Guidance Note
Guidance Note
In the UK, domestic gas meters gain type approval by following a series of steps to ensure that the meters comply with regulatory standards set by the Office for Product Safety and Standards. This is generally achieved by assessing the gas meter against a CEN TC/237 designated standard, which sets out a series of requirements, including testing. The harmonised standards for gas meters were written based on the predominant fuel for residential, commercial, and industrial applications in Europe being natural gas.
Therefore, the test gases recommended for use in these standards are geared towards this. When specifically considering the use of gas meters with hydrogen and hydrogen enriched natural gas (HENG), the designated standards currently do not explicitly provide recommendations on acceptable test gases to use. To address this, a review of the CEN/TC 237 standards was undertaken by an expert group with a view to suggesting any changes that should be made when considering the use of renewable gases, which included hydrogen and HENG.
Hydrogen is at the centre of discussions for reducing carbon emissions and meeting net zero targets. Blending hydrogen with natural gas represents one of the most promising near-term options for decarbonisation of the existing natural gas infrastructure.
While some gas meters and pipelines might accommodate hydrogen to an extent, the safety and performance however needs to be confirmed. Regulatory authorities in the UK and Europe are therefore working to amend measuring instrument regulations to account for new gases like hydrogen or other renewable alternatives to traditional natural gas.
With this changing fuel environment, industry needs confidence that gas meters can accurately measure flow, remain leak-tight, and operate reliably over time - whether handling pure hydrogen or HENG.
Domestic gas meters in the UK are subject to the Measuring Instruments Regulations 2016 (SI 2016/1153). These regulations closely follow the EU MID (2014/32/EU), ensuring that devices meet certain requirements for accuracy, durability, and safety. To comply, manufacturers can use:
By following these routes, organisations prove that their meters remain within the Maximum Permissible Error (MPE) levels required for gas measurement over time.
Most domestic gas meters are tested against CEN TC/237 standards, with each standard catering to a different meter type.
Until recently, these standards primarily focused on natural gas. While the “second family” gases referenced in the standards allow for up to 23% hydrogen by volume, it is incorrect to assume that meters are verified for metrological performance at this hydrogen content. This was because CEN TC/237 standards require the meters to be tested for the real gas the meter will be measuring, which in most cases is gases with hydrogen content less than 0.1% by volume.
When considering hydrogen or HENG, CEN TC/237 standards do not explicitly specify the test gases to use. Hence these standards will need to be updated to address the use of renewable gases.
An alternative to the CEN TC/237 series is OIML R 137, a set of international recommendations for performance and testing of gas meters. This standard is less explicit about the gas types, stating only that meters must be tested across the range of gases they intend to measure. Therefore, as with the CEN TC/237 standards, it is recommended that OIML R 137 be revised to include hydrogen and HENG in the family of gases used by the meters, with recommendations on suitable test gases.
PAS 4441:2024 a Publicly Available Specification from the British Standards Institution provides guidance on the use of hydrogen in domestic and commercial applications, including gas meters. It references a test gas called “G40,” with at least 99.9% hydrogen content. While acknowledging that some meter designs (eg diaphragm types) are fairly insensitive to gas composition, PAS 4441 stresses the need to test in hydrogen when the intended operational fluid is hydrogen. Only with enough data can manufacturers confidently substitute air or other fluids for testing.
Domestic gas meters have historically been tested with air or natural gas to check their metrological performance. As hydrogen enters the picture, the question emerges: Is it enough to test with standard reference gases, or do we need direct hydrogen testing?
A dedicated research project, NEWGASMET, recommended that, until strong evidence shows otherwise, meters intended for hydrogen or HENG should be verified using the actual blend or 100% hydrogen. This position stems from hydrogen’s unique density, diffusivity, and other properties that could affect the meter’s accuracy.
Proposed test gases for HENG include blends of either natural gas or methane with 10%, 20%, and 30% hydrogen by volume. For pure hydrogen, the recommended test gas is 99.9% or higher hydrogen.
