Multi-tank monitoring: reefer units, generators, and auxiliary tanks

The tanks nobody sees

Say "fuel monitoring" and everyone pictures the truck's main tank. But one walk through a logistics or contracting yard reveals a wider reality: reefer trucks carrying an independent tank for the cooling unit, generators at remote sites refuelled from drums, long-haul tractors with an auxiliary tank doubling their range, and machines that work in hours rather than kilometers. These secondary tanks consume a sizeable share of the fuel bill, yet they are mostly managed with a paper logbook — or not managed at all. Some have no meter whatsoever; others are refilled from a mobile bowser with no itemised invoice, removing the very first link of any later reconciliation.

The irony is that these are precisely the tanks most exposed to loss. A reefer unit burns fuel while the vehicle is parked, so a falling tank draws no attention. A generator sits at a distant site a supervisor rarely visits. An auxiliary tank tempts because its shortage never immobilises the truck. Monitoring the main tank alone is securing the front door while leaving the windows open.

Nor is the issue purely financial. With refrigerated cargo, a neglected tank becomes a full operational risk; on contracting sites, all work stops the moment the generator goes silent. Monitoring secondary tanks is therefore insurance for both revenue and operations — not a luxury line in the tracking project.

Three practical scenarios

The reefer truck: the cooling unit has its own diesel engine running for long hours while the truck sits in a yard or at a warehouse dock. Its consumption is a function of setpoint temperature, ambient heat, and door-opening behavior — never of distance. And because the cargo is sensitive, monitoring the unit's tank is not merely a financial matter: running out of reefer fuel on a summer night means a spoiled load before anyone knows. That is why box temperature and unit fuel are managed together: multi-zone temperature readings alongside the tank level make diagnosis immediate — did the temperature rise because fuel is running out, or because the doors stayed open too long? And fuel burned in a warehouse yard rather than on the road stays invisible to any formula that divides liters by kilometers.

The fixed or mobile generator: on construction sites, farms, and telecom stations, the generator tank is filled from drums or a mobile bowser, and the industry-familiar gap between what is purchased and what actually reaches the tank is nearly impossible to prove without measurement. A sensor on the generator tank settles both questions at once: how much really entered the tank, and how much does the generator burn per running hour? As the weeks accumulate, each generator gets its own baseline, and any deviation from its rate becomes a specific, investigable question rather than a vague impression that dissolves in meetings.

The auxiliary tank: in long-haul transport, an auxiliary tank extends range and connects to the main one through a manual or automatic transfer valve. Without a sensor on each tank, the "fuel level" becomes a puzzle: a drop in the main tank with a rise in the auxiliary may be a legitimate internal transfer — or perfect cover for an external drain that no aggregate figure will ever show. The answer is reading both tanks as parallel curves: an internal transfer keeps the combined total nearly constant, while an external drain reduces it — a distinction automatic detection catches and a single aggregated number never will.

The right measurement architecture: one sensor per tank

The governing principle is simple: every tank is an independent measurement entity with its own sensor, calibration table, and rules. The table below summarizes the architecture:

Technically, modern tracking devices support multiple sensor inputs per unit, and the protocol families supported on Pixa — 18 tested families including Omnicomm, Teltonika, and Ruptela — carry multi-tank readings within the same message; see the devices page for compatibility. On the platform, each tank is defined as an independent parameter with its own name, formula, and calibration table through the general sensor-abstraction layer, and is verified at installation with the raw-data viewer before sign-off. Never copy a calibration table between two tanks, even with identical stated capacities: a reefer unit's small cylindrical tank behaves nothing like the main sectioned tank, and each has its own height-to-volume curve. The tanks' curves and per-tank refill and drain events then appear together on the fuel screen.

Engine hours, not kilometers

The biggest methodological mistake in multi-tank monitoring is measuring everything in kilometers. A reefer unit can run 10 hours while the truck never moves a metre; dividing its consumption by distance produces mathematical nonsense, and folding it into traction consumption distorts the truck's rate — making its driver look wasteful while entirely innocent.

Proper separation rests on two complementary sources:

• Running-hour counters: engine hours and reefer or generator hours are read as independent parameters, so each tank gets its own liters-per-hour rate — and a creeping rise in that rate exposes the unit's declining efficiency before it fails outright.
• PTO signal and operating state: these establish when consumption was legitimate (unit running, cargo being cooled) and when a tank dropped with no operational justification — which is precisely the definition of a suspicious drain on a secondary tank.

The picture completes when maintenance plans are driven by the same actual measurements: reefer and generator servicing scheduled by real running hours rather than calendar guesswork — one sensor serving both fuel control and maintenance. And in businesses whose contracts bill by equipment running hours, the measured rate itself becomes a reference that settles discussions with numbers instead of memory.

Rules and alerts per tank

One generic alert rule applied to tanks of different natures produces noise on one side and blindness on the other — and noise kills trust while blindness kills the business case. The right approach is dedicated rules in the rules engine, with composite conditions and time windows per tank:

• Traction tank: drain alerts by volume, time, and location — tightened during quiet hours and outside approved refuelling sites.
• Reefer tank: a drop alert during long stops, plus a low-level alert with a safety margin that protects the cargo before run-dry — alongside multi-zone box-temperature alerts from the same parameter layer.
• Generator tank: a geofence around the site and an instant alert on any sharp drop outside scheduled running hours — because a drain at a remote site is never caught by eyesight.

Alerts are delivered over 5 channels — WhatsApp, SMS, email, app push, and browser notification — with escalation, acknowledgement tracking, and flood control, so a small tank's event never drowns in the day's traffic. Day-to-day follow-up happens from the vehicle card on the live tracking screen, where tank levels, temperatures, and running hours appear together — so the supervisor treats a reefer truck as one multi-measurement unit instead of hopping between three screens, two logs, and a paper notebook.

Starting right

Begin by inventorying the secondary tanks across your fleet and ranking them by spend: reefer units usually top the list, followed by continuously running generators. Fit the biggest spenders with a calibrated sensor and its own table, run a week to establish a liters-per-hour baseline, then enable the dedicated alert rules and reconcile each tank's measured refills against its invoices monthly. The gap that surfaces in the first month is usually the strongest argument for completing the rest of the fleet. Then expand wave by wave: each wave instruments the tanks most similar to those before it, benefiting from calibration tables built with the previous wave's experience and alert rules tuned on reality rather than assumption.

Are there tanks in your fleet that nobody is watching? Contact the Pixa team via the contact page for a working session to design the right measurement architecture for your reefer trucks, generators, and auxiliary tanks.