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Hot Wire Anemometer Flow Meter Principle: Thermodynamic Mass Airflow Sizing for Low Gases
Quick Answer: A hot wire anemometer flow meter measures gas mass flow by sensing heat loss from a heated PT100 sensor. For low gas flows below 1 kg/h or below 0.5 m/s average velocity, thermodynamic mass airflow sizing must include gas composition, pipe size DN, actual pressure in bar, and actual temperature in °C. Silver Instruments supplies inline and insertion thermal mass flow meters with 4-20 mA HART output and ATEX Zone 1 options for air, biogas, CO2, nitrogen, and natural gas.
How a Hot Wire Anemometer Flow Meter Works
A hot wire anemometer flow meter uses a heated sensor inserted into the gas stream. The sensor is usually a platinum RTD element such as a PT100 or a PT20.
Gas flows across the heated surface and removes heat. The control circuit measures this heat loss. More mass flow removes more heat. Less mass flow removes less heat.
The sensor follows the heat transfer correlation for a heated cylinder. Heat transfer depends on gas velocity, gas density, and gas thermal conductivity. The meter converts heat loss into a mass flow signal in kg/h or Nm3/h.
Most industrial hot wire probes run in constant temperature mode. The circuit adjusts power to keep the sensor at a fixed temperature above the gas temperature. A second PT100 sensor measures gas temperature. The difference in power gives a direct mass flow signal.
In practice, this principle works well for low gas flow because the heated sensor responds to molecular cooling, not mechanical movement. A turbine meter or vortex meter loses signal at low velocity. A hot wire sensor still sees heat removal even at 0.5 m/s or lower.
Thermodynamic Mass Airflow Sizing for Low Gases
Low gas flow sizing is different from liquid flow sizing. For air and gas, actual density at operating pressure and temperature changes the reading. A hot wire meter measures mass flow directly, so you do not need a separate pressure transmitter and temperature transmitter for compensation.
Send us these five data points: gas composition, operating pressure in bar, operating temperature in °C, pipe size in DN, and minimum to maximum flow in kg/h or Nm3/h.
Example from a project in Vietnam. A customer needed to measure 0.8 kg/h to 12 kg/h of compressed air in a DN15 pipe. Pressure was 6 bar, temperature was 30 °C. We recommended a SIL-TMF-DN15 inline thermal mass flow meter with a 0 to 20 kg/h range. The meter gave a stable 4-20 mA signal across the whole range.
Another project in Chile involved biogas from a digester. Flow was only 2 kg/h to 15 kg/h in a DN25 line at 45 °C. The user needed ATEX Zone 1 protection because the biogas contained methane. We supplied a SIL-TMF-DN25 inline thermal mass flow meter with 316L stainless steel wetted parts and an ATEX Zone 1 housing.
Most engineers skip the density correction step because the thermal meter reads mass flow. This approach works for a single gas. If the gas composition changes often, the meter may need a field calibration or a conversion factor.
Low Gas Flow Applications in Industry
We see hot wire anemometer flow meters used in wastewater aeration lines, biogas digesters, fermentation air supply, nitrogen blanketing, CO2 injection, and hydrogen pilot feeds.
A food and beverage plant in Thailand measures fermentation air at 1.5 kg/h to 20 kg/h in a DN20 pipe. The meter runs 24/7 with a
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A chemical plant in Saudi Arabia uses a thermal mass flow meter for nitrogen blanketing on solvent storage tanks. Flow rates are low, around 0.5 kg/h to 8 kg/h in DN15 and DN25 pipes. The meter detects line breaks and blocked filters faster than a pressure switch.
Marine customers use hot wire probes for engine room ventilation air and gas fuel purge lines. Low flow accuracy matters because a leak or blocked vent can lead to an unsafe condition.
Silver Instruments Thermal Mass Flow Meter Options
Silver Instruments offers the SIL-TMF inline series for DN15 to DN50 pipes and the SIL-TMF-INS insertion series for DN65 and larger ducts. The inline models suit low gas flows because the full gas stream passes across the sensor.
Our standard output is 4-20 mA with HART. Optional outputs include RS485 Modbus RTU and pulse. Process connections include G thread, NPT thread, flanges, and Tri-Clamp. Wetted materials are 316L stainless steel or Hastelloy for corrosive gas.
For hazardous areas, choose the ATEX Zone 1 or Zone 2 version. The transmitter housing can be aluminum or stainless steel. The sensor is a PT100 platinum RTD element. Accuracy is typically ±1 percent of full scale or ±0.5 percent of reading plus 0.5 percent of full scale, depending on the calibration range.
Tell us your gas type and flow range. If you only know volumetric flow in L/min or m3/h, include the pressure and temperature. We will convert the value to mass flow and size the meter for you.
Phone: +86-135-1213-1526. WhatsApp: 86-135-12131526. WeChat: +86 13512131526.
FAQ
What is the principle of a hot wire anemometer flow meter?
The hot wire sensor measures heat loss from a heated element placed in the gas flow. The heat loss changes with the mass flow rate. The transmitter converts this heat loss into a 4-20 mA or digital flow signal.
What flow range can a hot wire thermal mass flow meter measure?
SIL-TMF inline models for DN15 to DN25 pipes can measure from 0.3 kg/h to 50 kg/h for air. SIL-TMF-INS insertion models can measure larger flows in DN80 to DN300 pipes or ducts. The exact range depends on gas type, pressure, and temperature.
Can the meter measure low gas flow under pressure?
Yes. Provide the operating pressure in bar. A thermal mass flow meter measures mass flow, not volume flow. The pressure only affects the density. The signal remains stable at low flows because the signal does not depend on a mechanical rotor.
What are the common outputs for Silver Instruments hot wire flow meters?
Standard output is 4-20 mA with HART. Optional outputs include RS485 Modbus RTU and pulse output. The HART signal lets you read flow, temperature, and diagnostic status from the same two wires.
How do I get a quote for low gas flow measurement?
Send us your gas composition, pressure in bar, temperature in °C, pipe size in DN, and flow range in kg/h or Nm3/h. Use WhatsApp 86-135-12131526 or WeChat +86 13512131526. We will reply within 24 hours on business days.
Silver Automation Instruments supplies thermal mass flow meters for air, biogas, nitrogen, CO2, and natural gas. We support distributors and end users in Southeast Asia, Oceania, Latin America, Africa, and the Middle East. Send your gas flow data today and get a sizing sheet with a quote.
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