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Mass Flow Meter Principle: Thermal vs Coriolis
Quick Answer: Thermal mass flow meters use heat transfer to measure gas flow directly in mass units. Coriolis mass flow meters detect the twisting force on vibrating tubes to measure mass flow of liquids, gases, and slurries. For dry gas applications like compressed air or natural gas, a thermal meter is often the right choice. For viscous liquids, high accuracy custody transfer, or multi-phase fluids, a Coriolis meter delivers the performance engineers need.
How Thermal Mass Flow Meters Work
A thermal mass flow meter uses the principle of convective heat transfer. Inside the sensor, two PT100 RTD elements sit in the gas stream. One resistance temperature detector measures the process gas temperature as a reference. The other is heated with a constant energy input. As gas flows past the heated sensor, the gas carries away heat. The temperature difference between the two RTDs changes. The meter calculates mass flow in kg/h or Nm3/h directly from this temperature difference. No pressure or temperature compensation is needed for the mass flow output. Because the measurement depends on the gas thermal properties, the meter must be calibrated for a specific gas mix. A 4-20 mA HART signal and RS485 Modbus are standard outputs on Silver Instruments insertion and inline thermal meters.
How Coriolis Mass Flow Meters Work
A Coriolis mass flow meter makes use of the Coriolis effect. The fluid moves through one or two vibrating tubes inside the meter body. An excitation coil keeps the tubes vibrating at their natural frequency. When mass flows through the vibrating tube, the fluid inertia generates a small twist in the tube. Two electromagnetic pickoff sensors on the inlet and outlet sides detect a phase shift in the vibration. This phase shift is directly proportional to the mass flow rate. The meter also measures the natural frequency of the tubes to derive fluid density in kg/m3. From mass flow and density the meter can calculate volume flow. A single Coriolis meter gives you mass flow, density, temperature, and volume flow. Silver Instruments Coriolis meters handle liquids from 0.2 cP solvents to 50000 cP heavy oils and slurries. We have seen a paint manufacturer in Thailand measure resin at 500 cP and 60 °C with a DN25 Coriolis meter. The batch accuracy improved by 0.3 percent compared to a positive displacement meter.
Key Differences at a Glance
Thermal meters work best for single-phase dry gas. Coriolis meters handle liquids, compressed gases, and slurries. The typical accuracy of a thermal flow meter is plus minus 1 percent of reading over a limited range. A Coriolis meter achieves plus minus 0.1 percent to 0.2 percent of reading with a wide turndown ratio. Thermal meters need a specific calibration for the gas composition. Coriolis meters measure mass flow independently of fluid properties. In practice, if your gas has varying humidity or you need high accuracy for a liquid blend, choose Coriolis. Most engineers skip the thermal meter for anything other than clean, dry gas. We have seen this on customer sites many times. A wastewater plant in Mexico tried a thermal meter on digester gas with high moisture. The sensor lost accuracy within weeks. They moved to a Coriolis meter with Hastelloy tubes and the drift stopped.
When to Choose Thermal Mass Flow Meters
Select a thermal mass flow meter when your application is a dry, single-phase gas with stable composition. Common examples are compressed air systems, argon and nitrogen blanketing, flare gas, and natural gas sub-metering. Insertion thermal meters install easily via a weld-on ball valve for large pipe sizes from DN80 to DN300. Inline thermal meters cover small lines from DN6 to DN50. A thermal meter from Silver Instruments comes with built-in PT100 temperature sensors and ATEX Zone
When to Choose Coriolis Mass Flow Meters
Choose a Coriolis mass flow meter for liquids, high-value products, custody transfer, and any stream with changing density. Coriolis meters are the standard in food and beverage, chemical injection, marine fuel bunkering, and pharmaceutical processes. A Coriolis meter measures mass flow directly. The output does not depend on conductivity or Reynolds number. A single flowmeter gives you mass flow, density, and temperature. Typical line sizes range from DN1 to DN200. Silver Instruments offers stainless steel 316L and Hastelloy C22 wetted parts. A customer in Kenya uses a DN15 Coriolis meter with 4-20 mA HART and Modbus to fill water-treatment chemicals into 20 L jerrycans. The filling error is below 5 grams per can. This kind of accuracy requires a Coriolis meter.
Silver Instruments Recommendations
Silver Automation Instruments supplies both thermal mass flow meters and Coriolis mass flow meters from our factory to industrial end users and distributors in Southeast Asia, Oceania, Latin America, Africa, and the Middle East. If your fluid is a dry gas like compressed air, nitrogen, or natural gas, a thermal meter will cost less and work well. If your fluid is a liquid, slurry, or wet gas, a Coriolis meter is the reliable choice. For mixed applications we suggest keeping a small stock of both technologies. Send us your pressure in bar, temperature in degrees Celsius, pipe size DN, and flow range in kg/h. Our engineers will reply with a recommended model and price within one working day. See our inline Coriolis flow meter page for liquid chemicals or our insertion thermal mass flow meter for compressed air.
FAQ
Which mass flow meter is better for compressed air monitoring?
A thermal mass flow meter is better for dry compressed air in pipes up to DN300. The insertion style keeps installation simple. If the air carries oil aerosol or water from a compressor leak, a Coriolis meter avoids the sensor drift that thermal meters would suffer.
Can a Coriolis meter measure high viscosity fluids?
Yes. A Coriolis meter handles viscosities from 0.2 cP to over 50000 cP. A thermal meter cannot measure liquids at all. For heavy fuel oil, glucose syrup, or polymer melts, the Coriolis meter is the only mass flow choice between these two principles.
What is the typical accuracy difference between thermal and Coriolis mass flow meters?
A thermal meter delivers plus minus 1 percent of reading for most gas ranges. A Coriolis meter achieves plus minus 0.1 percent to 0.2 percent of reading for liquids and gases. For custody transfer of diesel or LPG, a Coriolis meter is the required standard.
Do thermal mass flow meters need a straight pipe run?
Inline thermal meters need some straight pipe upstream and downstream, similar to a turbine meter. Insertion thermal meters are more forgiving if the probe is placed in a section with a fully developed flow profile. Coriolis meters need no straight pipe run at all. This is a big advantage when space is tight on a skid.
How do I choose between an insertion and an inline thermal mass flow meter?
Use an inline thermal meter for small pipes from DN6 to DN50 where a spool piece is easy to install. Use an insertion thermal meter for pipes DN80 and larger. The insertion option costs less and allows hot tap installation without shutting down the process.
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