A MEMS (micro-electromechanical system)-based calorimetric sensor element, as shown in Figure 1a), is the core element of any microthermal flow sensor, such as those used in natural gas meters. The sensor element is located on a membrane on a silicon chip and consists of a micro-heating element and temperature sensors that are integrated upstream and downstream. When an electric current flows through the micro-heating element, it generates a temperature profile on the membrane. If no gas is flowing, the temperature is identical at the upstream and downstream temperature sensors (see Figure 1b)). If gas flows across the membrane, it generates a heat flow – in other words, causes the temperature profile on the membrane to change – resulting in a temperature change at the upstream and downstream temperature sensors (see Figure 1c)). The resulting temperature difference between the two sensors creates a precisely measurable sensor signal that is a function of the flow velocity: the greater the temperature difference, the greater the flow velocity of the gas over the sensor element.

The thermal flow measurement principle is based on the physical effect of heat convection,  so the sensor Signal depends on the flow velocity of the gas across the membrane on the one hand, and on the measured natural gas mixture’s thermal properties on the other.

A thermal flow sensor therefore provides accurate measurement data if it has either been calibrated for a particular gas mixture in advance or if it has a routine that dynamically takes into account varying gas mixtures when measuring flow.

Gas meters can be used for a very large number of potential natural gas mixtures, and the composition of These can vary over time. In practice, it is not feasible to individually calibrate for all possible natural gas mixtures in advance, which is why Sensirion’s microthermal technology for gas meters has a proprietary, dynamic natural gas
quality routine to ensure accurate flow measurements, even when the natural gas compositions vary.

The measurement data presented here was recorded using microthermal flow sensors with a dynamic natural gas quality routine. The routine is optimized for H, L, and E gases according to EN 437:2018 that contain up to 23% hydrogen. The flow sensors’ output signal is temperature- and pressure-compensated in standard liters per minute (slm).
The flow sensors were tested in a generic gas meter prototype housing, with the flow sensor positioned at the gas meter housing outlet (see Figure 2a)). An external gas supplier mixed the tested gas mixtures (see Table 1). Sonic nozzles were used as flow references and the measurements were conducted at room temperature. The measurement setup for the flow measurements is shown as a schematic diagram in Figure 2b).

Figure 2: View inside the gas meter prototype housing with the flow sensor at the gas meter outlet (a). Schematic diagram of the measurement setup for the flow measurements (b).

Table 1: Composition of the test gas.

Figure 3 shows the relative measurement errors at the reference gas flow of up to 100 standard liters per minute (slm) for three flow sensors in air, methane, and natural gas mixtures containing 5%, 10%, and 23% hydrogen. The error limits of ± 3.5% and ± 2.0% shown in red are the maximum permissible error limits according to European Directive 2016/32/EC on measuring instruments (MID) and the recommendations made by the International Organization of Legal Metrology OIML R 137 regarding accuracy temperature-compensated gas meters of accuracy class 1.5.

All the error curves of each of the three measured flow sensors are well within the maximum permissible error limits and also comply with the permitted air-gas relationship of 3% and 1.5% respectively according to the European Standard for ultrasonic gas meters EN 14236:2018 and the draft European Standard for thermal-mass flow-meter based natural gas meters (prEN 17526).

The test gas containing 23% hydrogen (the maximum amount used in this series of measurements) was test gas G222 as per EN 437:2018. G222 is the gas mixture with the maximum hydrogen content used for testing gas appliances for second family natural gas mixtures according to EN 437:2018. A hydrogen content of 23% is not, however, the upper limit for microthermal flow measurement principle. If more than 23% hydrogen in natural gas becomes foreseeable from an application angle or a market perspective, the measuring range for hydrogen can be extended as required.

Figure 3: Relative measurement error at the reference gas flow for three flow sensors in air (a), methane (b), and natural gas mixtures containing 5% (c), 10% (d), and 23% (e) hydrogen.

Sensirion microthermal flow sensors do not have any safety-related limitations when they are operated with natural gas mixtures containing any amount of hydrogen. Both the maximum temperature and the maximum stored thermal energy on the micro-sensor element are significantly below hydrogen/air mixtures’ ignition temperature or ignition energy, even if the flow sensor’s voltage regulation malfunctions. Sensirion microthermal flow measurement technology has therefore been successfully used for years in demanding gas analysis applications with 100% hydrogen.

