Short answer: neither technology is better in general; the right choice depends on whether you are designing a new station or renewing an existing orifice station, and each case asks different questions. In a new station, where piping, regulator location and filtration can be designed together, ultrasonic metering is a strong candidate for points with a wide or seasonally changing flow profile, a tight allowable pressure loss, bidirectional flow or a need for continuous condition monitoring. Orifice metering remains a balanced option where the flow range is narrow, where a removable, dimensionally inspectable primary element is preferred, and where maintenance teams know differential pressure technology.
At an existing orifice station, the first question is not "which technology" but "is the problem caused by the technology or by the installation". Problems often come from insufficient straight pipe, missing flow conditioning or a DP range that no longer fits the flow profile, and can be solved within the orifice concept. This guidance applies to single-phase, clean or filtered, non-pulsating natural gas; at custody transfer points the contract and applicable regulations have the final say.
Why the two scenarios lead to different decisions
In a new station the required conditions can be designed in: the meter goes upstream of or away from the regulator, inlet straight pipe and a flow conditioner are in the layout from the start, and filtration is sized for gas quality. The decision is driven mainly by the flow profile, the pressure budget and the verification policy.
At an existing station many variables are fixed: run length and flange spacing, regulator location, installed fitting type, spare parts stock, staff skills, flow computer configuration and historical measurement records. Changing technology is not just a meter swap; the verification regime, the calibration program and, in most cases, the counterparty's acceptance change as well. EN 1776 covers major changes of existing gas measuring systems as well as new ones, so a conversion should usually be treated like a new measuring system.
Criteria that change the decision
· Flow range and annual profile: With an orifice, flow is proportional to the square root of differential pressure. With the same plate, when flow falls to 1/10 the measured DP falls to 1/100, and at 1/5 of flow it is about 4% of its maximum; relative error grows at the bottom of the transmitter range. Annual operating hours at each flow therefore matter. With ultrasonic meters, resolution at very low gas velocity and noise and pressure loss at very high velocity are the limits; the numerical range must be confirmed with manufacturer data.
· Allowable pressure loss: Measured DP and permanent pressure loss are not the same. Part of the differential pressure across the plate is recovered downstream; the permanent loss depends on the beta ratio, and the 2022 edition of ISO 5167-1 contains an annex dedicated to this topic. A full-bore ultrasonic body has no element that restricts the flow, but if a flow conditioner is used, its loss must be added to the pressure budget.
· Straight pipe, flow conditioning and available run length: Both technologies need a developed, swirl-free velocity profile. The AGA 3 / API 14.3.2 abstract states that orifice installations not upgraded to the standard may have bias errors due to inadequate flow conditioning and straight pipe. The required length depends on the beta ratio for orifices and on path count and conditioner use for ultrasonic meters; at an existing station, the available run length is what decides.
· Regulator noise: Pressure regulators and control valves also generate noise above the audible range. This noise can reduce the signal-to-noise ratio of an ultrasonic meter; its spectrum depends on regulator type, trim, pressure drop and flow, and a regulator located upstream of the meter is the more critical arrangement. For ultrasonic metering this criterion is a screening question.
· Contamination and liquid carry-over: Both ISO 5167 and the standards for ultrasonic gas meters assume single-phase flow. With an orifice, edge damage or deposits on the plate bias the flow calculation, and the bias does not show up in the flow computer by itself. With ultrasonic meters, deposits and liquids affect the acoustic paths and the profile; diagnostics can reveal this but do not correct it. Settle filtration and liquid separation before choosing the technology.
· Diagnostics: Ultrasonic meters report per-path speed of sound, gain, signal-to-noise ratio and profile indicators; comparing measured speed of sound with the value calculated from gas composition is described in ISO 17089-1 as a consistency check. The 2022 edition of ISO 5167-1 also covers diagnostics and condition monitoring for differential pressure meters, but whether an existing orifice station has this infrastructure must be checked separately.
· Verification and calibration approach: Within standard geometry and application limits, orifice flow can be calculated with the standard equations without flow calibration; confidence rests on dimensional inspection of the plate and the meter tube. The 2022 edition of ISO 5167-2 also defines calibration as an alternative to the standard equations. For ultrasonic custody transfer meters, accuracy is established by flow calibration; ISO 17089-1 covers calibration and diagnostics for meter verification. The recalibration interval is set by the contract, regulations and the operator's measurement policy.
