TL;DR · DIRECT ANSWER
Natural gas is measured by four meter series. Turbine (LWGQ) covers city-gate billing DN25 to DN300 at ±0.5 to ±1.0 % of rate, 1:20 turndown, standard for AGA-7 custody installs. Roots PD (LQ) covers the small-bore residential tail down to DN25 at ±0.5 % and 1:30 turndown when low-flow registration matters most. Ultrasonic (CSUF) handles non-intrusive custody at DN25 to DN200, single-path / dual-path, 1:100 turndown, no straight-pipe constraint. Coriolis (ZLMFM) is the mass-direct universal: ±0.5 % on gas, top-tier price, kept in reserve for hydrogen, LNG, or odd-blend service.
1. Density physics and reference units
Natural gas measurement splits along three axes before the spec sheet matters: pipeline vs city-gate vs LNG; custody vs operational; gas-phase vs cryogenic. The first two decide which regulatory regime lands on the meter. The third decides whether this guide applies: LNG at −162 °C uses a cryogenic Coriolis with vacuum-jacketed sensor, not the four gas-phase series covered here.
Gas density swings strongly with temperature and pressure. Pipeline natural gas at 4 MPa and 15 °C is roughly 30 kg/m³, about 40 times denser than at atmospheric conditions, and a turbine on the high-pressure side reads volumetric flow at line conditions. Without T+P compensation the buyer invoices 30 times the actual heat content in a cold snap and 30 times less in a heat wave. The ideal-gas law also misses by 1 to 3 % at custody pressures: small on a process line, large enough to swing a million-dollar invoice over a year. The compressibility factor Z captures the deviation:
Z depends on the gas mix and line conditions. The custody-grade flow computer reads pressure and temperature live and resolves Z against a published equation of state: NIST REFPROP (US custody) or GERG-2008 (European, in ISO 20765-2). Both are buyer-arranged references, not ex-factory certifications. ProFlow meters ship with tabulated Z firmware; the buyer’s notified body verifies it against the chosen EOS at commissioning.
The contract is usually written in energy units (MJ, GJ, MMBTU, kWh), and the meter delivers a standard-volume reading paired with a heating value. Pipeline natural gas at 90 to 96 % methane sits near 39 MJ/Nm³ (HHV); industrial fuel gas off a cracker or refinery sits 30 to 48 MJ/Nm³, and the meter pairs with a gas chromatograph for the live value.
Three reference conditions show up in custody, and confusion among them is the most common invoice-dispute pattern in our quote logs:
| Unit | Reference | Definition | Where it shows up |
|---|---|---|---|
| Nm³/h | IUPAC STP | 0 °C and 101.325 kPa absolute | European custody, ISO contracts, China GB/T pipeline default |
| Sm³/h | 15 °C reference | 15 °C and 101.325 kPa absolute | Some US natural gas contracts, ISO 13443 standard reference |
| SCFM / SCFH | NIST | 60 °F (15.56 °C) and 14.73 psia | US natural gas custody, AGA reference base |
| Am³/h | Actual | At line temperature and pressure | Process monitoring; never billed |
1,000 Nm³/h at IUPAC STP equals 1,055 Sm³/h at 15 °C and 624 SCFM at NIST. On a 10,000 Nm³/h pipeline the difference between Nm³/h and Sm³/h swings the monthly invoice by tens of thousands of dollars over a year. Specify the reference at order; every ProFlow meter switches the base via HART.
2. The four meter series
Four meter series do real fiscal duty on natural gas. A fifth (vortex) covers non-custody industrial service. The decision tree in §3 maps service type to the right series.
Turbine
Rotor-and-blade; gas drag spins the rotor proportional to volumetric flow. T+P-compensated build outputs mass direct. Default for city-gate and station-yard billing.
Roots positive displacement
Twin-lobe rotor pair displaces a fixed volume per rotation. Low-flow tail registers gas a turbine would not start. Standard for residential and light-commercial billing; biogas / RNG fence-inlet AMR with the 3.6 V lithium battery option.
Ultrasonic transit-time
Paired piezoelectric crystals at oblique angle; the time difference between downstream and upstream pulses resolves mean axial velocity. No moving parts; bidirectional; near-zero pressure drop.
