Flow, level & control
is based on the physical effect of heat convection, so the sensor signal depends on both the gas’ flow velocity across the membrane and the measured gas mixture’s thermal properties. So, a thermal flow sensor provides accurate
measurement data if it has either been precalibrated for a particular gas mixture, 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 gas mixtures, and their composition can vary over time. In practice, it is not feasible to perform individual precalibrations for all the possible natural gas mixtures, which is why Sensirion’s thermal-mass sensors for gas meters have a proprietary, dynamic natural gas and hydrogen recognition routine to ensure accurate flow measurements, even when gas compositions vary.
TesT seTup
The measurement data presented here was recorded using thermal-mass flow sensors with a dynamic gas recognition routine. The routine is optimised for H, L and E gases according to EN 437:2018 that contain up to 23 per cent hydrogen, as well as for pure and near-pure hydrogen. The flow sensors’ output signal is temperature- and pressure-compensated in standard cubic meter per hour (m3
/h). 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).
Flow measuremenTs in naTural gas/hydrogen mixTures
Figure 3 shows the relative measurement errors at the reference gas flow of up to 6m3
/h (G4 meter) for five flow sensors in air,
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 test gases.
methane and natural gas mixtures containing five per cent, 10 per cent and 23 per cent hydrogen. The error limits of ± 3.5 per cent and ± 2.0 per cent shown in black are the maximum permissible error limits according to the 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 five
measured flow sensors are well within the maximum permissible error limits and also comply with the permitted air-gas relationship of three per cent and 1.5 per cent respectively according to the European Standard for thermal-mass gas meters, EN 17526, as well as EN 14236 for ultrasonic domestic gas meters.
The test gas containing 23 per cent
hydrogen is the test gas G 222 as per EN 437. G 222 is described as the “limit test gas” –a gas mixture with the maximum hydrogen content used for testing gas appliances for second-family natural gas mixtures according to EN 437.
Flow measuremenTs in hydrogen gases
Since hydrogen has a lower calorific value (lower than that of natural gas by a factor of three), significantly higher hydrogen flow rates are required to maintain the same energy flow through the meter. Figure 4 shows the relative measurement errors at the reference gas flow of up to 20 m3
/h for five flow sensors in 100
per cent H2 and 98 per cent H2 + two per cent CO2. Two per cent of impurities is the
Figure 3: Relative measurement error at the reference gas flow for five flow sensors in air (a), methane (b) and natural gas mixtures containing five per cent (c), 10 per cent (d) and 23 per cent (e) hydrogen.
Instrumentation Monthly January 2022
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