Sight gauges for indicating fuel level are simple and reliable, but a few things should be kept in mind.
First, the clear tube must be compatible with the fuel. Glass is compatible but fragile. Clear rigid PVC (vinyl) tubing can be used for straight runs. If flexible tubing must be used, it’s important to select the right type. There are several types of transparent flexible tubing specifically made for fuel, including yellow vinyl or blue polyurethane, but the tinted tubing can make it more difficult to see the fuel level inside the tube.
One way to make the fuel level more visible is to use a floating ball inside the tube. Plastics that are light enough to float in fuel are rare, but one option is the U10853-000 “Piper Fuel Ball” sold by Univair and other suppliers.
However, the fuel ball is quarter-inch diameter, meaning it won’t move freely inside a quarter-inch ID tube; you need a 5/16-inch ID tube … which is less common than quarter-inch ID tubing.
Clear polyurethane tubing is available in 5/16-inch ID, but it’s important to note that there are two different urethane formulations: ester-based (which is compatible with fuel) and ether-based (which is not). McMaster-Carr sells a 5/16-inch ID ester-based urethane tubing, part number 5108K53, which is what I used on my Fisher FP-404.
At the ends of the sight tube are two brass elbows connecting to the fuel lines behind the panel. These end fittings should have a restriction, both to keep the ball from getting stuck in the fitting and to slow fuel loss if the tube should fail for any reason. This can be easily accomplished by drilling a 1/16-inch or smaller hole in a brass plug and soldering it into the elbow. This will also damp fuel sloshing in the gauge for a steadier reading.
If it’s a tailwheel plane, there should be two sets of markings for the gauge: one set for the level flight attitude, and another set for when it’s on the ground in three-point attitude.
Regardless of what kind of tubing is used, it should be inspected during every preflight and replaced every couple of years or if any degradation is noticed.
It’s been sixty years since I owned an airplane with a sight gauge (a ’46 Champ), but I built a single-place helicopter a few years back. I added a sight-gauge to it and it worked quite well without the restriction plugs, probably because it experiences few off-vertical G-forces.
OTOH, I have several 55g. drums of water/gas/diesel/etc. for which a sight gauge would be quite handy (I’m not a fan of dip-sticks) and it never crossed my mind to add sight-gauges to them. They will need to be soldered in place, however.
Thanks for sharing your excellent write-up, especially the pointer to the float-ball!