When a chemical changes hands, the contract, not the meter, defines what quantity is being sold. Chemicals may be invoiced by mass, by volume corrected to a reference temperature, or by the amount of active ingredient, and that choice drives the measurement method. Meter-based custody transfer measurement is a sound option when the product stays single-phase and stable while it is metered, when the meter's wetted materials are proven compatible with it, when there is a practical way to verify the meter in the field, and when buyer and seller both accept that method. If the transaction falls under legal metrology, the measuring system must also meet whatever national regulations require.
The general make-up of a custody transfer skid for truck loading, and how such skids are verified, is covered on the custody transfer truck loading skids page. This page concentrates on chemical products: how the quantity basis is chosen, and when a meter can replace a truck scale or work alongside it.
Choosing the sales basis: mass, volume or active ingredient
Quantity basis | How it is determined | What to watch for with chemicals |
Mass | Gross minus tare weighing on a truck scale, or a mass flow meter | Independent of temperature; with a meter, entrained gas and material compatibility become the deciding factors |
Volume at reference temperature | Volumetric meter plus temperature measurement and product-specific expansion data | Petroleum volume correction tables were developed for crude oil, refined products and lubricating oils; for a chemical, the density-temperature relationship must come from product-specific data agreed by both parties |
Active ingredient or quantity at a stated concentration | Measured mass multiplied by concentration | The contract must state whether concentration comes from laboratory analysis or inline density, and which result prevails in a dispute |
Before designing the measuring system for a chemical terminal, it pays to obtain the unit of sale, the reference conditions and the concentration definition written into the sales contract. If the same product is sold to different customers on different bases, the system should be configured to report both.
Truck scale or flow meter?
Weighing a tanker on a truck scale before and after loading is long-established practice at many chemical plants. A key advantage of the scale is that it never touches the product, so material compatibility simply is not an issue with corrosive, crystallizing or very viscous products. International recommendations for weighing instruments state that maximum permissible errors apply to the gross load, and to the net load when a tare device is used, but not to net values that are calculated. A quantity obtained from a full-minus-empty weighing therefore carries the uncertainty contributions of two separate weighings.
A meter, by contrast, measures in real time and compartment by compartment: the batch controller stops the flow when the target is reached, each compartment is recorded separately, and the queue at the scale disappears. The trade-off is that the meter contacts the product, is sensitive to gas and deposits, and must itself be verified. Weigh-in-motion of road vehicles is a separate instrument category from static scales and is assessed against its own requirements.
Criterion | Truck scale | Meter-based measuring system |
Product contact | None, so no material compatibility question | Yes; wetted materials are selected from manufacturer data |
Quantity per compartment | Only with intermediate weighings | Each compartment and batch measured separately |
Stopping at the target quantity | Indirect; separate fill control is needed | Direct, through the batch controller |
Effect of gas and bubbles | Not affected | Can corrupt the measurement; gas elimination is needed |
Verification | Test weights and periodic inspection | Master meter, comparison with the scale, or a prover, depending on the product |
Operations | Two weighings per vehicle and a queue | Continuous flow at the loading bay |
When is meter-based custody transfer appropriate?
· Single-phase, stable flow: The product does not release gas, foam or leave deposits inside the meter during measurement.
· Proven material compatibility: The meter manufacturer has compatibility data for the actual concentration and temperature of the product; as explained on the chemical metering skids page, the meter material is not inferred from the pipe material.
· A workable verification method: The product's hazard profile allows field comparison against a prover or master meter; if not, a periodic comparison with the truck scale can be defined instead.
· Acceptance by both parties: Buyer and seller agree to invoice on the meter reading, and a written dispute procedure exists.
· Regulatory fit: Where the transaction is subject to legal metrology, the measuring system as a whole meets national requirements.
Where legal metrology applies
The international recommendation for dynamic measuring systems for liquids other than water lists liquid chemical products among the liquids it covers. It applies, however, to systems subject to legal metrological control, and whether a particular chemical sale is subject to such control is decided by national legislation. The same documents set separate error limits for the meter and for the complete measuring system. A meter's datasheet accuracy does not include gas elimination, temperature and density conversion or installation effects, so a meter approval does not by itself mean the whole measuring system complies.
Uncertainty sources specific to chemicals
· Unloading or purge gas: Nitrogen used to push product or blow down a line can pass through the meter and distort both mass and density readings.
· Line condition at the transfer point: Whether the line downstream of the meter is left full or empty must be the same at every delivery.
· Coating inside the tubes: With crystallizing or polymerizing products, a layer building up in the meter tubes can shift density and zero; cleaning and zero checks should be written into the procedure.
· Temperature and concentration changes: Temperature directly affects results on a volume basis, and concentration data directly affects results on an active-ingredient basis.
Records, sealing and reconciliation
In custody transfer, the record for each batch covers product, compartment, quantity, temperature, density, concentration where relevant, and alarms. The meter factor and calculation parameters are protected by a seal or access control, and every parameter change is written to an event log. When a scale and a meter are used together, the acceptable difference between their results and the action to take if it is exceeded should be agreed in advance. That comparison also happens to be a practical way to watch how the meter behaves in service.
How TLY Enerji can help
For chemical custody transfer projects, TLY Enerji can provide engineering support in selecting a measurement technology that matches the quantity basis, supplying meters and temperature and pressure instruments, integrating the batch controller and recording system with PLC or SCADA, and in site installation, testing and commissioning. Which organization handles legal metrology approval and verification services is determined separately for each project and country.
Information needed to settle the measurement basis
· The quantity unit in the sales contract: mass, volume at reference temperature, or active ingredient
· Reference temperature, concentration definition and how concentration is determined
· Existing truck scale infrastructure and whether the scale will remain the commercial basis
· The product's tendency to release gas, crystallize or polymerize
· Number of compartments per tanker and whether per-compartment records are required
· Whether the transaction falls under legal metrology
· Preferred verification method and verification frequency
Related pages
· Chemical metering skids: Meter technology and wetted material selection.
· Custody transfer truck loading skids: General layout of a custody transfer skid and its verification approach.
· Batch metering systems for chemicals: Stopping at the target quantity and keeping batch records.
· Chemical truck unloading systems: How gas at the end of unloading affects measurement.
Custody transfer FAQ
Are chemicals sold by mass or by volume?
That depends on the contract. Mass is a frequent choice because it does not depend on temperature, but chemicals are also sold by volume or by active ingredient. On a volume basis, the contract should define the reference temperature and the product-specific expansion data; on an active-ingredient basis, it should define how concentration is determined and which result prevails if the parties disagree.
Can a flow meter fully replace a truck scale?
A meter can serve as the commercial basis if the product is single-phase and stable, the meter materials are compatible with it, and a workable verification method exists. Even so, many sites keep the scale as a backup or comparison tool. Because the scale never touches the product, it has an edge with corrosive or crystallizing chemicals, while the meter offers per-compartment measurement and a clean stop at the target quantity.
Is a chemical sale subject to legal metrology?
It depends. The international recommendation includes liquid chemical products among the liquids covered by dynamic measuring systems, but national legislation decides whether a given sale is under legal control. In contract-based sales between businesses, the parties may agree on the measurement method themselves; the scope should still be confirmed against the metrology regulations of the country concerned.
Is the meter's density reading enough to set concentration for invoicing?
Usually not on its own. In two-component solutions, concentration can be calculated from density, but a third component or dissolved gas will skew the result. That is why active-ingredient sales are commonly based on laboratory analysis, with inline concentration used for continuous monitoring and deviation alerts. The contract should state which result is binding.