When adding hydrogen to natural gas, it is important to consider that the calorific value of hydrogen by volume is about three times lower than typical natural gas mixtures. In practice, this means that, if a gas appliance is operated with pure hydrogen instead of natural gas mixtures, a gas volume approximately three times greater must be supplied to achieve a comparable heating. In this case, gas meters originally designed for operation with natural gas need to be capable of measuring an increased gas volume due to the admixture of hydrogen, so purely volumetric natural gas meters may need to be chosen in a larger size. Larger meter designs can lead to higher costs and require more installation space. If larger gas volumes flow through a meter than it was originally designed for during operation with hydrogen-free natural gas, this can increase the wear on the meter mechanics, thus shortening the service life.

In contrast, microthermal flow measurement technology is a static measurement principle without any moving parts. Consequently, increased volume flow does not result in any additional wear and has no influence on the microthermal gas meter’s service life. Unlike with volumetric gas meters, microthermal gas meters can be the same size regardless of whether they are operated with natural gas or with any optional hydrogen content. In the microthermal measurement principle, the key parameter to be considered is not the gas volume flowing through the meter, but rather the relevant gas mixture’s Reynolds number. It is often used as a parameter in fluid dynamics and can provide information about whether turbulent (high Reynolds number) or laminar (low Reynolds number) flow conditions are formed in a system. Comparing the Reynolds numbers for pure methane ReCH4 (representing a natural gas mixture) and for pure hydrogen ReH2 shows that ReH2 is lower than ReCH4 by a factor of more than six for the same meter housing geometry. Even assuming that hydrogen flow increases by a factor of three (to compensate for the lower calorific value of hydrogen, three times lower than natural gas), ReH2 is still about two times lower than ReCH4. Compared to methane, the lower Reynolds number for hydrogen means that measuring conditions remain stable at all times with the same meter housing geometry, even if the volume flow increases by a factor of three. Consequently, the same meter size can be used without any problems for both natural gas and for operation with up to 100% hydrogen. The pressure drop caused by the gas meter at increased flow when using hydrogen also remains comparable with normal operation using natural gas. This is due to hydrogen being far less dense than natural gas.

The measurement data presented here shows that the microthermal measurement principle complies with the error limits for measurement accuracy and the air-gas relationship as stipulated by the MID for various natural gas/hydrogen mixtures. The 23% maximum hydrogen content in this series of measurements is not a technical upper limit for the measurement principle. Rather, it corresponds to the maximum hydrogen content in test gases according to EN 437:2018. The measuring range of the microthermal measurement principle for hydrogen can be extended as required. There are no limitations with regard to operational safety, even when operating with 100% hydrogen. The size of microthermal gas meters, which is already very compact, can be maintained regardless of the hydrogen admixtures. This is a distinct advantage when compared to purely volumetric meters. It eliminates the need for expensive, large meter designs, and keeps the logistics and installation of microthermal meters simple and affordable.

In recent years, technological progress in gas meters has mainly involved enabling them to communicate as smart meters. Hydrogen admixtures could drive further modernization in the gas meter industry, seeing a move away from old, mechanical, and volumetric measurement principles toward modern technologies that can offer significant advantages for operation with hydrogen. Over 4 million gas customers worldwide are already benefiting from reliable and fair billing through microthermal gas meters. Going forward, hydrogen admixtures may well promote the ever-faster uptake of this compact, static metering technology.

Sensirion AG

Michele Monitaro, Key Account Manager Industrial Market       

Moritz Mattmann, Sensior Product Manager Gas Metering

Headquartered in Stäfa, Switzerland, Sensirion AG is a leading manufacturer of digital microsensors and systems. Its product range includes gas and liquid flow sensors, differential pressure sensors and environmental sensors for the measurement of humidity and temperature, volatile organic compounds (VOC), carbon dioxide (CO₂) and particulate matter (PM2.5). An international network with sales offices in the USA, Europe, China, Taiwan, Japan and Korea supplies international customers with standard and custom sensor system solutions for a wide range of applications. Sensirion sensors are commonly used in the medical, industrial and automotive sectors, and in analytical instruments, consumer goods and HVAC products.

One of the hallmark features of Sensirion products is the use of its patented CMOSens^® Technology, which allows for intelligent system integration of the sensor element, logic, calibration data and digital interface on a single chip. Sensirion's credentials as a reliable supplier are evident from its loyal customer base, reputation for quality (ISO/TS 16949) and excellent customer pedigree