Published on July 15, 2026 at 6:31 AM
Measuring air velocity in pipes and ventilation ducts is one of the oldest tasks in fluid mechanics. In 1732, the French engineer Henri Pitot invented the tube that bears his name to measure the velocity of water in the Seine. 293 years later, the Pitot tube is still the STANDARD method for measuring fluid velocity — from aircraft airspeed to airflow in power plant flue gas ducts to HVAC system balancing.
Why a Pitot Tube for HVAC Applications
Alternative anemometer technologies have specific drawbacks for measurements inside ducts:
Vane anemometer (impeller):
- Sensor diameter typically 20-70 mm.
- Must be positioned at the duct opening.
- Not suitable for in-situ measurement through small openings.
- Best application: outlet openings, large ducts.
Hot-wire anemometer:
- Sensor probe 8-12 mm in diameter.
- Sensitive to vibration and particle contamination.
- Best application: precision measurement at low velocity (below 5 m/s).
Pitot tube (HD350):
- Tube diameter typically 3-6 mm.
- Ideal choice for IN-SITU measurement through duct drill holes.
- No moving parts — mechanically robust.
- Best application: high velocity range (5-100 m/s), duct balancing.
Bernoulli’s Equation — the Physics Behind It
A Pitot tube does not measure velocity directly — it measures PRESSURES, from which the velocity is calculated. The physics is based on the CONSERVATION of energy in a flowing fluid.
For an incompressible fluid, Bernoulli’s equation states:
P_total = P_static + ½ρv²
Where:
- P_total = total pressure (impact pressure, measured at the front opening pointing directly into the fluid flow).
- P_static = static pressure (ambient pressure of the flow, measured at the side openings perpendicular to the flow).
- ρ = fluid density (~1.225 kg/m³ for air at standard conditions).
- v = fluid velocity.
Rearranged for v:
v = √(2 × (P_total − P_static) / ρ) = √(2 × ΔP / ρ)
So if we can measure the DIFFERENTIAL PRESSURE ΔP, the velocity can be calculated.
How the Pitot Tube Is Constructed
The Pitot tube is a thin tube-in-tube design:
- Central tube opening (impact opening): At the foremost end, aimed directly into the flow. Fluid stagnates here and produces the full dynamic pressure. Connected to one side of the differential manometer.
- Static ring openings: Small holes on the sides of the tube, approx. 10 tube diameters behind the front tip. They measure the static (ambient) pressure of the flow. Connected to the other side of the differential manometer.
- Two connection tubes: Carry the two pressures separately to the HD350 instrument.
The HD350 instrument contains a precise differential pressure sensor (piezoresistive, MEMS technology) that measures the difference between the two pressures.
Application: HVAC Duct Balancing
Standard procedure for ventilation duct balancing according to VDI 3803:
- Drill a hole through the duct wall (typically Ø3-5 mm) at a location with stable flow (at least 10 duct diameters downstream of bends or filters).
- Insert the Pitot tube so that the impact opening is positioned at the center of the duct.
- Align the tube with the flow direction (front tip pointing directly into the fluid flow).
- Wait for the instrument’s pressure to stabilize (5-10 seconds).
- Read the velocity measurement.
- Repeat at multiple points across the duct cross-section (typically 8-16 points, distributed log-linearly).
- The average velocity yields the volume flow rate (v × A, where A is the duct cross-sectional area).
The log-linear point distribution accounts for the boundary-layer slowdown at the duct wall. Without multiple points, the average velocity can deviate 5-15 % from the true value.
Simultaneous Display: Pressure + Velocity + Temperature
The HD350 displays simultaneously:
- Differential pressure (5 selectable units: Pa, mbar, mmH2O, inH2O, psi).
- Calculated air velocity (m/s, km/h, ft/min).
- Volume flow rate (with manual entry of the duct cross-sectional area).
- Temperature (from an additional sensor on the Pitot tube).
Temperature is important for accuracy — the calculated air density ρ in Bernoulli’s equation depends on temperature and air pressure. The standard value of 1.225 kg/m³ applies at 15 °C and 1013 mbar. At 25 °C, the density drops to 1.184 kg/m³, which means a 1.7 % higher velocity reading error if not compensated.
The HD350 compensates automatically when the temperature is entered.
Accuracy and Ranges
- Differential pressure range: ±0.7252 psi (±50 mbar).
- Accuracy: typically ±0.3 % of range.
- Calculated velocity: 5-100 m/s practically usable.