Coriolis mass flow
Resonant vibrating tube measures mass direct via Coriolis force; density and temperature are byproducts of the same measurement. The universal meter for hydrogen H35/H70 dispensers, LNG cryogenic custody, and mass-direct fiscal at small DN. Top-tier price; kept in reserve.
Vortex (LUGB) covers non-custody industrial work. The LUGB vortex covers DN15 to DN600 with 1:70 turndown, ±0.5 to ±1.5 % of rate on gas, and built-in T+P. The 2024 VRd model tolerates pipe vibration around a regulator station that a turbine bearing would not survive. Vortex suits industrial fuel-gas headers feeding boilers, kilns, or process heaters where the buyer wants one instrument reporting mass flow without an external flow computer.
3. Decision tree and worked example
Answer in order; the first decisive answer routes the duty. LNG is out of scope and goes to the cryogenic Coriolis page.
5 questions, answer in order
WORKED EXAMPLE: INDUSTRIAL SUB-METER
Service: DN50 fuel-gas header, four boiler trains. 1.6 MPa gauge. 0 to 2,000 Nm³/h peak. ±1.0 % target. ~2D upstream straight pipe. Budget < USD 4,500.
Tree: Q1-Q3 No. Q4 industrial sub-meter, no custody role? YES.
Recommendation: Roots LQ G100 at DN80. Convert peak demand to actual flow:
Operating pressure: 1.6 MPa gauge ≈ 1.7 MPa absolute → P ratio ≈ 16.8
Operating temperature: ~20 °C (293 K) → T ratio ≈ 1.07
Peak actual flow ≈ 2,000 ÷ (16.8 / 1.07) ≈ 125 m³/h actual
G100 (2.5 to 160 m³/h actual) covers 125 m³/h with headroom; G65 (2 to 100 m³/h) does not.
Specify G100 at DN80 with DN80×DN50 concentric reducers. The 1:30 turndown handles overnight idle. Roots PD is indifferent to velocity profile, so the straight-pipe shortage that would disqualify a turbine is not a constraint here.
4. Install rules and custody regime
Natural gas service spans five orders of magnitude in pressure, and the tier decides both the meter series and the regulatory regime:
| Tier | Pressure | Typical meter series |
|---|---|---|
| Burner-tip headers | 5 to 50 kPa gauge | Thermal mass; vortex on industrial fuel-gas |
| Commercial distribution | 0.05 to 0.4 MPa | Roots LQ; vortex; turbine LWGQ at upper end |
| City-gate handover | 0.4 to 1.6 MPa | Turbine LWGQ default; ultrasonic CSUF retrofit |
| Transmission | 2 to 10 MPa | Multi-path ultrasonic AGA-9; rotary-piston PD |
| CNG dispensing | 20 to 25 MPa | Coriolis ZLMFM custom build |
| H35 / H70 hydrogen | 35 to 70 MPa | Coriolis high-pressure hydrogen-threaded series |
DN coverage: Turbine DN25 to DN300. Roots DN25 to DN150. Ultrasonic CSUF DN25 to DN200; multi-path AGA-9 DN300 to DN800 is a different price lane. Coriolis ZLMFM DN3 to DN150; cost curve breaks above DN50.
Accuracy tiers: ±1.5 % of rate is check-meter grade. ±1.0 % covers commercial distribution and most non-fiscal industrial duty. ±0.5 % is the fiscal floor on LWGQ turbine and Class A Roots, and is what most city-gate custody specs call for. Top-tier fiscal duty climbs to ±0.25 % and ±0.15 % of flow: multi-path ultrasonic AGA-9 territory.
Install rules across the four series:
- Coalescing filter upstream: 5 µm, sized for the duty, with one DN of straight pipe between filter and meter inlet flange.
- Pressure regulator upstream: meter sits downstream so it sees stable working pressure. If pulsation is unavoidable, add an accumulator vessel.
