All field notes
Every fleet operator in Qatar has the same story. The monthly fuel bill is eight per cent higher than the routes justify, nobody can say which vehicle it went into, and by the time anyone looks the tank has been refilled twice.
The naive detector is a threshold: alert when the level drops more than fifteen per cent. It fires on every hill, every hard brake, every vehicle parked on a camber, and within a week the dispatcher has muted it. An alarm nobody reads is worse than no alarm, because it is also a line item.
A tank does not empty smoothly
Consumption is a slope. A refill is a step up. A siphon is a step down — and the three are distinguishable by shape long before they are distinguishable by magnitude. The interesting signal is not how much left the tank but how fast, and whether the engine was running while it did.
- Consumption: a gradual decline, correlated with distance and engine hours.
- Refuelling: a rise of tens of litres inside two minutes, almost always stationary, almost always at a known location.
- A siphon: a fall of the same magnitude at the same speed, with the ignition off and the vehicle somewhere it does not normally stop.
That last clause is the one that does the work. FMS holds the last known position, the ignition state and the tank level on the same timeline, so the rule is not "the level dropped" but "the level dropped while nothing was running, away from a depot".
40s the window a sudden-loss rule watches, end to end
Why the alert has to name the vehicle and the place
An alert that says "fuel anomaly detected" is a request for somebody else to do the investigation. The ones that get acted on carry the plate, the litres, the timestamp and a map pin — everything a supervisor needs to make one phone call rather than open four screens.
The alarm is not the product. The phone call the alarm makes possible is the product.
This is also why sudden fuel loss is a separate alert type in FMS rather than a threshold on a sensor reading. A threshold tells you a number crossed a line. An alert type knows what the crossing means, which fields to carry, and who to wake.
What it costs to get wrong
Both directions cost something, and they do not cost the same. A missed siphon is a few hundred riyals. A false positive at every legitimate refuelling stop is the credibility of the whole alerting system, and once that is gone the genuine alerts go unread with the rest.
Thresholds are per-sensor and per-fleet in FMS for exactly this reason: a refrigerated trailer and a light van do not share a definition of "sudden".