According to the MID, meters must keep their measurement error within prescribed MPE limits for each meter class. CEN standards interpret this requirement by describing specific methods for calibration and verification, often allowing air as a proxy in certain scenarios.
Key meter types and their considerations:
These meters are commonly considered less sensitive to fluid properties. Meter calibrations undertaken as part of the NEWGASMET project, along with other tests by TÜV SÜD and PTB, demonstrated that air and nitrogen gas calibrations were transferable for hydrogen and methane/hydrogen blends (up to 30% volume fraction), where the errors remained within the MPE limits for Class 1.5 meters.
Another study which investigated HENG up to 23% hydrogen also found the diaphragm meters performance were within the Class 1.5 MPE limits.
For these meters, testing with air is permitted if the assessment of the gas-air relationship meets the requirement specified in the standard. The calibrations must consider speed of sound, attenuation, viscosity, and density. Therefore, when used with hydrogen and HENG it was recommended that appropriate test gases are selected which ensure the meter performance is maintained across the range seen for each gas property.
While some studies investigating HENG with up to 23% hydrogen found that ultrasonic meters performed within the Class 1.5 MPE limits, tests at TÜV SÜD with methane and 20% hydrogen blends demonstrated that meters not designed for hydrogen service exhibited inconsistent results when the hydrogen concentration was above 15%, with errors exceeding the Class 1.5 MPE limits.
These rely on King’s law (gas density times specific heat capacity). Large variations in these properties from natural gas to hydrogen can cause bigger shifts in performance.
Whilst differences of 1 % to 2% were observed between nitrogen and hydrogen gas calibration of a thermal mass meter at TÜV SÜD, the errors remained within the Class 1.5 MPE limits, and the meter also met the criteria for the gas-air relationship specified in EN 17526.
Studies with hydrogen gas blends (such as HENG or methane/hydrogen blends) have shown that thermal mass meter error increases with higher hydrogen concentrations. While some tests with inconsistent results still had meter errors within Class 1.5 MPE limits, other tests have showed errors exceeding these limits when the hydrogen concentration is above the manufacturer’s specifications.
Meters must remain stable over years of operation. The MID highlights this in Annex 1 – 5 which says over the time period estimated by the manufacturer, the meter must maintain an adequate stability of its metrological performance under the intended environmental conditions.
Durability testing conducted as part of the NEWGASMET project found that the metrological performance of diaphragm meters only changed by a maximum of 1% after 12 months of exposure to hydrogen gas. In contrast, the performance of thermal mass meters exhibited greater variation, with differences exceeding 2% at lower flow rates.
Another study investigating the effect of HENG on the durability of diaphragm meters and a thermal mass meter found that prolonged exposure to HENG with up to 15% hydrogen by volume did not impact the meters metrological performance.
Leak tightness is partly about safety—external leakage increases the chance of gas build-up and potential fire or explosion. Equally, internal leakage can cause under-measurement (unbilled gas) or performance issues. The CEN TC/237 standards allow air as the default gas for leak checks at or above the intended operating pressure. Yet hydrogen’s propensity to escape through small gaps raises new questions.
As part of the NEWGASMET project, PTB developed a new leak testing procedure for domestic gas meters intended for hydrogen applications. The study recommended that existing leak test methods using alternative gases to hydrogen remain applicable but should adopt a stricter acceptance criterion, where the leak rate for hydrogen use should be 75% of the current limit applied for natural gas. However, this approach does not account for the lower ignition energy of hydrogen or its rapid dispersion properties, hence further refinements may be necessary.
As hydrogen gains prominence, it is not just about swapping out test gases. It involves:
The information above is a summary of what is in the full Guidance Note: The Use of Domestic Gas Meters with Hydrogen and Hydrogen Enriched Natural Gas. If your organisation is involved in gas metering, gas distribution, meter manufacturing, compliance, or any phase of energy transition planning, this document will be of interest.
It includes:
Find out more about our hydrogen domestic gas flow facility which offers flow analysis, measurement, testing and control and other services for the hydrogen industry.
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