· Spare parts and field maintenance skills: Orifice maintenance means plate inspection, plate and seal stock, DP transmitter checks and impulse line upkeep. For ultrasonic meters, transducer and electronics spares, staff able to interpret diagnostic data and access to manufacturer support are decisive.
· Contract and standards: ISO 5167-1 and -2 define the scope for differential pressure devices and orifice plates, AGA 3 / API 14.3 for orifice metering of natural gas, and AGA 9 and ISO 17089-1 for multipath ultrasonic gas meters. If the contract requires a specific method, a technology change needs the counterparty's acceptance.
Unweighted decision matrix for two scenarios
The matrix below has no scores or weights. For each row, mark your own station as "favors orifice", "favors ultrasonic", "neutral" or "unknown". There is no scoring because a single screening question, such as unsolvable liquid carry-over or a contractually required method, can override all other rows.
Questions per criterion for a new station and for an existing orifice station
Criterion | Question for a new station | Question for an existing orifice station | Which way does the answer point? |
Flow profile | Can the annual flow distribution be covered by one plate and one DP range? | Is the problem at low flow? Would a second DP transmitter, a plate change or a parallel run be enough? | A wide, continuously changing profile favors ultrasonic; a narrow profile favors orifice |
Permanent pressure loss | How much loss can the pressure budget accept for metering? | Does the present loss really limit operation, or is it being confused with measured DP? | A tight pressure budget favors ultrasonic |
Run length and flow conditioning | Does the layout allow the required straight pipe and conditioner? | Does the existing run comply with the standard? Would a new meter and conditioner fit the same space? | A layout that does not fit is a measurement risk for both technologies |
Regulator location | Can the meter be placed upstream of or away from the regulator? | Can signal quality be verified at commissioning without moving the regulator? | A nearby regulator that cannot be moved is a screening question for ultrasonic |
Contamination and liquids | Are filtration and liquid separation included in the design? | Do plate inspection records show deposits or traces of liquid? | Unsolved liquid carry-over limits both technologies |
Need for diagnostics | Are continuous condition monitoring and remote assessment required? | Can the existing control system receive and store diagnostic data? | A requirement for continuous monitoring favors ultrasonic |
Verification approach | Does the operator prefer dimensional inspection or flow calibration? | Can a new calibration program and backup metering during calibration be planned? | A preference for physical inspection in the field favors orifice |
Maintenance skills | Which technology can the team maintain independently in the field? | Team and spares are set up for orifice; would a change require training? | Installed skills favor the existing technology |
Contract and standards | Which metering standard does the contract reference? | Are counterparty acceptance and a transition period defined for a technology change? | If a specific method is required, the contract decides |
Screening questions first
1. Does the contract or regulation require a specific metering method or standard?
2. Can single-phase, non-pulsating flow be ensured with filtration and liquid separation?
3. For an orifice, are the ISO 5167-2 application limits (50 mm–1000 mm pipe diameter, Reynolds number above 5000) met?
4. For ultrasonic, are the meter-to-regulator arrangement and the available run length acceptable?
Existing orifice station: diagnose first, convert second
Example scenario: at a city gate station the gap between winter and summer flows has widened, the DP transmitter runs at the bottom of its range in summer, and the operator is considering an ultrasonic conversion. The scenario is illustrative, not a real project.
The first step is to define the problem within the orifice setup: plate inspection records, beta ratio, DP transmitter ranges, straight pipe and flow conditioner condition are reviewed together. A flow profile problem can be solved with a staged second DP transmitter, seasonal plates with a different beta ratio, or a parallel run. If plate changes should be possible without shutting down the line, the fitting type is reviewed too; for example, the manufacturer document for EPS Leading meter2 states that an installed meter1 single-chamber fitting can be upgraded to dual-chamber while in the pipeline. Since system accuracy comes from the plate and meter tube together, an upgrade should not look at the plate alone.
Conversion makes sense when these improvements cannot meet the flow profile, pressure budget or monitoring need. The conversion scope should then cover: whether the existing run length is enough for the new meter and conditioner, the effect of the regulator location on signal quality, flow computer configuration and communication, agreement with the counterparty on the transition period, comparative operation of the old and new metering for a period, and continuity with historical records.
