Good to know
vlittle said:
George: I want to point out to anyone considering an in-line fuel filter to be very careful: Many automotive types will block water. This means that water in the filter will also stop the flow of fuel... not a good thing. The Subie guys use filters all the time, I'm sure that they'd share their insight. Vern Little RV-9A
I did not know that, thanks. Where does it say that. I know the Non-sube RV guys have debated this. I know a lot of guys use in-line filters coming off each wing tank. I guess the idea is drain the tanks so water is not introduced, but still that would not be good to have a "water block". I know that there is some controversy about the filter element absorbing water and freeezing. Some have tested this theory and found it not true.
I don't understand how that would cause a problem if the filter is a wire mesh (for example: 60 micron (pelted) stainless steel screen); if you get a slug of water I would think it would pass thru the in-line filter, where the gascolator would pick it up. These in-line filters are really just a Fat tube with a wire mesh cone. I wonder if differnt filter materials can block water? If it does you are right, they are no good, but the ones I have seen are wire mesh. Any paper type filter would be a no-no. I don't know how metal would not allow water to pass thru. Regardless if you us an in-line filter test it with water, drain and than freeze the thing, than try to pass chilled (-0C) fuel. This would be the acid test, so to speak.
To eliminate any filter issue, you could remove the in-line filter element and just use the shell. The idea is little area for some air to be trapped to absorb and surges (air is compressible), not filter the fuel. Instead of a in-line fuel filter, you can put a "T" in the line and run the spare tube leg so air is trapped in it and cap it off. This makes a little accumulator. Matronics makes a little fuel accumulator sphere for this purpose.
On second thought an in-line filter is another maintenance / service requirment and being in the cockpit, servicing and fuel spilling would be a pain. Also using a tiny filter to pass all the fuel is against standard aircraft filter design, where you should use larger more course filters close to the tank and get smaller (finer) filters towards the engine. Regardless of the water issue, keep it simple, light and per plans is a good idea.
I am going to try the system as I discribed without any in-line filter and add an accumulator if there is any large scale error. The only filter will be the gascolator.
N2prise: As far as mounting the FF sensor between the gascolator and the engine driven fuel pump. I can see and advantage in that you isolate the sensor form the electric pump (which is the source of pulses). That is why some see erroneous readings when the electric pump is on. The mechanical pump is steady? If it works it works! Cool. Good to know. I like the idea of having in the cool floor, but your design looks like a good alternative. Bottom line if it works it works. However the instructions do discuss having straight runs in and out of the FF sensor. The idea is as long as you have solid flow, not turbulent or unsteady pulsing flow, you are good to go.
G