What Is a LACT Unit?
A LACT unit (Lease Automatic Custody Transfer) is a skid-mounted measurement package that automatically meters, samples, and quality-checks crude oil as it changes hands from a producer's lease to a pipeline or trucking company. It replaces manual tank gauging with a continuous, auditable, unattended measurement point.
LACT Unit in one line: A LACT unit is an automated skid that meters crude oil volume, corrects it for temperature and water content, and grabs representative samples so ownership can transfer without an operator manually gauging a tank.
What a LACT Unit Does
A LACT (pronounced "lact") unit sits between the lease tank battery and the buyer's pipeline or truck. Its job is to answer one question with legal defensibility: exactly how many net standard barrels of merchantable crude were delivered? To do that it measures gross volume with a flow meter, records temperature and sometimes pressure, and continuously monitors basic sediment and water (BS&W) so that off-spec oil high in water is diverted rather than sold.
The measured gross volume is converted to net standard volume by applying correction factors: a volume correction factor for temperature (per API MPMS / API 11.1), a meter factor from the last proving, and a deduction for entrained water. The result is the number that appears on the run ticket and the sales invoice, which is why the skid must be metrologically sound and tamper-evident.
Typical LACT Skid Components
A conventional LACT skid includes a charge pump, an air eliminator and strainer to protect the meter, a positive-displacement or Coriolis meter for volume or mass, a temperature element and sometimes an RTD/pressure transmitter, a BS&W probe or capacitance water monitor, an automatic sampler that collects a proportional composite for lab analysis, a back-pressure valve to keep the meter full and liquid, and a diverter valve that rejects oil that fails the water spec.
Increasingly the volume element is a Coriolis meter, which reports mass flow and density directly and needs no proving loop hardware of its own, though it is still proved against a master meter or prover. A flow computer on the skid handles the API volume corrections, holds meter factors, drives the sampler pacing, and logs the run tickets. Because the flow computer typically speaks Modbus, a cloud SCADA platform can poll gross and net totals, temperature, BS&W, and alarm state directly from it.
Frequently Asked Questions
What does LACT stand for?
LACT stands for Lease Automatic Custody Transfer. "Lease" because it sits at the production lease, "automatic" because it operates unattended, and "custody transfer" because it is the metering point where ownership of the crude passes from seller to buyer.
How is a LACT unit's meter kept accurate?
The volume meter is periodically "proved" against a known reference - a bidirectional prover, small-volume prover, or master meter - to derive a meter factor that corrects for wear and drift. Provings are scheduled on a throughput or calendar basis per the sales contract, and the resulting meter factor is entered in the flow computer so every subsequent barrel is corrected.
Can a SCADA system monitor a LACT unit?
Yes. The skid's flow computer exposes gross and net totals, temperature, BS&W, diverter and pump status, and alarms, usually over Modbus. A cloud-native SCADA platform like Merobix can poll those registers to trend production, catch a stuck diverter or failed sampler, and alert operators to off-spec runs without anyone standing at the skid.
Sources and verification
This page references the protocol specifications published by the organizations below. Editions, product capabilities, and documentation change over time - confirm current requirements and specifications directly with the source.
- API Manual of Petroleum Measurement Standards (MPMS) - American Petroleum Institute
- Modbus Application Protocol Specification - Modbus Organization
Merobix is not affiliated with, endorsed by, or sponsored by these organizations; their names are used only to identify the standards and products discussed.
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