With the Napa Gold fuel test kits you can check for fuel oxidation, water, rust, scale, sludge by basic visual methods.

Standard kit # 4383 has 10 test swabs, instructions, color card and extension stick

Pro kit # 4384 has 60 test swabs instructions, color card and extension stick.

How To Use The Napa Diesel Fuel Test Kit

Attach the swab to the extension stick clamp, lower into fuel tank bottom to sample contamination (if any).

Remove extension stick and compare swab to images as per above.

Summary

These are a very basic test kit, at the price point they are value as they are cheap but as said they are basic, so ensure they are fit for purpose for your testing needs.