Weighbridge and flowmeter disagree on quantity: which records should you compare?

Short answer: when a weighbridge and a meter (flowmeter) show different quantities for the same delivery, the first step is not to declare one of them faulty, but to check, record by record, whether both results describe the same quantity, the same system boundary and the same time window. Work through these items in order: the unit and concept of the quantity, the density and temperature used for conversion, the transfer point and loading arm fill, the tare and any product left in the tanker, weighing conditions, gas and product changeovers, the meter factor and verification dates, time stamps, and the tolerance band formed by the maximum permissible errors of both instruments.

This sequence applies to a single delivery of a single tanker, a static vehicle weighbridge and a liquid product. With LPG and LNG, vapor return and phase change enter the balance as separate items. Choosing between a weighbridge and a meter is not the subject here; this article covers data reconciliation when both measurements are already installed and their results disagree.

Mass balance: what does each instrument actually measure?

The weighbridge gives the difference between the loaded and empty weighings of the vehicle, so that difference includes everything that changed on the vehicle between the two weighings. The meter counts only the product that passes through it and reaches the transfer point. For both results to agree, this balance must hold: weighbridge net = quantity through the meter ± the start-to-end change in loading arm and hose fill between meter and tanker + other masses that changed on the vehicle between weighings (fuel, water, snow, mud, people, equipment) ± the errors of both instruments.

Comparing results without writing out this balance often makes a boundary difference look like an instrument error. A practical pointer: if the difference stays at roughly the same number of kilograms on every delivery, look first at boundary items (tare, arm fill, product left in the tanker); if it repeats as a percentage proportional to the delivered quantity, density, temperature conversion and the meter factor move up the list. This is not a diagnostic rule; it shows where to start.

First make sure you are comparing the same quantity

A meter delivery ticket may carry more than one quantity: volume at metering conditions, volume converted to base conditions, and mass. OIML R 117-1 separates metering conditions (temperature and pressure of the liquid at the point of measurement) from base conditions and defines the conversion device, which converts volume to base volume or to mass, as part of the measuring system. The value to compare with the weighbridge is mass; if the meter measures volume, read from the record which density and which temperature were used to get to mass.

Density and temperature: operating or base conditions?

A common inconsistency is multiplying volume at metering conditions by density at base temperature. A consistent pairing is either metering volume × density at the same conditions, or base volume × base density. Also ask where the density came from: online measurement, laboratory analysis or a fixed table value, and which batch the sample was taken from. For petroleum products the correction algorithm is given in API MPMS 11.1, which does not cover LPG or NGL; for chemicals the density-temperature relationship is set with product-specific data and agreed between the parties. API MPMS 12.2 aims, through calculation sequence and rounding rules, for the same data to give the same result whoever performs the calculation; check whether both parties use the same calculation method.

A weighbridge result is a weighing-in-air value

OIML R 76-1 states that mass in weighing instruments is preferably used in the sense of conventional mass, the conventional value of the result of weighing in air; OIML R 117-1 likewise requires mass indications of liquid measuring systems to be labeled as mass or conventional mass. A Coriolis meter measures the mass of the product directly. Using the defining conditions of OIML D 28 (air 1.2 kg/m³, reference density 8 000 kg/m³), the difference between the two concepts for a liquid of 860 kg/m³ is about 0.12%, or about 32 kg on a delivery of 25 300 kg. Check which concept the ticket reports and which one the contract is based on.

System boundary: transfer point, arm fill and tare

·         Transfer point: Under OIML R 117-1 a measuring system includes at least one transfer point and is designed either as an empty hose or a full hose system; the term hose also covers rigid pipework. The pipework between the meter and the transfer point is to be kept full during measurement and shutdown. The question is: were the arm, hose and intermediate piping in the same fill state at the start and end of this delivery?

·         Line pack: On the first loading after maintenance, draining or a product changeover, the meter also counts the product that refills the line, but that product never reaches the tanker. Conversely, if product left in the arm from the previous delivery drains into the tanker, the weighbridge weighs product the meter did not count on this delivery. Arm, hose and intermediate pipe volumes can be calculated from drawings and compared with the difference.

·         Tare and tare timing: Was the empty weighing done immediately before loading, or was a stored tare used? A stored tare does not capture fuel consumption, the driver, vehicle equipment or changes in product left in the tanker. OIML R 76-1 states that maximum permissible errors apply to gross loads and, when a tare device is in operation, to net loads, but do not apply to calculated net values when a preset tare device is in operation.

·         Product left in the tanker: Product left in a compartment or tanker piping from a previous delivery does not enter the net if it was weighed in the tare; it creates a difference if it was drained after the tare was taken or if the stored tare contained a different amount.

·         Weighing conditions: Are all axles on the platform, is the vehicle touching anything off the platform, are the platform edges clean? Wind and water, snow or mud accumulated on the vehicle enter the result. OIML R 76-1 separately tests eccentric loading and tilting; these tests do not replace correct positioning of the vehicle on the platform. Weighing in motion is a separate instrument category under OIML R 134-1.

Air, vapor, product changeover and sampling

OIML R 117-1 treats the gas elimination device, which keeps air or undissolved gas in the liquid out of the measurement, as part of the measuring system, and notes that gas pockets can form through thermal contraction during shutdown or when the supply tank runs empty; a systematic air inlet is not considered a disturbance. For reconciliation, request these records:

·         Gas separator or gas indicator alarms, pump suction pressure drops, low level in the supply tank;

·         For Coriolis meters, sudden fluctuations in density and diagnostic indications during loading;

·         Product changeover: previous product left in the line, density used during the transition and product-specific parameter selection;

·         Sampling: batch and point of sampling, analysis temperature and report date.

