The billing unit for liquefied petroleum gas (LPG) is set by national regulations and the sales contract, not by the meter. Quantity may be reported as mass, as volume corrected to a reference temperature, or both. Once the unit is fixed, four decisions define custody transfer measurement: where density comes from, which LPG-specific method corrects for temperature and pressure, how product returning through the vapor line is accounted for, and how the result is verified. If the contract leaves any of these open, the parties will end up with different numbers no matter how good the meter is.
Keeping the product liquid in the meter is a hardware question covered on the LPG Metering Skids page; this page follows how a measured LPG value becomes a billable quantity.
Billing basis: mass or reference volume?
LPG density depends strongly on both the propane to butane ratio and temperature, so the same volume can represent quite different masses. That is why mass is a common basis in LPG trade, while some markets and contracts use volume corrected to a reference temperature. The chosen basis shapes the measurement chain:
Billing basis | Typical measurement chain | Input that drives the result |
Mass, dynamic | Direct mass from a Coriolis meter, or mass inferred from a volumetric meter plus in-line density | Meter operating in the liquid phase; for inferred mass, the accuracy of density measurement |
Volume at reference temperature | Volumetric meter with temperature and pressure measurement and an LPG-specific correction, or mass divided by reference density | Correct reference density and the correction method applied |
Mass, static weighing | Tanker weighed on a weighbridge before and after loading | Metrological status of the scale and conditions of gross and tare weighings |
Equivalent liquid volume by component | Component volumes calculated from measured mass and a representative compositional analysis | Sample representativeness and analysis quality |
The last row is used mostly for mixed natural gas liquids but can appear in composition-based LPG contracts. Settle the basis early: meter technology, density measurement and proving method all follow from it.
Where density comes from
Density is the bridge between mass and volume, and for LPG its source should be written into the contract. There are three options:
· In-line measurement: density from the Coriolis meter or a separate densitometer. The value stays current as product changes, but it is measured at line conditions and has to be corrected to reference conditions.
· Laboratory analysis: analysis of a sample drawn from pressurized LPG. Methods such as floating piston cylinders are used so that light components are not lost; a poor sample means wrong density and composition.
· Calculation from composition: an international method calculates density and vapor pressure from component factors, but it is intended for product quality specifications, not for custody transfer quantities.
A calculated density printed on a quality certificate should not be the basis of the invoiced quantity unless both parties have expressly agreed to it.
Temperature and pressure correction for LPG
The generalized volume correction factors used for crude oil and refined products do not cover LPG or natural gas liquids. The American Petroleum Institute's Manual of Petroleum Measurement Standards (API MPMS) has a separate temperature correction procedure for LPG, and it takes the product's density at reference temperature as an input. Pressure correction likewise requires the equilibrium (bubble point) pressure at metering conditions. Because LPG vapor pressure is usually not measured directly, a separate method defines how to estimate it from routinely measured data.
Reference temperature is not a single fixed value either: the relevant standards provide calculations at 15 °C, 20 °C and 60 °F. National regulations and the contract decide which one applies, and it must be settled before the flow computer or batch controller is configured.
How vapor return affects the delivered quantity
In closed LPG loading, vapor in the tanker flows back to storage through the vapor return line as liquid enters, and that vapor carries product mass. The International Organization of Legal Metrology recommendation OIML R 117-1 accepts a gas-phase connection between supplying and receiving tanks for liquefied gases under pressure only if the returned gas is measured and subtracted from the delivered quantity; on measuring systems mounted on road tankers, such a connection is prohibited outright. Some custody skids therefore add a second meter on the vapor line and calculate net delivery as liquid in minus vapor returned. Outside legal metrology, the parties decide how returned vapor is treated, and if that decision is not in writing, reconciliation differences are inevitable.
