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How Does a Thermal Mass Flow Meter Work: Boundary Layer Heat Dissipation Physics
Quick Answer. A thermal mass flow meter measures gas mass flow directly by using a heated sensor and a reference sensor. The gas flow cools the heated sensor, and the electronics measure the power needed to keep the temperature difference steady. That power signal is proportional to mass flow.
Silver Automation Instruments supplies these meters for compressed air, natural gas, biogas, nitrogen, oxygen, and flare gas. Inline models cover DN15 to DN100. Insertion models cover large pipes and ducts.
Silver Automation Instruments is a thermal mass flow meter supplier for gas flow measurement in Southeast Asia, Oceania, Latin America, Africa, and the Middle East.
The Core Physics: Boundary Layer Heat Transfer
A thin gas layer forms next to the heated sensor surface. This is the boundary layer. Heat moves from the sensor through this layer into the moving gas. At low velocity, the boundary layer is thick. Heat loss is low. At high velocity, the boundary layer becomes thin. Heat loss rises fast.
The cooling effect depends on gas density, thermal conductivity, viscosity, and velocity. Because mass flow is the product of gas density, velocity, and pipe area, the heat loss signal tracks mass flow directly. This is the boundary layer heat dissipation physics behind the thermal mass flow meter.
Most engineers skip the physics and watch the 4-20 mA signal. But the signal quality depends on this boundary layer effect. If gas composition changes, the meter needs a correction factor.
How the Sensor Pair Works
Industrial thermal mass flow meters use two platinum RTD sensors. Usually these are PT100 elements. One sensor measures gas temperature. The other sensor is heated to a fixed temperature above the gas. The electronics hold this temperature difference, often from 20 °C to 50 °C. As flow increases, the gas removes more heat. The electronics supply more current to maintain the difference. That current or power signal is converted to mass flow units.
Some meters use constant power mode. The heater power stays fixed. The sensor temperature drops as flow increases. Constant temperature difference is more common in Silver Instruments inline and insertion models because it responds faster and handles a wider turndown.
Inline vs Insertion Thermal Mass Flow Meters
An inline thermal mass flow meter puts the sensor in a flow tube. The tube may include a flow conditioner to shape the gas profile. Inline versions fit DN15, DN20, DN25, DN40, DN50, DN65, DN80, and DN100. Accuracy is typically 1 percent of full scale. Some versions reach 0.5 percent of reading for critical lines.
An insertion thermal mass flow meter uses a probe. The probe is installed through a ball valve or welded nozzle. It reads the gas velocity at one point. The transmitter calculates mass flow using pipe area and a profile factor. Insertion meters suit pipes from DN100 to DN2000 or rectangular ducts. We have seen this on customer sites many times in aeration air lines and biogas headers.
Here is the thing. Insertion meters are easier to install on large pipes. But they n
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Gas Types and Process Limits
Thermal mass flow meters work for dry gases and gas mixtures. Common examples include compressed air, nitrogen, oxygen, argon, natural gas, biogas, digester gas, flare gas, stack gas, and aeration air. Saturated wet gas or gas with droplets can cause unstable readings. Liquid carryover is not suitable for this meter. For liquids, Silver Automation Instruments offers electromagnetic and Coriolis mass flow meters.
Process limits vary by model. Typical inline versions handle gas temperature from -20 °C to 200 °C. Pressure ratings go to 40 bar. Insertion probes handle similar temperatures. ATEX Zone 1 and Zone 2 approvals are available for hazardous areas. Outputs include 4-20 mA HART, Modbus RTU, and pulse.
Last year a customer in Vietnam asked us to check a biogas line. The pipe was DN80, gas temperature was 40 °C, pressure was 0.3 bar, and flow range was 20 to 200 Nm3/h. We selected an inline model with PT100 sensors and 4-20 mA HART output. Before that, a paint manufacturer in Southeast Asia requested a similar meter for solvent vapor on a DN50 pipe at 5 to 50 kg/h.
What to Check Before You Request a Quote
Gas composition matters. A meter calibrated for air will read differently on methane or biogas. Tell us the gas or gas mixture. Tell us the minimum and maximum flow rate. We need pipe size in DN or inches. We need pressure in bar and temperature in °C. For insertion meters, we also ask for straight run length.
Do not send a general contact form message. Instead, send us your gas type, pressure in bar, temperature in °C, pipe size in DN, and flow range in kg/h or Nm3/h. We will reply with a model, accuracy statement, and price range.
Quick Questions and Answers
Can a thermal mass flow meter measure liquid flow?
No. This meter is for gases only. For liquids, select an electromagnetic flow meter or a Coriolis mass flow meter from Silver Instruments.
What is the difference between mass flow and volumetric flow?
Volumetric flow changes with temperature and pressure. Mass flow stays the same. A thermal meter reads mass flow directly without a separate pressure and temperature compensation system.
Can I use a thermal mass flow meter on wet biogas?
Moisture droplets cause unstable readings. Install a moisture separator or condensate trap upstream. For wet biogas, ask us about a Coriolis or insertion model with a dirt shield.
What approvals and outputs are available?
Standard output is 4-20 mA HART. Modbus RTU and pulse are optional. ATEX Zone 1 and Zone 2 models are available for gas lines in hazardous areas.
What information do I need to send for a price?
Send gas type, pressure in bar, temperature in °C, pipe size in DN, and flow range in kg/h or Nm3/h. We will quote an inline or insertion thermal mass flow meter with a delivery time.
Contact Silver Automation Instruments. Tel: +86-135-1213-1526. WhatsApp: 86-135-12131526. WeChat: +86 13512131526. Website: flow-meter.com.au.
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