4.2 Airflow Instruments: Hoods, Anemometers, and Pitot Tubes

Airflow Instruments and Method Selection

Match the method to the boundary

No single instrument is best for every air measurement. The measurement boundary, velocity range, profile, temperature, density, access, and required uncertainty determine the method. A duct traverse can establish main flow when a suitable plane exists. A capture hood can measure many terminal devices. A rotating vane or thermal anemometer can sample a face when its range and manufacturer method fit. An airflow station is useful only after its installation and signal are verified.

Before testing, identify the design quantity and ask whether the chosen method measures the whole airstream without double counting or omission.

Pitot-static tube and differential meter

A Pitot-static tube derives velocity pressure from total minus static pressure. Inspect the tube for straightness, clean openings, burrs, and damage. Connect impact pressure to the high port and static pressure to the low port, zero the meter, and keep the tube aligned as the selected procedure requires. If swirl or yaw exceeds the method's limit, improve the location or use an approved directional method rather than applying an invented correction.

Use a meter range that resolves the smallest expected velocity pressure without risk of overpressure. Convert each point's velocity pressure to velocity before averaging. Pitot traverses are weak at very low velocity pressure and in highly distorted flow.

Capture hoods

A capture hood encloses a terminal and reports volume through its calibrated base. Select a hood and skirt that fully seal around the device without blocking adjacent outlets. Center and support the hood consistently, allow the reading to stabilize, and repeat it in a second orientation when the manufacturer or field condition warrants.

The hood adds resistance and may change terminal flow. The magnitude depends on the terminal, branch pressure, hood, and flow. A backpressure-compensated hood uses its defined procedure to estimate the unhooded flow; do not call the result exact. Cross-check unusual terminals against a branch traverse or another approved method. Linear slots, very large grilles, high-induction devices, and low-pressure branches may require adapters or an alternate method.

Vane and thermal anemometers

A rotating vane averages velocity over its swept area and is useful for many grilles, filters, coils, and large openings within its range. Follow the manufacturer orientation, spacing, and sweep pattern. Bearing condition, starting threshold, turbulence, and blockage affect the result.

A thermal anemometer is sensitive at low velocity but its small sensor responds to direction, temperature, contamination, and handling. Use it only in conditions covered by its specifications. Do not impose a blanket ban on outdoor or return air; instead assess dust, moisture, temperature, and sensor protection.

For a grille or diffuser, airflow may be Q = Vavg × Ak when the manufacturer supplies an effective area for the exact model, size, blade position, and test method. Gross face area is not a substitute. For 600 FPM and Ak 3.2 sq ft, Q = 1,920 CFM.

Calibration and field checks

NEBB's current instrument requirements list measurement functions and intervals; many TAB electronic functions use a 12-month interval with specified traceability. Apply the current table to the actual instrument and function and also follow manufacturer requirements. Do not assume that every item has the same interval or that calibration must be performed by the factory.

An instrument record should include:

• instrument type, make, model, serial number, and range;
• measurement function and applicable requirement;
• calibration date, due date, and traceability record;
• pre-use inspection and zero check; and
• field comparison or damage discovered during use.

A Pitot tube's geometry is fundamental, but the technician still inspects it and follows any applicable calibration or verification requirement. A current sticker does not excuse a blocked port, damaged sensor, leaking hood, or zero drift.

Cross-checks and reconciliation

Compare main-duct flow with terminal sums only when system mode and boundaries match. Differences can result from duct leakage, omitted devices, diversity, simultaneous readings taken at different times, poor profiles, density treatment, or instrument effects. Establish a reasonable uncertainty band from the methods rather than forcing totals to agree.

When two methods disagree materially, repeat zero and setup checks, verify dimensions and effective-area factors, confirm the operating mode, and choose the method with the better defined boundary and field condition. Record both readings and the reason for the accepted result.