Life-cycle cost items
The table has no amounts; it ensures the same items are filled in for both options when comparing bids.
Cost items to compare for the orifice and ultrasonic options
Item | When does it arise? | What drives it for orifice? | What drives it for ultrasonic? |
Meter run equipment | Initial investment | Fitting type, meter tube, plate set, DP/P/T transmitters | Meter body and electronics, flow conditioner, P/T measurement |
Flow calibration and transport | Initial investment and periodic | Usually none when standard equations are used; applies if the calibration route is chosen | Calibration facility, transport, calibration together with the meter run if required |
Conversion and downtime | Upgrade project | Scope of fitting upgrade or added transmitter | Line modification, piping changes, commissioning time |
Operating effect of pressure loss | Throughout operation | Permanent loss depending on beta ratio | Flow conditioner and filter loss |
Periodic verification | Throughout operation | Plate inspection, measuring tools, transmitter checks | Diagnostic monitoring, recalibration, check meter upkeep if present |
Spare parts | Throughout operation | Plate, seal and transmitter spares | Transducers, electronic boards, manufacturer support |
Training and software | Commissioning and after | Plate inspection and record-keeping discipline | Diagnostic software and data interpretation |
Metering during calibration or maintenance | Periodic | Need for a spare run or bypass, depending on fitting type | Need for a spare meter or spare run while the meter is removed |
Records and audit | Throughout operation | Plate change and parameter records | Diagnostic archive, event logs, calibration certificates |
Unsuitable conditions and limits
· With pulsating, two-phase or continuously liquid-laden flow, the standard bases of neither technology apply; the source must be removed first.
· An orifice application outside the ISO 5167-2 limits cannot be treated as a measurement calculated with the standard equation.
· A regulator close to the meter, operating at high pressure drop and impossible to relocate, makes an ultrasonic solution risky unless countermeasures are assessed for that specific meter and regulator pair.
· This article does not replace sizing software, an uncertainty budget or vendor selection; accuracy, range and straight pipe values are confirmed with manufacturer data.
· For custody transfer, legal metrology requirements and acceptance procedures vary by country and contract.
Data to prepare for a technical review
· Minimum, normal and maximum flow with the definition of reference conditions; if possible, the annual hourly flow distribution
· Operating pressure and temperature range, line size and pressure class
· Gas composition, history of liquid or dust carry-over and the existing filtration arrangement
· Allowable permanent pressure loss and downstream pressure requirement
· Existing line geometry: isometric drawing, elbows and valves, usable straight length, flange spacing
· Location and pressure drops of regulators and control valves
· For an existing orifice station: fitting type, bore, beta ratio, DP transmitter ranges and plate inspection records
· Purpose of measurement (custody transfer or process), the standard referenced in the contract and the flow computer/control system interface
Related pages
· Orifice gas metering skids: Components of an orifice skid and a feature-level comparison of the two technologies.
· Ultrasonic gas metering skids: Path layout, installation effects and diagnostic parameters in detail.
· Custody transfer natural gas metering stations: Station architecture and record-keeping obligations for billing measurement.
· Choosing single-chamber, dual-chamber or double-isolation orifice fittings: Useful when improving an existing orifice station instead of converting it.
· Orifice beta ratio and DP range: To check whether a low-flow problem can be solved within the orifice concept.
Frequently asked questions
How much pressure loss is saved by moving from orifice to ultrasonic?
Estimating it from measured DP is misleading, because part of the measured differential pressure is recovered downstream of the plate. Compare the permanent loss of the existing plate, which depends on its beta ratio, with the total loss of the ultrasonic arrangement including the flow conditioner and filter.
Can we switch to ultrasonic if the contract references AGA 3 or ISO 5167?
It may be technically possible, but because the contractual metering method changes, the counterparty's written acceptance, a transition period and the new verification approach must be defined in advance. At points under legal metrology, national regulations must also be checked.
Should the old and new meters run together for a while after conversion?
The contract and operating policy decide whether it is mandatory. Where possible, side-by-side monitoring for a period supports continuity with historical records and reveals installation effects early.