There are limits to the distance that the Air Purge Blower (APB) can deliver the required air to the infra-red sensor which are mainly dependant on the APB specification and duct bore size. Other factors such as ducting surface characteristic and the air temperature/pressure also have an effect.
Table A to the left indicates typical limitations for smooth bore PVC ducting with the worst case air parameters of minus 20°C at sea level pressure. (Warmer Air enables greater distance)
The table takes into account 1m flexible hose connection at Sensor, and 1m at the Blower. (2m Total)