Test Instruments and Measurement Techniques
Choose the instrument for the range and the job
No single instrument covers every airflow measurement. A pitot-static tube is accurate and direction-sensitive, but below roughly 600 fpm the velocity pressure signal becomes smaller than typical gauge resolution, so a thermal anemometer or a capture hood is the better choice at diffusers and low-velocity ducts. Flow hoods require the manufacturer's correction factors for the specific diffuser, and hood readings are affected by backpressure and by edge leakage around the hood skirt.
Pressure instruments include inclined manometers, digital differential manometers, and micromanometers; always zero them, check the range, and confirm hose routing and polarity before taking a reading. Temperature measurement requires calibrated thermocouples, RTDs, or thermistors with adequate immersion depth and radiation shielding; averaging probes give representative duct temperatures during a traverse. Humidity instruments range from capacitive sensors to chilled-mirror devices, with quite different drift characteristics. Motor and fan checks use a tachometer or strobe for RPM and a true power meter for kW; measuring amps alone and assuming power factor is a common mistake.
Calibration records should be traceable to a national standard and current, and the calibration sticker or certificate must be available on site. Accuracy and precision are not the same: a precise reading from an uncalibrated instrument can be consistently wrong.
Rule of thumb
Measurement uncertainty propagates through calculations. Airflow uncertainty from a traverse is commonly two to three times the velocity uncertainty, so conservative point counts (16 minimum, more on large ducts) and careful probe alignment within about 10 degrees of the duct axis pay off.