Smart App is compatible with Bluetooth®-enabled measuring instruments from Testo - All Testo Smart Probes - Digital manifolds testo 550s 557s 550i and testo 550 557 - Digital refrigerant scale testo 560i - Flue gas analyzer testo 300 - Vacuum gauge testo 552 - Clamp meter testo 770-3 - Volume flow hood testo 420.