The tanks of the Sling are bolted and riveted to the wing. That makes it difficult to remove the tanks for fixing a leak later down the road. Therefore I want to be sure that the tanks have no leak.
The manual says to do a soap bubble test, which can be accurate but depends on the patience and accuracy of the observer. Doing an additional test with a water manometer will result in an absolute value that can be checked against the leak rate table.
Since the manometer test is dependant on the temperature of the air in the tank and the barometric pressure it’s best to do the test in a temperature stable room.
Remove the rubber around the valve with a knife and insert the valve in the fuel tube, the other piece goes around the fuel vent. Use a bit of fuel lube and 2 hose clamps to ensure the connections are leak free.
Step 4 – Apply pressure to the tank
Use a bicycle pump to add pressure to the tank to max 1 PSI, which is 70,3 cm H2O, so 35,15 cm on the right side!
Note: in an older version of the construction manual is a mistake, instead 1 PSI it says 1 BAR, which is way too much and can damage your tank!
Note: it can take a couple of seconds before a bubble becomes visible so take your time!
Step 6 – Endurance test
Make sure the test setup is in a temperature stable room. To record the barometric pressure and temperature a WIFI weather station is convenient like this Netatmo.
Record at the start and each hour these values:
– manometer value
– barometric pressure
– temperature on fuel tank (better would be to measure the temperature in the tank, but that’s not practicable)
Do that for 24 or better 48 hours.
Add the values in Excel and correct the manometer values:
– 10,197 mm for each mbar change
– 36,5 mm for each degree Celsius change
Contact me if you need the Excel file.
Step 7 – Result
The graph already show a good indication if there’s a leak.
It’s now possible to calculate the leak rate with this formula:
Q = V * ΔP / ΔT
Q is leak rate in milibar*l/s
V is volume in liter (Sling 2 fuel tank is 84 liter)
ΔP is the delta of the corrected pressure at the end minus the pressure at the beginning
ΔT is the time of the test in seconds
A result of 10-4 to 10-5 is considered fuel tight.