Key takeaways
- A fuel sensor tracks the level in the tank over time, so what matters is the change, not any single reading.
- A capacitive probe gives a clean, dedicated line. A CAN-bus tap is quicker to fit but only as sharp as the factory gauge.
- Driving drops the line gently. A siphon drops it like a cliff while the truck sits parked, and that shape is what raises a flag.
- Calibration maps the reading to real litres for that one tank, so the graph shows true fuel and not a rough guess.
- Because fuel data carries a time and a place, a drop becomes a named spot and minute you can act on.
What a fuel sensor actually measures
A fuel sensor has one job. Tell you how much fuel is in the tank, again and again, all day.
On its own a single reading means little. Forty litres at noon tells you nothing. But forty litres at noon and twenty litres at half past noon, with the truck parked the whole time, tells you a great deal.
So the sensor is really measuring change over time. It plots a line, and that line is where the truth lives. See how Otrac fuel monitoring works.
Probe sensors versus CAN-bus reading
There are two common ways to get that reading, and they are not the same quality.
The first is a capacitive probe. This is a thin rod fitted down into the tank. As fuel rises and falls around it, the electrical reading on the rod changes, and that change maps to a level. It is a dedicated instrument, fitted for one purpose, so it gives a clean and sharp line.
The second is a CAN-bus reading. Every modern truck already measures its own fuel for the dashboard gauge, and that number travels on the vehicle's internal data line, the CAN-bus. A tracker can tap that line and read the figure the truck already knows.
CAN-bus is faster to fit because you are not opening the tank. The trade-off is that you only get what the factory gauge gives, and factory gauges are built to be roughly right, not exactly right. For tight fuel control on a truck carrying real diesel, a calibrated probe usually wins on sharpness.
There is a fleet reason too. Older trucks and some imported units do not put a usable fuel figure on the CAN-bus at all, so the probe is the only way to get a clean reading on a mixed fleet. Standardising on the probe means every truck reports the same way, which keeps your reports comparable across the yard.
The probe also sits where you control it. A CAN-bus reading can change if a workshop touches the truck's wiring during a service. A dedicated probe keeps reporting the same way no matter who works on the engine, which matters when you are leaning on the data to question a driver.
Reading the fuel graph
Once the data flows, you get a graph. Time across the bottom, litres up the side. Learning to read it takes about a minute.
A refill is a sharp line going up. The truck pulled into a station and the tank filled, so the graph jumps. That is normal and expected.
Driving is a gentle line going down. The engine sips fuel as the truck moves, so the line slopes slowly. Also normal.
The shape you are hunting for is different. It is a cliff going down while the truck is not moving.
A truck cannot burn forty litres standing still. When the graph drops like a cliff and the wheels are not turning, that fuel did not get used. It got taken.
How a sudden drop flags a siphon
This is the part that catches the theft. A siphon is fast by nature, because nobody wants to stand at a parked truck with a hose for long.
So the siphon shows up as a steep drop over a few minutes, with the engine off and the truck stationary. The system sees that shape, knows it does not match driving or a normal refill, and raises a flag.
You do not have to stare at graphs all day waiting for it. The alert finds you. By the time you open the dashboard, the drop is already marked, and the fuel that walked off is already counted in litres.
The short-fill is the other shape worth knowing. A driver is given money for a full tank but only puts in part of it and pockets the rest. On the graph the refill line goes up, but it stops short of full. Pair that with the receipt the driver hands you and the gap is plain to see.
We walk through how drivers run siphons and short-fills in the guide to stopping fuel theft.
Why calibration is the whole game
A sensor is only as honest as its calibration, and this is where cheap setups fall down.
A tank is not a tidy rectangle. It is shaped to fit under the truck, with curves and humps. That means a litre of diesel raises the level by different amounts depending on whether the tank is near empty or near full.
Calibration is the work of mapping the raw sensor reading to true litres for that one specific tank. Our team does it on the truck the sensor is fitted to, filling in known amounts and recording how the reading responds.
Skip that step and the graph lies. You get false alarms that train you to ignore the system, and real thefts that slip past because the reading was too rough to notice. Calibration is what turns a rough gauge into evidence.
Pairing fuel data with location
A fuel drop on its own tells you that diesel left. Pairing it with location tells you where and when, and that is what makes it actionable.
Because the fuel sensor lives inside the same tracking unit, every fuel event carries a timestamp and a place. So the cliff on the graph is not just forty litres gone. It is forty litres gone at this depot, at 2am, while the truck sat parked off its route.
That is no longer a suspicion you cannot prove. It is a named time and a named place, which is exactly what you need to have a real conversation with a driver. For owners running several trucks, this data feeds straight into the fleet management screen, and a fuller setup lives under Otrac fleet management.
One last point worth naming. Diesel siphoned off trucks does not vanish, it gets resold into the downstream fuel market that the NMDPRA regulates. A fuel sensor is where you first catch your own diesel before it disappears into that trade.
