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Pitot Tube Flow Sensors and Annubar Flow Meters for Air Duct Measurement

Quick Answer: A multi-port averaging pitot tube or Annubar flow meter samples air velocity at several points across a duct. The probe sends one stable differential pressure signal to a DP transmitter with 4-20 mA HART output. Silver Instruments supplies these meters for circular and rectangular air ducts from DN100 to DN3000.

What Are Pitot Tube Flow Sensors and Annubar Flow Meters?

A pitot tube flow sensor measures fluid velocity from the difference between total pressure and static pressure. A single port pitot tube reads one point. An Annubar flow meter is a type of averaging pitot tube. It has multiple sensing holes across the duct. The holes face upstream and downstream. The upstream holes sense total pressure. The downstream holes sense a reference static pressure. The sensor averages these pressures inside the probe body. A differential pressure transmitter reads the average value. This signal goes to a flow computer or control system as a linear differential pressure reading. Silver Automation Instruments supplies Annubar flow meters with 4-20 mA HART transmitters for air ducts in many industries.

Why Multi-Port Averaging Arrays Work Better in Air Ducts

In large air ducts, velocity is not uniform. A single point pitot tube in a 1000 mm duct misses swirl and profile distortion. An averaging array with four, six, or eight ports spans the duct cross section. It captures a representative velocity profile. For rectangular ducts above 600 mm, a multi-port averaging array is the standard solution. Silver Instruments builds insertion probes with port spacing matched to the duct dimensions. This reduces the need for long straight runs. Common straight run requirements are 5 duct diameters upstream and 2 duct diameters downstream. Field accuracy after calibration is typically plus or minus 1 percent of flow rate for a properly installed Annubar flow meter. In practice, HVAC engineers use these probes on air handling units, paint booth exhausts, and combustion air ducts. We have seen this on customer sites many times. A food drying line in Thailand used a single point pitot tube for years. The readings drifted as fan loads changed. After the plant switched to a multi-port Annubar flow meter, the air flow signal stabilized within one day.

Key Specs to Send for a Quote

Send us the duct dimensions, duct shape, static pressure in Pa or mbar, air temperature in degrees C, flow range in m3/h or kg/h, and the required output. Mention if you need ATEX Zone 1 protection or a local display. A typical Silver Instruments Annubar flow meter model SI-AN-300 handles duct sizes from DN100 to DN3000. The model SI-PT-200 insertion pitot tube flow sensor suits smaller circular ducts below DN300. Both models connect to a DP transmitter with 4-20 mA HART. Contact Silver Automation Instruments for a quote. Tel: +86-25-68650347. Whatsapp: +86-25-52155837. WeChat: +86 15365082610. Send your duct size, static pressure, air temperature, and flow range. Our engineers will select the probe length, port layout, and transmitter range for your installation.

What About Dirty Air or Condensation in the Duct?

Most engineers skip this part during the first purchase. But dirty air streams cause many pitot tube failures. A paint manufacturer in Southeast Asia asked us to replace a damaged pitot tube flow sensor on a spray booth exhaust. Paint particles had blocked the single sensing hole. The new Annubar flow meter uses larger sensing ports and a purge port option. For air with dust, moisture, or oil mist, select a probe with a blowdown valve. Mount the probe horizontally with a slight slope to drain condensate. Silver Instruments supplies Annubar flow meters with stainless steel 316 wetted parts and optional Teflon coating for corrosive air streams. This works well in chemical plant fume hoo