Why can two compliant instruments read differently? Permissible errors

Even when both instruments operate within their own maximum permissible error (MPE), their results may differ. For instruments under legal metrology control the framework is set out in OIML Recommendations:

·         Static weighbridge (OIML R 76-1): MPE is expressed in verification scale intervals, e. For a class III instrument at initial verification it is ±0.5 e for loads up to 500 e, ±1.0 e between 500 e and 2 000 e, and ±1.5 e between 2 000 e and 10 000 e; the MPE in service is twice the MPE on initial verification.

·         Liquid measuring system (OIML R 117-1): Systems not assigned another class in the table are class 0.5; measuring systems on road tankers and the unloading of road tankers are listed explicitly in this class. In class 0.5 the MPE is 0.5% of the measured quantity for the complete measuring system and 0.3% for the meter itself. Liquids with a viscosity above 1 000 mPa·s and liquids measured below −10 °C or above +50 °C fall into class 1.0.

The class of the installed instrument and whether the transaction is under legal metrology control are determined by the data plate, approval certificate, verification record and national legislation; in contract-based applications the acceptance limit is defined in the contract. The table below is an educational example that only conveys orders of magnitude.

Example tolerance band (educational assumptions)

A linear sum is not an uncertainty calculation; it shows the worst case in which both instruments sit at the edge of their limits at the same time and in opposite directions. Under the example conditions, a difference of 100 kg does not by itself prove that either instrument is out of limits; conversely, a difference inside the band that repeats in the same direction on every delivery calls for a search for a systematic cause.

Example scenario: same volume, different density and temperature assumptions

The figures below are not real product or project data; they were chosen only to show the size of a conversion error. Assumptions: a volumetric meter shows 30 000 L at metering conditions; product density at 15 °C is 0.860 kg/L; product temperature is 35 °C; a simplified linear volume correction VCF = 1 − β × (T − 15 °C) with β = 0.0010 1/°C. Density at 35 °C is then 0.860 × 0.980 = 0.8428 kg/L. The weighbridge net for the same tanker is 25 300 kg.

Four mass calculations for the same 30 000 L (example scenario)

The pairing error in row B produces a difference of about 500 kg (about 2%) against the weighbridge and looks like a meter fault, although the meter counted the volume correctly. Row D shows that, under this assumption, each 1 °C deviation in the temperature record shifts the mass by about 0.1%. In real products expansion depends on the product and temperature; this example does not replace the calculation method defined in the contract or a calculation software.

Transaction record comparison form

The form puts both sides' records for a single delivery side by side. For every row where the match check fails, ask whether that item explains the difference; only the unexplained remainder is taken forward to instrument verification.

Transaction record comparison form

Review sequence

1.       Match the records: same vehicle, compartments, batch and time window.

2.       Align unit and concept: compare mass with mass; note the conventional mass difference.

3.       Recalculate the conversion: do volume, density and temperature refer to the same conditions?

4.       Separate boundary items: tare timing, product left in the tanker, arm fill.

5.       Check gas, product changeover and alarm records.

6.       Compare the remaining difference with the tolerance band and track it over several deliveries: constant kilograms or constant percentage?

7.       If a systematic difference persists, have the meter verified by the defined method (prover or master meter) and the weighbridge with test weights. Do not change the meter factor based on a single weighbridge comparison; sealed parameters and adjustments under legal metrology are changed through the authorized procedure.

Limits of this approach

·         The form does not make a remote diagnosis; it shows possible causes and which record to examine.

·         The weighbridge is not automatically the reference; the basis of measurement and the steps for disputes are defined in the contract and legislation. Terminal-level stock control and settlement of measurement disputes are covered separately in industry guides such as EI HM 32.

·         With LPG and LNG, vapor return and phase change are separate items; this form alone is not sufficient.

·         In multi-compartment loading the weighbridge gives only the total; per-compartment differences can be separated only by intermediate weighing.

·         A difference on a single delivery may not show a trend; a series of deliveries is needed for a decision.

Data to share for a review

·         Weighing tickets for the deliveries showing the difference, and the meter delivery tickets for the same transactions;

·         Delivery series for recent weeks: date, product, both quantities and difference (kg and %);

·         Density source and conversion method used;

·         Meter factor history and last verification report; weighbridge class, e value and last verification date;

·         Transfer point type, arm and intermediate pipe volume; alarm, maintenance and changeover records;

·         Basis of measurement and acceptance limit in the contract.

Depending on project scope, TLY Enerji can provide engineering support in structuring record comparison, measurement point and instrument selection, integration of the meter and recording system, and testing and commissioning. Legal metrology verifications and calibrations requiring accreditation are carried out by authorized bodies.

Related pages

·         custody transfer metering for chemicals: Covers choosing between weighbridge and meter and setting the basis of quantity.

·         custody transfer truck loading skids: Meter verification methods, sealing and delivery records.

·         Coriolis mass flow and density meters: Direct measurement of mass and density, gas bubbles and zero stability.

·         Coriolis versus positive displacement meters for truck loading: Discusses whether to measure mass or volume.

·         flow computer versus PLC: Delivery records, calculation traceability and event logs.

Frequently asked questions

Above what value is a weighbridge-meter difference a problem?

There is no universal value. The lower threshold is the band formed by the permissible errors of both instruments; the acceptance limit is defined in the contract or legislation. A difference inside the band that repeats in the same direction should still be investigated.

Can the meter factor be corrected to match the weighbridge?

Correction based on a single comparison is not recommended. First eliminate the record items; if a systematic difference persists, verify the meter by the defined method.

If a Coriolis meter measures mass, why do density and temperature still matter?

When mass is measured directly, conversion error disappears; but if base volume is reported, density and temperature determine that value. Gas bubbles and zero stability can also affect the mass reading, and since the weighbridge gives conventional mass, a small conceptual difference remains.