. Example: The boat is under way and a power loss is experienced or in worse case the engine dies when making way, under way. The only way for the captain to know if the filter is fouled is by inspecting the bowl and removing the filter for inspection.
Racor filter with a gauge gives the captain a good reasonable condition of the filter element. The gauge has three needles to read. (1) The present needle. This is set based on what the engine manufacture determine what the highest vacuum should be without harming the pump or engine performance. Normal setting is 8in mg to 10in mg. The (2) needle is the telltale. This needle can be zeroed after each day or left alone until the needle reaches a higher than normal vacuum reading. The telltale needle is moved by the (3) needle, This is the gauge needle that reads actual real time operating vacuum and moves needle (2) to the maximum reading and that needle stays in that position until the captain resets it. Vacuums can change based on engine speed and pump speed. The engine is using more fuel when at 3000rpm when compared to 1800 rpm so the vacuum can increase even if the filter is clean. This maximum value should be known and recorded when a new filter element is installed. This gives the captain a reference as to a start point with a clean filter.
My use of the vacuum gauge. I set the max needle (1) to 9in mg, I reset the telltale to zero when I have a new filter element installed. I run the engine at max WOT on the first day with new filter element. When I end the day of cruising I check the telltale reading. I record this value. This is the total vacuum created by the fuel system with a clean filter with maximum engine fuel flow. This value on my boat is 2in mg to 3 in mg is normal but I always check to make sure there are no other restrictions that have developed in the fuel system.
Examples: collapse fuel line hose, kinks, fuel valve position moved or not opened completely, full pick up obstructed.
Once I have determined the fuel system is operating at normal values I reset the telltale needle to zero and then monitor it when I do engine checks.
If and when I do see a increase of vacuum reading of 4 to 6 in mg I reset the needle and operate the engine for a cruising day and recheck the gauge. If the value returns to the elevated reading again it is time to inspect the filter, drain and replace the element.
The gauge can be used to troubleshoot the fuel system condition. Just because the gauge reads a high vacuum does not always mean that the reading is cause from a fouled filter. Normally it is but as I stated previously the can be other factors that can contribute to a fouled fuel system.
I believe a vacuum gauge should be standard equipment with diesel fuel systems.