Specifying flow measurement for a NOx generator test rig often means choosing between a Venturi tube with a DP transmitter (low upfront cost, hidden engineering in piping, compensation and commissioning) and premium Coriolis or imported thermal meters (great performance, but three units blow the budget).
A more pragmatic route: three thermal mass flow meters in different configurations, one per zone, all 316 SS wetted parts, 24 VDC and 4–20 mA output.
Zone A — Combustion Air (DN100 Insertion)
Blower suction side, 110.3 mm ID, 80–140 m³/h at 20°C, near atmospheric and slightly negative. Extremely low pressure loss required.
Recommended: DN100 insertion thermal mass flow meter, 316 SS. No pipe cutting, negligible pressure drop, far cheaper than inline at this diameter. Outputs mass flow directly — no temperature/pressure compensation, just one clean 4–20 mA signal to the PLC. Typical accuracy ±1–2% of reading.
Zone B — Heated Air (DN25 Inline, High-Temperature)
21 mm ID, 4.2–7.0 m³/h at 20°C up to 6.8–11.3 m³/h at 200°C. Normal max 200°C, absolute max 400°C, slight positive pressure.
Recommended: DN25 inline high-temperature thermal mass flow meter, 316 SS, split design. The 400°C limit is the binding constraint — standard sensors are rated below 200°C, so a high-temperature probe is mandatory. Split electronics keep the transmitter away from the hot pipe. State "sensor probe rated to 400°C" explicitly in the RFQ.
Zone C — Dutch Natural Gas (G-gas), DN15 Inline
15 mm ID, 7.3–12.2 m³/h at 20°C atmospheric, up to 200 mbar(g), methane-rich gas. Must suit flammable service, be calibrated for G-gas composition and offer low pressure drop.
Recommended: DN15 inline thermal mass flow meter, 316 SS, flanged. Two hard requirements: Ex or intrinsically safe certification for flammable service, and calibration on the actual gas composition — thermal meters rely on specific heat and thermal conductivity, so an air-calibrated unit on natural gas carries significant error. Provide the typical G-gas composition (>85% methane with minor N₂ and CO₂) and require calibration to it.
A DN25 vortex meter was also considered, but at DN25 on gas it has weak signal at low flow and needs temperature/pressure compensation for mass flow. Thermal remains simpler if 7.3 m³/h gives adequate velocity in DN15.
Common Requirements
24 VDC; 4–20 mA proportional to mass flow or Nm³/h; accuracy ±1–2% of reading; SS316/316L wetted parts; very low pressure drop; CE with documentation; standard 4–20 mA PLC input.
Indicative pricing: economy thermal meters in DN15/DN25 inline typically fall in the low-thousands RMB range per unit; the DN100 insertion unit is somewhat higher due to probe length and mounting hardware. Firm quotes require the duty parameters above.
Versus Venturi + DP
DP has an advantage in Zone B's 400°C service since the transmitter can sit away from the hot tap, but the Venturi must still withstand the temperature, and micro-differential measurement at a few mbar positive pressure has poor signal-to-noise. The three-thermal-meter approach wins on simplicity: independent mass flow output per meter, no compensation math, minimal PLC configuration. For a small-batch test rig where commissioning time is precious, simplification is itself a value.
Next step: send the RFQ to two or three domestic economy thermal meter suppliers in parallel, flagging Zone B's 400°C probe requirement and Zone C's G-gas calibration. Compare lead times and warranty, then award.
If you have similar requirements for precision flow meter or industrial gas flow meter selection and quotation, feel free to contact Winny:
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