- Straight pipe per series: Turbine LWGQ wants ≥3D upstream and ≥1D downstream (AGA-7 layers on 10D + 5D for full custody). Ultrasonic CSUF wants ≥10D upstream and ≥5D downstream. Roots LQ is indifferent to velocity profile. Coriolis ZLMFM needs no straight pipe.
- Isolation valves plus bypass: custody installs carry isolation valves on either side and a parallel bypass (valve closed in normal service). Operational installs can skip the bypass where shutdown for meter service is tolerable.
The custody regime is buyer-arranged with the local notified body. None of these reference standards ship from our factory as a certification:
- AGA-7: US turbine geometry. ProFlow LWGQ turbines are dimensionally compatible; the buyer arranges the witness sign-off through the fiscal authority.
- AGA-9: US multi-path ultrasonic geometry. The CSUF single-path / dual-path build is not an AGA-9 multi-path meter; for AGA-9 duty the buyer specifies a multi-path build through engineering quote.
- OIML R 137: international gas meter standard, dominant non-US fiscal reference.
- MID MI-002: EU Gas Meter Directive, required for European fiscal natural gas duty.
- NIST REFPROP and GERG-2008: reference EOS for Z-factor resolution, verified against the flow computer during buyer-arranged sign-off.
This split (meter ships with calibration cert and CNEX Ex d documentation; fiscal regime layered on by the buyer’s notified body) keeps the meter cost honest for the city-gate and industrial duties where most natural gas measurement actually happens.
5. Frequently asked questions
Can the same meter measure biogas and natural gas?
Mechanically yes; commercially no without recalibration. Pipeline natural gas runs 90 to 96 % methane near 39 MJ/Nm³. Post-upgrade biomethane sits in the same composition spec and uses the same turbine, Roots, or ultrasonic body, but the field K-factor is re-confirmed because residual CO₂ and N₂ shift density a percent or two. Raw biogas from a digester at 55 to 65 % CH₄ is a different problem: density drifts with feedstock, heat content swings 10 to 15 %, and the gas is wet with H₂S. That stream goes to a vortex meter sized for wet gas, or to a Coriolis on the dry side of the upgrader.
Does wet gas damage a turbine or ultrasonic meter?
It damages both. Turbines run on bearings that expect clean dry gas: entrained droplets at 5 to 10 m/s erode rotor blade tips and wash bearing lubricant, and the K-factor drifts within 6 to 12 months. Ultrasonic transit-time relies on a clean acoustic path; condensate film on the transducer port attenuates the signal. Install a coalescing filter upstream and a drip-leg downstream. For sustained wet-gas service, route the duty to a vortex meter or move measurement to the dry side of the dehydrator.
Is LNG the same measurement problem as natural gas?
No. LNG is a cryogenic liquid at −162 °C and 0.1 to 0.7 MPa, and the meter series changes accordingly. Gas-phase natural gas uses turbine, Roots, ultrasonic, or Coriolis configured for room-temperature service. LNG uses a cryogenic Coriolis with vacuum-jacketed sensor, or a positive-displacement liquid meter rated to −196 °C. The custody calculation references the heat content of the regasified gas at standard conditions, not the volume of liquid on the loading arm. LNG marine bunker, truck loading, and tank-farm transfer go to the cryogenic Coriolis page.
6. Final Thoughts
Natural gas measurement is rarely about finding the most accurate meter. It is about matching the right series to the right pressure tier, DN range, and fiscal regime, so the meter the buyer pays for is the meter the contract needs.
- Use turbine, Roots, or ultrasonic when the duty is custody or commercial billing at city-gate or station-yard pressure.
- Use Coriolis when no other series can handle the duty: hydrogen H35 / H70, LNG cryogenic, mass-direct fiscal at small DN.
A meter that does not belong on its service is the most expensive meter on the bill of materials. The four series each cover a defensible service window, and being able to defend the selection in front of procurement and the local fiscal authority is what makes the spec sheet survive contact with the city-gate.
Sizing a natural gas line? Send line data and our engineering team returns a sized quote within 4 business hours.
Specifying a natural gas meter? Send us the line data.
Pipeline pressure, flow range, line size, composition, accuracy target, and the fiscal regime you are working against. We will size the right series and return a quote within 4 business hours. The engineering team picks up direct.