Vacuum guage question.

[Rappey]

I am going to fit a vacuum guage between my fuel filter and lift pump. It measures 0 to 30 inHg. It seems from what I've read that at10 inHg the filter needs changing.
The question is about the tube that will be teed into the fuel line which will be about one meter long going vertical to the guage.
Does it have to be purged of air or does it make no difference to the guage reading ?

 

[Roberto]

I have the same gauge but I haven't fitted it yet: more than an erratic reading I am more worried by air along the measure hose to migrate inside the fuel line and stop the engine. I intend to fill it with diesel, but haven't figured out how to do it so far, placement of the dial, position of the hose, etc.

 

[Rappey]

Add the new measure hose.. you will have to bleed the system so that will fill the measure hose. Once full screw on guage . Even if their is a little air at the guage end it can't be sucked into the main fuel line as it will be fighting against a vacuum.

 

[greeny]

Even if you bleed the tube you will always have some air in there. In theory it should not affect the gauge reading. The air should de-compress/ compress to equal the liquid pressure in the line. In practice people argue both ways. In industry that gauge would normally be fitted as close to level or below the level of the fuel line and bled to remove air. If you're worried,
Try before you fit. Fit the gauge to flying tube and tye wrap it in both positions to test it.

 

[Daverw]

I would fit the gauge on a tee which would allow you to purge air from above the gauge connection

 

[RivalRedwing]

I fitted one to the 'spare' port on a CAV filter head using compression fittings and a short length of copper tubing, never gave a thought to bleeding it beyond the 'normal' filter changing routine and didn't have any issues either.

 

[IanCC]

I gave mine a suck to see if it worked then just t'd it in. Never thought about air. Just made sure all jubilee clips proper tight.