A fuel level sensor reports how much fuel is in the tank, message by message. Polygon watches that level over time. A sudden rise is a filling. A sudden fall while the vehicle stands still is a drain. The slow fall in between is fuel consumed. The device's Fuel Config tab holds the settings for each step. Read this page before you tune them, so you know which setting answers which complaint.
The app needs a fuel level in litres, or in the unit your sensor reports. Usually that is a fuel sensor with a calibration table that turns the probe's raw reading into litres. See Calibration tables. With more than one fuel sensor, reports use the one marked as the total. Without such a mark, they use the first fuel sensor in the list.
Fillings, drains and consumption appear in the fuel reports and the fuel chart. The app works them out as messages arrive and keeps them. When you save the Fuel Config tab or change the sensors, it works out the last 60 days again.
A probe in a moving tank swings by several litres from one message to the next. Before it looks for events, the app smooths the level with a median filter: each reading becomes the middle value of the readings around it.
Window Size sets how strongly it smooths. It is a level, not a number of messages:
| Window Size | Messages smoothed together |
|---|---|
| 1 | 5 |
| 2 | 9 |
| 3 | 15 |
| 4 | 19 |
| 0, or never saved | 3 |
Each odd level covers five times its value; each even level one message fewer. A wider window hides more sloshing, but it also flattens the peak of a fast drain. With Enable median filter off, the app uses the raw readings.
A drain begins when the smoothed level drops by at least 10 litres between two messages. A slow fall never starts one, however far it goes in total, because that is the engine burning fuel. No setting changes those 10 litres.
By default the app looks for drains only while the unit stands still:
Turn on Detect fuel drains in motion to drop both conditions.
The drain goes on while the level keeps falling. It ends when the level rises again, or when it stays flat for Timeout to separate consecutive drains (s). A pause in the middle of siphoning therefore does not split one drain into two.
The volume is the level before minus the level after. With Calculate drain volume by raw data on, the app uses the unsmoothed readings when they show a larger volume.
Last, the app subtracts the fuel the vehicle would have burned during the drain anyway. Unless the device says otherwise, that is 1 litre for every 5 km driven and 1 litre for every 3 hours standing. The app reports the drain only if what remains is at least Min. Theft Volume.
Any rise in the smoothed level can start a filling. With Detect fuel fillings only during stops on, the app ignores a rise while the unit is moving.
The filling goes on while the level keeps rising. It ends when the level falls, or when it stays flat for Timeout to separate consecutive fillings (s). A pump that stops and starts again is still one filling.
The volume is the level after minus the level before. With Calculate filling volume by raw data on, which is the default, the app uses the unsmoothed readings when they show a larger volume. The app reports the filling only if the volume is at least Min. Refill Volume.
Every fall of the smoothed level outside a filling or a drain counts as fuel consumed. The app splits it into driving, idling and parked. Telling idling from parked needs an ignition sensor; without one, all standing time counts as parked.
Drained fuel never counts as consumed, whatever Exclude drains from fuel consumption says.
A tank that sloshes, or a probe that sticks, can show a filling and then at once a drain of the same size. It can also happen the other way round. Turn on Exclude consecutive draining/refilling to drop such pairs. The app drops both events when:
The percentage compares the tank level before and after, not the two volumes. With this option on, an event appears only once that time has passed without a matching opposite event. Reports therefore show fillings and drains that much later.
Fuel theft and fuel refill alerts use a second, faster check that runs on each message. It follows the same rules and settings, with three differences:
An alert and a report can therefore disagree about a borderline event.