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Low Pressure Air Flow Meter Systems: Sizing Averaging Pitot Tubes for Commercial HVAC Ventilation Ducts
Quick Answer: For commercial HVAC ducts between 300 mm and 1800 mm, an averaging pitot tube with a low range 4-20 mA HART differential pressure transmitter gives repeatable air flow measurement. Size the probe by duct internal width and insertion direction, not by flow rate alone. Send your duct width, duct height, air velocity in m/s, static pressure in Pa, and air temperature in degree C to Silver Automation Instruments for a same-day size confirmation.
Why Averaging Pitot Tubes Fit Low Pressure Air Flow Systems
Commercial HVAC ducts often run at static pressures from 50 Pa to 500 Pa. Many flow meters add too much pressure loss. An averaging pitot tube inserts multiple sensing ports across the duct. It averages the velocity pressure profile. The permanent pressure loss is usually below 25 Pa. That matters for fans and energy use.
In practice, most engineers skip orifice plates for duct air because the permanent loss is too high. An averaging pitot tube gives you a simple insertion solution. You drill one port, insert the probe, and connect a low range differential pressure transmitter.
We have seen this on customer sites many times. A facility manager in Singapore replaced an old vane anemometer with a duct mounted averaging pitot tube. The building management system got a stable 4-20 mA signal and the fan energy did not increase.
Inputs You Need Before Sizing
Start with the duct internal width and height in mm. For round ducts, use the DN size such as DN300, DN500, or DN800. Then define the insertion direction. For most rectangular ducts, the probe inserts across the shorter dimension. This reduces unsupported length and vibration. A duct that is 1200 mm wide and 800 mm high often uses an 800 mm insertion probe.
Next, record the minimum and maximum air velocity. HVAC supply ducts usually run between 3 m/s and 12 m/s. Exhaust ducts may run lower. Tell us the air temperature and static pressure. Air density at 22 degree C and 101.3 kPa is about 1.2 kg/m3. That value changes the velocity pressure calculation.
Also check the straight run available. Averaging pitot tubes need less straight run than orifice plates. A typical rule is 10D upstream and 4D downstream, where D is the duct equivalent diameter. In tight plant rooms we have used 5D upstream with a small correction factor.
Sizing Method Step by Step
First, calculate the velocity pressure. Use the formula 0.5 x air density x velocity squared. For air at 1.2 kg/m3 and 8 m/s, the velocity pressure is 38.4 Pa. The transmitter span should be close to the maximum velocity pressure, not ten times higher. A 0 to 60 Pa or 0 to 100 Pa transmitter works for many HVAC ducts.
Second, pick the probe insertion length. Measure the duct internal dimension along the insertion axis. Add 20 mm to 30 mm for mounting hardware and cap clearance. If the duct wall is 40 mm thick, add that to the rigid section outside the duct.
Third, check the probe K factor. Silver Instruments supplies a calibration sheet with each probe. The transmitter converts differential pressure to flow using a square root function. Most building management systems accept a 4-20 mA signal with a square root already applied at the transmitter.
Fourth, confirm the low flow cut-off. In HVAC systems, velocities below 1 m/s generate very small differential pressures. A cut-off at 0.5 m/s avoids noisy readings at night or when fans cycle off.
Example Sizing for a 1200 mm by 800 mm Supply Duct
A recent project in Southeast Asia had these conditions. Duct internal size 1200 mm wide by 800 mm high. Air velocity 7 m/s at 24 degree C. Static pressure 180 Pa. The velocity pressure was 29.4 Pa. We supplied a 800 mm insertion probe, a 0 to 50 Pa differential transmitter, and a local digital indicator.
The installed pressure loss was 18 Pa. The
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Most engineers skip this part. They focus only on the probe length. The transmitter range is just as important. A 0 to 500 Pa transmitter on a 29 Pa application will lose resolution. That gives a noisy signal at low velocity.
Installation Notes for Low Pressure Systems
Mount the probe in a straight duct section. Avoid elbows, dampers, and transitions upstream. Use a drill template to keep the probe perpendicular to the duct wall. Seal the mounting port with a compression fitting or flange.
Connect the high and low pressure ports to the transmitter. Keep impulse tubing short and sloped to avoid condensation in humid climates. In a commercial kitchen exhaust duct, use a purge air system if the air has grease or moisture. Silver Instruments can supply the manifold and valve kit for dirty air.
For outdoor air intake ducts in coastal areas, specify stainless steel probe material. Salt air corrodes carbon steel fast. We have supplied 316L stainless probes for rooftop AHU ducts in Saudi Arabia and Oman.
Transmitter and Output Selection
Use a differential pressure transmitter with a 4-20 mA HART output. For building management systems, 4-20 mA is the common option. Some sites use Modbus RTU or BACnet MS/TP via a converter. Ask us for the transmitter type that matches your controller.
Set the square root output at the transmitter. This gives a linear 4-20 mA signal proportional to air velocity or flow. If the duct cross-sectional area is fixed, the flow in m3/h equals velocity multiplied by area. The transmitter can scale the output to m3/h directly. If your site reports air flow in kg/h, we can also scale the output with density correction. A PT100 temperature sensor and a static pressure transmitter can be added for that purpose.
Model Recommendation
Silver Automation Instruments supplies averaging pitot tubes with insertion lengths from 300 mm to 3000 mm. We match each probe with a low range differential pressure transmitter. Typical spans include 0 to 25 Pa, 0 to 50 Pa, 0 to 100 Pa, and 0 to 250 Pa. Tell us your duct width, duct height, air velocity, static pressure, and air temperature. We will send a quote with a full sizing sheet.
Call Silver Automation Instruments at Tel: +86-25-68650347. Message us on WhatsApp: +86-25-52155837. Or use WeChat: +86 15365082610. Send us your static pressure in Pa or bar, temperature in degree C, pipe size in mm or DN, and flow range in m3/h or m/s.
FAQ
What duct sizes suit an averaging pitot tube?
We recommend averaging pitot tubes for duct widths from 300 mm to 1800 mm in commercial HVAC. For very small ducts below 250 mm, the blockage effect can become high. For large industrial ducts above 2000 mm, we can supply multi-probe arrays.
How do I calculate the differential pressure for low pressure HVAC air?
Use 0.5 x air density x velocity squared. At 1.2 kg/m3 and 5 m/s, the velocity pressure is 15 Pa. At 10 m/s, it is 60 Pa. Send these values to us and we will select the transmitter span for you.
Can I install the probe in a short straight run?
Yes, but accuracy drops. Try to keep 10D upstream and 4D downstream. If you only have 5D upstream, we can still size the probe but we will add a correction factor or state a wider accuracy band.
What output signal do I need for building management systems?
Most building management systems accept 4-20 mA. If your controller uses Modbus or BACnet, we can add a converter. Specify your output when you request a quote.
How much pressure loss does an averaging pitot tube add?
Permanent pressure loss is usually below 25 Pa in commercial HVAC ducts. That is much lower than an orifice plate. The exact value depends on probe diameter and duct size.
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