The FT029 is a replacement part for the FT742-SM range of wind sensors. Located around the base of the sensor, this EPDM O-ring (2.62 CS x 36.17 ID) is the primary water ingress protection. This O-ring is provided with the FT742-SM to provide additional water ingress protection. In the event of damage (or general replacement), it can be replaced with care by a technician.
The O-ring sits within the groove in the base of the sensor, marked below with a yellow arrow:
Refer to the FT742-SM product manual or further information.