I've often wondered why some fuel pressure gauges are fluid filled? If it matters, wouldn't the inevitable air bubble within throw off any perceived accuracy that may be "gained" by the fluid?

 

It's not accuracy. The fluid is a mechanical way to dampen the movement so the needle won't bounce - you get a more steady reading that smooths out any pulses in the pressure input.
e.g., I have a non-liquid pressure gauge on my air compressor tank because that's what I had handy. While filling, the needle bounces +/-20PSI. So can't really tell where it is in the charge process until it shuts off.

 

It's to dampen the needle movement, which allows it to be read easier. A mechanical fuel pump will have a pulsed output which can make an undamped gauge needle bounce around a lot.

 

However the temp of the fluid ( internal pressure) can change the readings ( hot engine compartments vs cold ).

 

Thanks, guys; now I know!

Learn something new every day!

 

2x, Aeromotive advertises a fuel pressure gauge which fixes this, I'm not sure if others have fixed theirs.

 

Yes, the fluid filling dampens the needle movement to make it easier to read; however, there is another major benefit...

The gauge lasts many times longer because less motion means the internal gears don't wear out so quickly.

 

ShiftByEar - thanks for posting the video!

 

Exactly right...which is why liquid-filled gauges have a rubber plug on the top.

"Burp" the rubber plug before taking a reading.

Some plugs have a tiny nipple molded-into them. IF (big IF) the nipple is facing up when the gauge is installed, and IF it's used in a "clean" environment, and IF the gauge isn't subjected to G-forces, you could poke a pin through to create a permanent vent. I have liquid-filled gauges on my home water pressure system--one before the regulator, and one after I considered venting the rubber plugs, but decided against it.

 

Thanks for contributing to the discussion, Schurkey.