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Fuel sensors · Free advisor

Fuel-Theft Detectability Advisor: Smallest Drain a Sensor Can See

A sensor cannot flag a drain smaller than its own noise. Enter the tank, the sensor resolution, the slosh and the report interval. Get the smallest drain the setup can see, and the drain, fill and filter settings to start from.

  • Runs in your browser
  • Parked and moving
  • Your sensor values
L

Usable volume of the tank the probe sits in.

mm

Tank inner height covered by the probe after cutting.

L/mm

From the calibration table, for tanks that are not straight-sided. Leave empty to use volume ÷ length.

mm

Smallest level step the sensor reports. From the sensor datasheet. Example value.

%

Absolute accuracy as the datasheet states it. Used for the level band, not for steps. Example value.

mm

Standard deviation of the level in a parked log. Example value.

mm

Typical swing of the level on your roads, before any filter. Example value.

s

How often the device records the level.

s

Time the device or platform averages before it decides. Example value.

L/h

Average burn of this vehicle. Zero if unknown.

L

Optional. The tool says whether your setup can see it.

Typical values. Replace resolution, accuracy, noise and slosh with your sensor datasheet and a short log from your vehicle.

Smallest drain and thresholdsParked and moving

Keep these settings with the sensor record

A report with the smallest drain, the thresholds and the filter time.

Fuel sensor settings report

Set thresholds from physics, not from habit

  • Smallest drain seen parked and moving
  • Drain, fill and filter values to start from
  • Your current thresholds checked
  • Drain sizes the setup sees and misses
  • Method and the values you entered

Why do small drains go unseen?

A level sensor reports steps, and every reading moves a little even when nothing happens. Set the drain alert below that movement and the fleet gets false alarms. Set it far above and real drains pass quietly. The right threshold sits just above the noise you can measure.

Probe in the tankparkedsloshdrain thresholdmoving: noise and sloshdrain: a step downLevel over time
Fig. 1 A drain shows as a step in the level. The step must be larger than the reading noise when parked, and larger than the leftover slosh after the filter when moving.

How does the tool calculate the smallest drain?

It turns millimetres into litres, finds the smallest step that beats the noise, and adds a safety factor.

k = tank volume / measuring length (L per mm)Parked step = max(2 x resolution, 3 x noise)Moving step = max(2 x resolution, 3 x noise + slosh / sqrt(n)), n = filter window / reading intervalDrain threshold = step x k x 1.5, rounded up to a litreMoving drain threshold adds fuel use x window / 3600Example: 400 L, 600 mm, 1 mm, noise 1 mm, slosh 20 mm, 10 s, 120 s, 25 L/h -> parked 3 L, moving 10 L
  • Two resolution steps are needed so that a one-count flicker is not an event.
  • Three times the noise standard deviation is a common rule that keeps random readings below the threshold.
  • Averaging n readings shrinks random slosh by the square root of n.
  • The safety factor is a page setting, 1.5 by default.

Parked, moving and filling

The same sensor needs different settings in three states.

Most drains

Parked

Ignition off

Fuel surface
Still
What limits you
Noise and resolution
Threshold
Smallest
Set the drain threshold for this state first.

Moving

Ignition on, driving

Fuel surface
Sloshing
What limits you
Slosh after the filter
Threshold
Larger
Also subtract the fuel the engine burns.

Filling

At the pump

Fuel surface
Rising, then still
What limits you
Settling time
Threshold
Same step as parked
Compare each fill with the fuel card later.

How do you measure noise and slosh?

  1. Log the vehicle parked

    Park on level ground for an hour with the device recording. The spread of the level readings is your noise.

  2. Log a normal trip

    Drive your usual roads. The swing of the raw level, before the filter, is your slosh.

  3. Enter both here

    Use the numbers in millimetres. If the export is in litres, divide by the litres per mm of the tank.

How do you tune the thresholds?

Start from the advised values, then watch a week of events.

  • Many false drains while driving: lengthen the filter window.
  • False drains when parked on a slope: check the probe position and the parking spot.
  • A drain you know of was missed: lower the threshold one step and check again.

Assumptions and limits

  • The tank is treated as straight-sided. For a shaped tank, enter the litres per mm of the section you watch.
  • Resolution, accuracy, noise and slosh are your inputs. The starting values are typical examples, not Tracom specifications.
  • Slosh is treated as random. A tank that rocks in one rhythm may need a longer window.
  • Temperature changes the fuel volume. A slow change over hours is not a drain.
  • A slow leak below the threshold is not caught by steps. Compare fuel used with the distance over days for that.
A starting point, not a guarantee

The thresholds here are starting values from your inputs. Check them against a week of real events before you act on alerts.

Related Tracom solution

Fit the probe right, then set the numbers

Resolution, accuracy and filtering options differ by sensor model. Ask Tracom for the datasheet of the fuel sensor you plan to fit and how its readings reach the device.

  • Sensor datasheet gives resolution and accuracy.
  • Cut length sets the measuring length in the tank.
  • Calibration table turns millimetres into litres.
  • A week of logs confirms the thresholds on your roads.
Questions

Questions about fuel drain detection

Planning fuel sensors for a fleet? Contact Tracom.

Why not set the drain alert at 1 litre?

Because the reading moves by more than that on its own. A threshold below the noise sends alerts that nobody can act on, and people stop reading them.

Is accuracy the same as resolution?

No. Accuracy says how close the level is to the truth. Resolution says how small a change the sensor reports. Drain detection depends on resolution and noise. Accuracy matters when you compare litres with fuel cards.

Why is the moving threshold larger?

Fuel sloshes while driving, and the engine burns fuel during the filter window. Both look like a drop, so the threshold has to clear them.

Can this tool tell me if fuel was stolen?

No. It tells you the smallest drain the setup can see and where to set the alerts. Whether a drop was theft needs the event, the location and a check on site.

What is Tracom's role here?

Tracom supplies the device and sensor datasheets and installation guidance. The thresholds are yours to set and test on your vehicles.

Related tools and templates

Fit and calibrate the sensor first

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