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Hot Wire Flow Meter: Thermal Anemometers for Precision Low Velocity Ducting
Quick Answer: A hot wire flow meter is a thermal anemometer that measures low gas velocity from about 0.1 m/s to 30 m/s in ducts, stacks, and pipes. Silver Automation Instruments supplies insertion hot wire thermal mass flow meters with 4-20 mA HART output for duct diameters from DN40 to DN2000. Send us your duct diameter, gas type, pressure, temperature, and velocity range for a fixed quotation.
What a Hot Wire Flow Meter Does in a Duct
A hot wire flow meter uses a heated sensor exposed to gas flow. The moving gas cools the sensor. The instrument measures the cooling effect and converts it to a velocity signal.
The sensor head includes a heated wire element and a PT100 reference RTD. The transmitter reads the temperature difference between the heated wire and the reference sensor. That difference changes with gas velocity and gas density.
Do not confuse this with a room comfort thermal anemometer from an HVAC supply store. Industrial hot wire meters use 316L stainless steel probes and process transmitters with HART or Modbus output.
In practice, most engineers skip the heat transfer equations. They check the velocity range, duct size, gas composition, and output signal.
Why Low Velocity Ducting Measurement Fails with Other Meters
Here is the thing. Low velocity ducting often means flow below 2 m/s. Differential pressure meters lose signal because the pressure drop becomes tiny. Averaging pitot tubes may read zero or drift around 0.5 m/s.
We have seen this on customer sites many times. A paint manufacturer in Vietnam had a DN300 exhaust duct running at 1.2 m/s. A pitot tube gave unstable readings no matter how the transmitter was ranged.
But the hot wire signal stays stable. The cooling effect does not rely on a square root extraction. Because the sensor still loses heat at very low flow, the meter keeps resolution.
Silver Instruments Hot Wire Thermal Anemometer Range
For low velocity ducting, Silver Instruments recommends the TF-100 insertion thermal mass flow meter. This model works with air, nitrogen, biogas, flue gas, and other clean gases.
The standard flow range is 0.1 m/s to 30 m/s. Accuracy is ±1.5 percent of reading plus 0.3 percent of full scale from 0.3 to 30 m/s. Repeatability is ±0.5 percent of reading. The turndown ratio is 100 to 1 under field conditions.
Output cards include 4-20 mA HART and RS485 Modbus RTU. The standard process temperature range is -20 °C to 120 °C. A high temperature version is available for 180 °C. The probe body is 316L stainless steel. The housing is IP65 aluminum.
Where Low Velocity Hot Wire Meters Are Used
Typical applications include HVAC outside air intake, paint booth exhaust, stack gas monitoring, wastewater aeration air, biogas lines, and cleanroom pressurization ducting.
Last year a customer in Oman used the TF-100 on an aeration blower line with only 1.0 m/s air flow. The plant needed a reliable totalized air mass flow for blower efficiency checks.
In Australia, HVAC contractors use hot wire meters for ventilation duct
Installation Without Process Shutdown
The TF-100 can be installed through a ball valve assembly. The technician drills a hole in the duct wall, mounts the valve, inserts the probe, and seals the fitting. There is no need to stop the process flow.
Probe insertion depth should place the sensor at the average velocity point. For most round ducts, this point is around 0.5 times the duct inner diameter from the wall.
For rectangular ducts, send us the duct cross section. We will calculate the correct probe length and probe insertion depth.
Calibration and Real Gas Effects
Silver Instruments calibrates each hot wire flow meter on an air reference bench. If your gas is not air, send the gas composition with your inquiry. We adjust the correction factor in the transmitter before delivery.
Do not use a hot wire meter for wet gas, saturated gas with droplets, or gas with sticky dust. For those services, ask us about a thermal mass flow meter with a different sensor protection design.
Output and Integration
Most plants wire the 4-20 mA signal to a PLC or a flow totalizer. The HART digital signal gives velocity, temperature, totalized mass flow, and diagnostic status. The Modbus output works well for SCADA systems in water and wastewater plants.
A local display is optional. The display shows m/s, kg/h, standard cubic meters per hour, or another engineering unit.
Recommended Model and Quote Request
For duct diameters from DN40 to DN2000, start with the Silver Instruments TF-100 insertion hot wire thermal mass flow meter.
Send us your duct diameter, duct wall thickness, gas type, pressure, temperature, and velocity range. Mention if you need ATEX Zone 1 or Zone 2. We will reply with model selection, probe length, and price.
Contact Silver Instruments. Tel: +86-135-1213-1526. WhatsApp: 86-135-12131526. WeChat: +86 13512131526.
FAQ: Hot Wire Flow Meter for Low Velocity Ducting
Q: What is the minimum velocity a hot wire flow meter can measure?
A: The TF-100 starts at 0.1 m/s. For stable accuracy, Silver Instruments recommends 0.3 m/s and above.
Q: Can this meter be used in rectangular ducts?
A: Yes. Provide the duct width, height, and wall thickness. We will size the probe length and insertion depth.
Q: What gases work with a hot wire thermal anemometer?
A: Clean dry gases such as air, nitrogen, carbon dioxide, biogas, and flue gas. Avoid wet gas or gas with droplets.
Q: What output signal does the TF-100 provide?
A: The standard output is 4-20 mA HART. RS485 Modbus RTU is available as an optional output.
Q: How do you install the sensor into an existing duct?
A: Use a hot-tap ball valve assembly. Drill the duct, mount the isolation valve, insert the probe, and seal the compression fitting. The process can stay online.
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