Testo 425 Thermal Hot-Wire Anemometer

The Testo 425 is a handheld thermal (hot-wire) anemometer for measuring air velocity, air temperature, and volume flow inside ventilation ducts. The sensor sits at the tip of a cable-connected telescoping probe that extends to 32.3 in (820 mm) on a shaft 0.3 in (7.5 mm) in diameter, so a technician can traverse a trunk line or reach an overhead duct through a small drilled test port. The meter has its own display and connects to a phone or tablet over Bluetooth 5.0.

Low-Velocity Accuracy for Duct Traverses

The hot-wire sensor reads from 2 to 5,906 fpm (0.01 to 30 m/s) at 2 fpm (0.01 m/s) resolution. Accuracy is ±(6 fpm + 4% of reading) up to 3,937 fpm (20 m/s) and ±(98 fpm + 5% of reading) above that. The low end is where a thermal sensor earns its place: at 200 fpm, the 425's rated velocity accuracy works out to ±14 fpm, or 7% of the reading, while the Testo 410i vane probe, rated at ±(40 fpm + 2% of reading), works out to ±44 fpm, or 22%, at the same velocity. The built-in NTC sensor reads air temperature from -4 to 158 °F (-20 to 70 °C) at ±0.9 °F (±0.5 °C) with air moving at 197 to 5,906 fpm (1 to 30 m/s). Volume flow in cfm or m³/h is calculated from the velocity and the duct dimensions, with timed and multi-point averaging for traverses.

measureQuick Wireless Integration

The Testo 425 connects to measureQuick over Bluetooth, so air velocity, air temperature, and calculated volume flow go straight into the test record with no hand transcription. The measured airflow is recorded with the static pressure, temperature split, and refrigerant readings from the same test, giving the technician a measured number to check against the 350 to 450 CFM per ton range most systems are designed around.

Build and Field Readiness

The meter measures 5.3 x 2.4 x 1.1 in (135 x 60 x 28 mm), weighs 9.5 oz (268 g), and uses an ABS/PC and TPE housing rated IP40; the probe is rated IP20. Three AA batteries run it for about 35 hours. The probe connects to the meter on a 4.9 ft (1.5 m) cable. The 425 ships with a carrying bag, a calibration protocol, and batteries. The free testo Smart App for iOS and Android adds a second-screen display, graph history, and PDF or CSV reports.