Three Thermal Mass Flow Meters for a NOx Test Rig Simpler Than Venturi + DP
When specifying flow measurement for a NOx generator test rig, the choice often gets stuck between two extremes: a Venturi tube plus differential pressure transmitter (low headline cost, but hidden engineering in piping modifications, temperature/pressure compensation and commissioning), or premium Coriolis/imported thermal meters (excellent performance, but three units blow the budget).
The customer's technical team proposed a more pragmatic approach: three thermal mass flow meters in different configurations, one per measurement zone, all with 316 stainless wetted parts, 24 VDC supply and 4–20 mA output. Here is a concise summary for similar applications.
Zone A — Combustion Air (DN100 Insertion Type)
Duty: Blower suction side, 110.3 mm ID, 80–140 m³/h at 20°C, near atmospheric (slightly negative), extremely low pressure loss required. Standard DN100 acceptable if priced better.
Recommendation: DN100 insertion thermal mass flow meter, 316 SS. Insertion design avoids cutting the pipe for a flanged spool, adds negligible pressure drop, and costs far less than inline meters at this diameter. Thermal technology outputs mass flow or normalized volume flow directly — no temperature/pressure compensation needed, just one clean 4–20 mA signal to the PLC.
Accuracy and pressure drop: Typical economy thermal meters deliver plus or minus 1–2 percent of reading; the insertion probe's obstruction to airflow is minimal, so pressure loss is effectively negligible — critical on a blower suction line.
Zone B — Heated Air to Gas Treatment Module (DN25 Inline, High-Temperature)
Duty: 21 mm ID (DN25), 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 (a few mbar). Sensor and probe must withstand 400°C; split design with separate electronics is acceptable.
Recommendation: DN25 inline high-temperature thermal mass flow meter, 316 SS, split design. The 400°C limit is the binding constraint — standard thermal sensors are typically rated below 200°C, so a high-temperature probe is mandatory. Split electronics keep the transmitter away from the hot pipe. Inline is preferable to insertion at DN25 for compactness and stability.
Specifying note: State "sensor probe rated to 400°C" explicitly in the RFQ and request material compatibility documentation. Some economy high-temp models reach 350–400°C at the probe — confirm it is the probe itself, not just a heat sink.
Zone C — Dutch Natural Gas (G-gas), DN15 Inline
Duty: 15 mm ID (DN15), 7.3–12.2 m³/h at 20°C atmospheric, supply pressure up to 200 mbar(g), methane-rich pipeline gas. Must be suitable for flammable gas, calibrated for G-gas composition, and low pressure drop.
Recommendation: DN15 inline thermal mass flow meter, 316 SS, flanged. Two hard requirements for natural gas: suitability for flammable service (Ex or intrinsically safe certification depending on the installation zone), and calibration on the actual gas composition — thermal meters rely on the gas's specific heat and thermal conductivity, so an air-calibrated unit used on natural gas will carry significant error. Provide the typical G-gas composition (usually more than 85 percent methane with minor nitrogen and CO₂) and require calibration to it.
Alternative considered: A DN25 vortex meter (316 SS, flanged, 400°C) was also mentioned. At DN25 on gas, vortex meters have weak signal at low flow and require temperature/pressure compensation to obtain mass flow — more system complexity than thermal. If the Zone C lower flow of 7.3 m³/h gives adequate velocity in DN15, thermal remains the simpler choice.
Common Requirements for the RFQ
Power: 24 VDC. Output: 4–20 mA proportional to mass flow or Nm³/h. Accuracy: plus or minus 1–2 percent of reading. Wetted parts: SS316/316L. Pressure drop: very low, negligible for insertion and to be confirmed for inline. Certification: CE with technical documentation. PLC interface: standard 4–20 mA input.
Indicative pricing: Economy thermal mass flow meters in DN15 and DN25 inline versions typically fall in the low-thousands RMB range per unit; the DN100 insertion unit is somewhat higher due to probe length and mounting hardware, but the three-unit total stays well within budget. Firm quotes require the duty parameters above.
Versus Venturi + DP
The customer was comparing a Venturi plus differential pressure transmitter system. To be fair, DP has an advantage in Zone B's 400°C service — the transmitter can sit away from the hot tap — but the Venturi itself 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: each meter outputs mass flow independently, no temperature/pressure compensation math, and minimal PLC configuration effort. For a non-standard, 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, explicitly flagging Zone B's 400°C probe requirement and Zone C's G-gas composition calibration.
If you also have similar requirements for precision flow meter or industrial gas flow meter selection and quotation, feel free to contact
CIXIFM Winny:
Email: [email protected], WhatsApp: 86 18049841995
