I recently had to service my fuel level sensor and took the opportunity to measure my diesel fuel tank and fuel sensor characteristics. I also measured the tank level to fuel gauge calibration, the graph below shows the Fuel-gauge reading verses the fuel level. Conclusion -The fuel sensor only starts to register the fuel level when the level reaches 23 Litres of fuel (When the fuel-sensor-float starts to float). This corresponds to the gauge level at the left of the RED band. The gauge rises reasonably linearly until the sensor gets to maximum at 90 Litres (When the float hits the top of the sensor travel), this corresponds to the gauge at the right of the BLUE band. The 1/4, 1/2 and 3/4 positions are equally spaced across the whole of the gauge scale. That is between the left of the RED band to the RIGHT of the BLUE band.
Warning – do not assume your tank and gauge are the same as mine!
The fuel tank is quoted as having a capacity of 93 litres. I did not measure the position of the fuel take off pipe in the tank and must assume that it is NOT right at the tank bottom otherwise water and debris at the bottom of the tank would be drawn into the system. As the fuel sensor reports empty when there are approximately 22 Litres of fuel left in the tank one should take this level as near empty. The bottom of the fuel sensor is about 30mm from the bottom of the tank, if one assumes that the the fuel take off is also at this level then there are about 10 Litres of inaccessible capacity. Therefore I guess the useable tank capacity is a maximum of 93L – 10L = 83L !
| Gauge Position |
My Litres of Diesel |
Calculated Litres |
Guess of useable Litres of Diesel |
| Empty (Left of RED band) |
0 to 22.5 |
0-22.5 | 0-12.5 |
| Right of Red band | 30 | 20 | |
| 1/4 (0.25) | 42 | 40 | 32 |
| 1/2 (0.5) | 61.5 | 56 | 51.5 |
| 3/4 (0.75) | 78 | 73 | 68 |
| Left of Blue band | 85.5 | 75.5 | |
| Full (Right of Blue Band) | 90 to 93 | 90-93 | 80-83 |
Gauge showing expected tank capacity (top number) and probable useable capacity (bottom number)
Note
If you suspect that you have a diesel leak from the tank area you should inspect the tank and connections. These are situated in the cockpit locker under the locker floor boards. It is difficult to locate the source of any leak as the fuel tends to sit in any low-point. It is useful to pad the top of the tank with tissue/paper towel, this will soak up any diesel and help you to locate the leak source. If the leak is from the connections or the fuel sensor then the diesel may only leak with a full tank and the boat healed. It is also useful to put a wad of towel in the channel in the left side of the wet-locker in the heads, this will indicate if diesel is leaking from the tank (it will also get wet from drain-off from the aft locker).
Fuel Sensor sealing
Later boats seem to have their fuel sensor bedded on a sealant so should be leak-proof.
Mick Nelmes