Accuracy class and legal metrology
OIML R 117-1:2019 places liquefied gases under pressure measured at or above −10 °C in accuracy class 1.0. In that class the maximum permissible error is 1.0 % for the complete measuring system and 0.6 % for the meter itself. The limit is wider than class 0.5, which is typical for fuel loading, and reflects LPG-specific effects such as vapor-phase control and density uncertainty. These figures are not a compliance promise: national legal metrology rules decide which class and which approval and verification procedure apply, and type approval of the meter alone does not cover the whole system.
How LPG meters are proved
Prover and master meter methods are defined for single-phase liquid hydrocarbons, which for LPG means keeping the product above its vapor pressure during proving as well. The prover or master meter circuit has to be pressure-rated, and product left at the end of proving goes back into the closed system rather than to atmosphere. Coriolis meters can also serve as master meters. Some sites periodically compare meter results with weighbridge readings; that is a useful check, but scale error limits apply to gross and tare weighings, not to the net value calculated from them. Periodic checks of the densitometer and temperature measurement belong in the same verification plan.
What an LPG delivery ticket should record
· Product identity and density source: in-line, laboratory or a value fixed by contract
· Gross liquid quantity, quantity returned through the vapor line and net delivery
· Average temperature, meter outlet pressure, line density and reference density
· Correction method and reference temperature used
· Meter identity, current meter factor and date of the last proving
· An event log showing parameter changes and alarms
Recognized practice for electronic liquid measurement covers audit trail, reporting, verification and security alongside the calculation, including inferred mass systems that combine a volumetric meter with an in-line densitometer.
How TLY Enerji contributes
TLY Enerji provides design, procurement and engineering for LPG truck loading and unloading stations, and its product range includes Coriolis mass flow and density meters, temperature and pressure instruments and control valves. Depending on project scope, support can cover selecting measurement technology to suit the billing basis, supplying instruments for the liquid and vapor lines, installing and commissioning flow meters, and data acquisition and reporting through PLC and SCADA. Legal metrological inspection and verification are carried out by authorized bodies.
What to share for a custody transfer review
· Billing unit and reference conditions in the contract
· Legal metrology regime at the site and the expected accuracy class
· Source of density and composition data; sampling arrangement
· Whether a vapor return line exists and how returned vapor is accounted for
· Existing weighbridge, prover or master meter facilities
· Delivery ticket format and the systems that receive the data
Related pages
· LPG metering skids: Back pressure and gas elimination hardware that keeps the meter in the liquid phase.
· LPG vapor return systems: Function and sizing of the vapor line and its effect on metering.
· Propane and butane metering: How product differences affect density and correction.
· Custody transfer truck loading skids: The fluid-independent custody skid and its proving approach.
· LPG truck loading systems: The closed loading chain and fill limits around the metering point.
LPG custody measurement questions
Is LPG invoiced by mass or by volume?
Both are used, and national regulations and the contract decide. Because LPG density varies noticeably with composition and temperature, mass, which those changes do not affect, is a common basis. On a volume basis, the quantity is corrected to reference temperature with an LPG-specific method, and getting reference density right has a direct effect on the result.
Can the density on a quality certificate be used for billing?
Use caution. Quality certificates often show density calculated from composition, and that method is intended for product specifications, not custody transfer quantities. Unless both parties expressly accept such a value, the invoiced quantity should rest on density measured in-line or determined in the laboratory from a representative sample.
Can a loading system with a vapor return line be used under legal metrology?
Under OIML R 117-1, except for systems mounted on road tankers, a gas-phase connection is acceptable if the returned gas is measured and subtracted from the delivered quantity. Separate measurement on the vapor line and a net calculation are therefore planned in. How national rules apply this requirement varies by country and should be checked at project start.
If meter and weighbridge results differ, which one governs?
The contract should state in advance which measurement governs. If the difference is systematic, meter factor, density source, the treatment of returned vapor and the metrological status of the scale are reviewed together.
Can an LPG meter be proved the same way as a fuel meter?
The methods are similar but the conditions differ. The prover or master meter circuit must run under pressure and keep the product above its vapor pressure throughout; otherwise two-phase flow corrupts the proving result.