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Pitot Tube Air Flow Meter: HVAC Ventilation Duct Velocity Probes
Quick Answer: A pitot tube air flow meter measures velocity pressure in a ventilation duct and converts it to air speed and volumetric flow. Silver Instruments supplies averaging pitot tubes with 4-20 mA output that install directly into rectangular or round HVAC ducts up to DN800. Send us your duct cross section, air temperature, and typical velocity range for a sensor and price recommendation.
Why Pitot Tubes Still Dominate HVAC Air Flow Measurement
Engineers keep coming back to pitot tube probes for HVAC because the sensor itself costs little, has no moving parts, and does not block the duct like an orifice plate. In clean air applications a simple L-type pitot tube gives you a velocity reading accurate enough for building automation or filter monitoring. Many of our customers in Southeast Asia order a single averaging pitot tube and pair it with a differential pressure transmitter to track airflow in paint booth exhausts or kitchen extraction ducts.
Here is the thing about duct velocity probes. A basic single-point pitot tube measures dynamic pressure only at one spot. That spot may not represent the average air speed across a duct 1200 mm wide. A rectangular duct in a commercial building in Riyadh we worked on had a flow profile so skewed that a single-point probe under-reported airflow by 20 percent until the maintenance team switched to an averaging design.
Averaging Pitot Tube Design for Rectangular and Round Ducts
An averaging pitot tube air flow meter has multiple total pressure sensing holes along its length facing upstream and a separate static pressure port. The tube spans the full duct width or diameter. The measured differential pressure represents the average velocity pressure across that section. We see this applied on HVAC ducts from 200 mm up to 2000 mm. Silver Instruments offers averaging probes in stainless steel 316L with mounting flanges suitable for sheet metal ducts or spiral-wound round ducts.
Most engineers skip this part but probe insertion depth matters. An averaging pitot tube must sit perpendicular to the flow and the sensing holes must cover the full internal dimension. For a round duct of DN400 the probe should have a sensing length of 400 mm with holes at positions defined by equal area method as per ISO 3966. We pre-drill the holes at the factory according to the duct size you provide. If you give us the inner diameter or the width and height of a rectangular duct we ship the probe ready to mount.
Output Options and Transmitter Integration
The pitot tube itself generates a tiny differential pressure usually in the range of 0 to 500 Pa for typical HVAC velocities. You need a differential pressure transmitter to convert that to an analog signal. Silver Instruments pairs the probe with a 4-20 mA HART transmitter with ranges as low as 0-100 Pa up to 0-10 kPa. The transmitter outputs a current signal proportional to airflow velocity or volumetric flow and can also provide a totalized value via pulse output. In larger ventilation systems we have seen a paint manufacturer in Malaysia integrate the 4-20 mA signal directly into a Siemens PLC to control variable speed fans and maintain positive pressure in a cleanroom corridor.
For outdoor air intake ducts or hot exhaust we add a PT100 temperature sensor inside the probe tip. The transmitter then applies density correction and outputs mass flow in kg/h. Last year a customer in Vietnam asked us to deliver a complete pitot tube assembly for a coffee roasting exhaust duct at 180 degrees Celsius. We supplied a high-temperature version with a ceramic static pressure tube and a transmitter rated for ambient temperature up to 85 degrees Celsius in a remote-mounted enclosure.
Installation Considerations That Avoid Rework
Straight duct runs upstream and downstream of the pitot tube decide your measurement accuracy. With an averaging pitot tube the minimum s
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Leak-tight compression fittings prevent false readings. For rectangular ducts we supply a welded flange with a gasket. For round spiral ducts a saddle clamp with an O-ring seal works well. Make sure the static pressure port is clean and free of any burr because even a small burr creates a local pressure disturbance. In practice we recommend blowing the impulse lines with dry compressed air once every quarter in dust-laden ventilation.
Silver Instruments Pitot Tube Flow Meter Package
We ship the averaging pitot tube, mounting hardware, a differential pressure transmitter, and a calibration certificate. The probe is calibrated together with the transmitter at a NIST-traceable lab so the 4-20 mA signal corresponds directly to a velocity range you specify for example 0 to 20 m/s or 0 to 5000 m³/h. This plug-and-play approach saves commissioning time for HVAC contractors and facility managers in Australia, UAE, and Singapore.
Our typical lead time for a standard duct velocity probe set is 2 weeks. For large rectangular ducts above 1500 mm width we fabricate a reinforced probe with a truss support to prevent vibration fatigue. Contact us with your duct dimensions, air temperature, and required flow range. Email [email protected] or call Tel +86-135-1213-1526. You can also reach our application team on WhatsApp 86-135-12131526 or WeChat +86 13512131526. Send us your duct size, air temperature, and velocity range and we will reply with a specific model number, price, and delivery schedule.
Frequently Asked Questions
Can a pitot tube air flow meter work in dirty or greasy exhaust air
Yes but only an averaging pitot tube with a purge system or regular maintenance will keep the sensing holes open. For kitchen exhaust ducts we supply a probe with enlarged 2.5 mm diameter holes and recommend a monthly water wash. In cement dust applications we have provided an automatic back-purge solenoid controller that blasts compressed air through the impulse lines every 4 hours.
What is the minimum air velocity you can measure with a pitot tube in HVAC
With a high-sensitivity differential pressure transmitter we can resolve velocities as low as 1.5 m/s. Below that the dynamic pressure becomes less than 1 Pa and thermal dispersion probes deliver better repeatability. For typical comfort ventilation between 3 and 15 m/s the pitot tube performs reliably.
Do I need temperature and pressure compensation for HVAC airflow
For standard building ventilation no. Air density changes less than 5 percent for normal indoor temperature swings and most BMS systems accept volumetric flow without correction. For a process exhaust at elevated temperatures or high altitude a PT100 input to the transmitter applies true density correction and outputs mass flow.
How do you mount an averaging pitot tube in a rectangular duct without cutting a full opening
We provide a welded mounting flange with a gland fitting. You drill a 25 mm hole in the duct wall, weld or screw the flange, insert the probe, and tighten the gland. This method works for sheet metal thicknesses from 0.8 mm to 3 mm. For insulated ducts we add an extended neck to clear the insulation thickness.
What maintenance does a duct velocity probe need
Quarterly visual inspection of the impulse lines for condensation and cleaning of the static port with a non-metallic pick. Annual zero check of the transmitter and a blowdown of the pressure lines. Over a 5-year span we see less than 1.5 percent drift on the transmitter in clean indoor HVAC applications.
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