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 hoods and marine engine room ventilation systems.

Pitot Tube vs Annubar vs Orifice Plate for Air Ducts

An orifice plate creates high permanent pressure loss in a low pressure air line. A pitot tube flow sensor or Annubar flow meter creates almost no pressure drop. That matters in air ducts where fans operate at 500 Pa to 3000 Pa static pressure. Because pressure loss is low, energy use stays low. Orifice plates need long straight runs and flanges. Insertion probes install through a 25 mm to 50 mm hole. You can hot tap the duct while the fan runs. This saves shutdown time. For rectangular ducts, orifice plates are not practical. A multi-port averaging array fits rectangular ducts from 300 mm by 300 mm to 3000 mm by 4000 mm. The Annubar flow meter becomes the economical choice for large air headers and ventilation shafts.

Installation and Field Checks

Install the probe with the upstream holes facing the flow direction. Use a compression fitting, flange connection, or isolation valve. For ducts under pressure, use a retractable probe assembly. Check the impulse lines for leaks. Zero the DP transmitter with both impulse lines at equal pressure. A drift of 0.1 mbar changes the air flow reading at low velocities. In a wastewater plant in Australia, a loose impulse line caused a 15 percent error on an aeration blower air flow meter. The fix took 10 minutes in the field. Silver Instruments includes a step by step installation guide with each pitot tube flow sensor and Annubar flow meter order.

FAQ: Five Critical Questions About Pitot Tube Flow Sensors and Annubar Flow Meters

1. What is the difference between a pitot tube and an Annubar flow meter?

A pitot tube flow sensor reads one point. An Annubar flow meter averages multiple points across the duct. For ducts above DN300, the averaging type gives a more stable and representative air flow signal.

2. Can an Annubar flow meter measure air flow in a rectangular duct?

Yes. Multi-port averaging arrays work in rectangular and circular ducts. Silver Instruments builds probes matched to the width and depth of your duct. Common sizes range from 300 mm by 300 mm to 3000 mm by 4000 mm.

3. What output signal does the flow meter provide?

The standard output is 4-20 mA HART from a differential pressure transmitter. You can add Modbus RS485, pulse, or a local display. Tell us your control system input before we size the transmitter.

4. How long does installation take on an existing air duct?

In most cases, one technician installs the probe in 2 to 4 hours. You need a drilled hole, a mounting boss, and an isolation valve if the duct runs under pressure. No heavy flanges or spool pieces are required.

5. What information do I need to request a quote?

Send duct shape, duct dimensions, air pressure in Pa, air temperature in degrees C, flow range in m3/h, and required output. Contact Silver Automation Instruments at Tel: +86-25-68650347, Whatsapp: +86-25-52155837, or WeChat: +86 15365082610.

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