Archived technical note, republished as independent reference material. Not affiliated with any manufacturer; no product is offered.
The M40 is a 4-inch (DN100) rotary positive-displacement meter — a high-flow member of the Liquid Controls M-series built for depot loading and large petroleum transfer. This note explains what the size is for.
High-Flow Custody Transfer
A 4-inch bore lets the M40 pass high volume at sensible velocity, keeping the PD element in its accurate window at loading rates that would over-run a 2-inch meter. It is a custody-transfer instrument: accuracy in the ±0.2% of reading region, high repeatability, and provability against a pipe prover, meeting the intent of OIML R117 and the API MPMS.
Part of a Loading System
Like all bulk meters, an M40 is engineered into a skid: a large air eliminator to strip vapour on pump start-up, a strainer to guard the rotors, a preset controller and control valve for clean batch cut-off, and temperature compensation to correct volume to a reference temperature. The bigger the flow, the more these accessories matter — a vapour slug or a valve slam at 4-inch flow is a large measurement error.
Proving and Maintenance
High-flow custody meters are proved regularly — often against a dedicated pipe prover installed at the terminal — because at these volumes even a fraction of a percent of error represents a meaningful quantity of product over a day's loading. Design the connections for a prover from the outset, keep the strainer clean, and watch the differential pressure across the meter as an early warning of a fouled element. With disciplined proving, an M40-class meter holds custody-grade accuracy across many years of heavy service.
Comparison in the Range
The M40 sits above the mid-range M7 and M10. For the family context see the Liquid Controls overview; for proving and accuracy classes see calibration and accuracy.