- Lower limit: below 5 m/s, the differential pressure becomes too small (< 0.15 mbar) for precise measurement. For slower flows, use a hot-wire anemometer.
- Upper limit: above 100 m/s, compressible effects begin, and Bernoulli’s equation can no longer be applied accurately.
Zero Function — Critical for Low Pressures
When measuring small differential pressures (typically 0.1-1 mbar for slow flow), atmospheric pressure fluctuations affect the reading dramatically. The zero function:
- Leave both tubes open to the atmosphere before measuring.
- Press the Zero button. The instrument sets the current pressure reading as the 0 point.
- Afterward, the instrument shows only the DEVIATION from this zero point.
This eliminates atmospheric baseline drift and optimizes accuracy at low differential pressures.
Max/Min/AVG Recording and Timestamps
With fluctuating flows (turbulent zones, pulsating fans), individual instantaneous values are not representative. The HD350 records:
- Maximum value since start.
- Minimum value since start.
- Average value since start (from all measurements in the interval).
- Relative timestamps for each extreme.
A 30-second measurement at a single position yields an AVG value that is considerably more reliable than a single reading.
USB Port and PC Software
Windows software for:
- Graphical display of measurements over time.
- Saving measurements with metadata (location, time, duct dimensions).
- Volume flow calculation with variable duct geometries.
- Report generation for HVAC balancing documentation.
99-Reading Memory per Mode
The HD350 stores up to 99 readings each for pressure, velocity AND volume flow. For a typical HVAC balancing project (balancing a mid-sized office building with ~50 outlets), the memory is sufficient.
Applications
HVAC balancing: Standard application for commissioning and maintenance of ventilation systems. Verification that each outlet receives the designed air volume.
Industrial extraction systems: Balancing of point extraction at welding, grinding, and chemical workstations (BGR 121).
Cleanroom verification: Laminar flow velocities in semiconductor and pharmaceutical production.
Flue gas flow measurement: Emission measurement in power plants and incineration facilities.
Wind tunnel verification: Verification of wind tunnel velocities in automotive, aerodynamics, and structural engineering research.
Building airtightness testing: Use in conjunction with blower door test systems.
Fire service smoke propagation analysis: Verification of flow direction and velocity in fire protection systems.
Agricultural and livestock barn ventilation: Verification of ventilation rates in animal housing facilities.
Train windshield testing: Aerodynamic measurements on test train cars.
Aviation testing: Wind velocity measurement on aircraft models in the wind tunnel.
Comparison with Alternatives
HD350 Pitot tube: 465 EUR. Best choice for high velocity (5-100 m/s), in-duct measurement, no moving parts.
SDL350 hot-wire (see earlier h-de articles): Best choice for low velocity (0.1-25 m/s), generous probe for open spaces.
Vane anemometer: Best value for simple outlet measurements. Not suitable for in-duct use.
Ultrasonic anemometer: For stationary weather stations. Very expensive, oversized for portable use.
Scope of Delivery
- HD350 instrument.
- Pitot tube (Ø3-4 mm typical diameter).
- Two connection tubes, 85 cm (33.5 in) each.
- 9V battery.
- Windows software.
- USB cable.
- Hard-shell carrying case.
Specifications
- Differential pressure range: ±0.7252 psi (±50 mbar).
- Accuracy: ±0.3 % of range.
- Pressure units: 5 selectable (Pa, mbar, mmH2O, inH2O, psi).
- Calculated velocity: 5-100 m/s.
- Volume flow calculation: manual with duct area.
- Temperature compensation: automatic with external sensor.
- Max/Min/AVG recording with timestamps.
- Zero function for offset correction.
- 99-reading memory per mode.
- USB port.
- Large backlit LCD display.
- Data hold and auto power-off.
What You Get for the Money
Extech HD350 Pitot tube anemometer with differential manometer, ±0.7252 psi range, 5 selectable pressure units, simultaneous display of pressure + air velocity + volume flow + temperature, zero function for low-pressure measurements, Max/Min/AVG recording with relative timestamps, 99-reading memory per mode, USB port with Windows software, complete with Pitot tube, two 85 cm connection tubes, 9V battery, and hard-shell carrying case. 465 EUR.
The standard tool for HVAC balancing engineers, industrial extraction system verification, cleanroom certification, flue gas measurement, and other applications where air velocity must be measured INSIDE a pipe or duct (not at the outlet). The higher price compared to a vane anemometer is justified by the unique ability to measure through small openings in the duct wall — which is unavoidable for most HVAC balancing tasks.
