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Quick Answer: Sizing Insertion Air Flow Meters for HVAC Ducts
Insertion Air Flow Meter: Sizing Thermal Dispersion Probes for Commercial HVAC Ventilation Ducts
For commercial HVAC ventilation ducts from 150 mm to 900 mm, one insertion thermal dispersion probe covers most applications when the probe tip reaches the duct centerline. For ducts from 900 mm to 2000 mm, use two or more probes and average the 4-20 mA or Modbus signals. Select a probe length equal to half the duct depth plus 100 mm for the compression fitting, housing, and wiring clearance.
Silver Instruments supplies insertion air flow meters for air handling units, fresh air ducts, return air ducts, and building exhaust shafts. Send your duct width, duct depth, velocity range, air temperature, and required output to Silver Instruments. We will confirm the probe insertion length and transmitter settings.
Why Thermal Dispersion Probes Fit Commercial HVAC Ventilation Ducts
Thermal dispersion probes read mass air velocity directly. They do not need separate pressure transmitters or temperature sensors for density correction. In practice, this means fewer ports, fewer leak points, and simpler wiring inside mechanical plant rooms.
A typical insertion probe has two RTD sensing elements. One element tracks the air temperature. The other element is heated. Moving air cools the heated element. The transmitter converts the cooling rate into a velocity signal. This method works well at low duct velocities from about 0.3 m/s to 20 m/s with a standard probe.
How Probe Length Is Sized
The probe should reach the center of the duct. For a round duct of 600 mm diameter, use a 300 mm insertion length plus 100 mm allowance. The total probe assembly length will be around 400 mm. For a rectangular duct with a depth of 800 mm in the insertion axis, use a 400 mm insertion length plus 100 mm allowance.
Here is the thing. Many engineers order a probe that is too short. The sensor then sits near the duct wall where the velocity profile is lower and less stable. That creates a permanent measurement offset. Silver Instruments advises centerline insertion for single probe installations.
Rectangular Duct Sizing Examples
Example one: A 500 mm x 500 mm fresh air duct in Singapore runs at 1.8 m/s average air velocity. One TMF-200 insertion probe with 250 mm insertion length is enough. The transmitter outputs 4-20 mA scaled to 0 to 5 m/s and Modbus RTU for the BMS.
Example two: A 1200 mm x 800 mm return air duct in a Manila office tower runs at 2.5 m/s. This duct needs two probes. The probes enter from opposite sides and overlap slightly. The two signals go to a Silver Instruments flow transmitter with dual input averaging. The output is one 4-20 mA signal scaled to 0 to 10 m/s.
We have seen this on customer sites many times. A facility management contractor in Vietnam ordered 12 TMF-200 meters for 600 mm x 600 mm fresh air ducts. The readings stayed within 1 percent of the reference value after two years.
Velocity Range and Temperature Compensation
Standard HVAC probes measure 0 to 20 m/s with an accuracy of plus or minus 1.5 percent of reading plus 0.3 percent of full scale. Some projects need 0 to 30 m/s for exhaust shafts or laboratory ventilation. Silver Instruments can adjust the upper range limit before shipment.
The standard temperature range for HVAC ducts is -20 to 60 degrees Celsius. For kitchen exhaust or smoke spill ducts, a high temperature probe up to 120 degrees Celsius is available. State the duct temperature when you request a quote. The transmitter needs this value for proper temperature compensation.
Outputs for BMS and Industrial Panels
Building management systems in Australia, Southeast Asia, and the Middle East often ask for Modbus RTU or BACnet MS/TP. Industrial panels may still use 4-20 mA. Silver
The relay alarms help with filter status or low duct flow alerts. For a commercial HVAC retrofit, the same probe can send 4-20 mA to the chiller plant controller and Modbus RTU to the BMS. This saves money because you install one probe instead of two.
Installation Notes for Large Ducts
Thermal dispersion probes need straight run upstream and downstream for best accuracy. A common rule is 10 duct diameters upstream and 5 duct diameters downstream. In tight mechanical plant rooms, you may only get 3 to 5 diameters upstream. In that case, use a dual probe layout or ask Silver Instruments for a flow conditioning correction.
For rectangular ducts wider than 2000 mm, do not use one very long probe. Long probes vibrate and can bend. Instead, use multiple probes mounted from different sides. Each probe monitors a section of the duct. The flow transmitter sums the airflow values by the area weighting method.
Recommended Silver Instruments Models
Silver Instruments TMF-200 is the standard insertion thermal dispersion air flow meter for commercial HVAC ducts from 150 mm to 900 mm. It has a 316L stainless steel probe, a cast aluminum transmitter housing, and 4-20 mA plus Modbus RTU. The insertion length is configurable from 100 mm to 500 mm.
For large ducts from 900 mm to 2000 mm or multi-point averaging, Silver Instruments recommends the TMF-400 with dual probe inputs. The TMF-400 accepts two or four probes. It calculates average velocity and total mass flow in kg/h or m3/h. This model also supports BACnet MS/TP for building automation networks.
Both models suit air handling units, fan coil plants, car park ventilation, and process air lines. For aggressive or wet air streams, specify the PTFE-coated RTD option.
How to Request a Quote
Send the following data to Silver Instruments. Include duct width and depth in mm, duct shape, air velocity range in m/s, air temperature, operating pressure, required output, and probe material preference. A typical customer query looks like: 800 mm x 600 mm duct, 0 to 5 m/s, 24 degrees Celsius, Modbus RTU, 316L probe. We will reply with a model, probe length, and price.
Contact Silver Automation Instruments by tel at +86-25-68650347 or by WhatsApp at +86-25-52155837. WeChat is +86 15365082610. Our sales team covers Southeast Asia, Oceania, Latin America, Africa, and the Middle East. Product data sheets are available at flow-meter.com.au.
FAQ: Insertion Air Flow Meter Sizing for HVAC Ducts
What probe length do I need for a 600 mm round duct?
Use a 300 mm insertion length for centerline position. Add 100 mm for the compression fitting and housing. The total probe assembly is around 400 mm.
When do I need two insertion probes?
Use two probes when the duct is wider than 900 mm or when straight run is short. Two probes average the velocity profile and reduce errors from swirl or asymmetry.
Can the meter send data to a BACnet network?
Yes. The Silver Instruments TMF-400 supports BACnet MS/TP. The TMF-200 supports Modbus RTU and 4-20 mA. Specify the output protocol when you order.
What is the lowest air velocity the probe can read?
A standard probe reads from about 0.3 m/s. Below this value the signal becomes small. For low velocity ducts such as displacement ventilation, tell Silver Instruments. We can configure a special low velocity range of 0 to 2 m/s.
What accuracy can I expect after installation?
Typical accuracy is plus or minus 1.5 percent of reading plus 0.3 percent of full scale under stable temperature conditions. Field calibration with a reference anemometer can improve this. We have seen repeatability better than 0.5 percent on customer sites in clean HVAC air streams.
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