An apparatus (10) for sensing a condition, such as humidity, through an opening (20) in a surface (12) such as a duct or wall. A housing (14) includes an outer section (16) mounted on the surface and a probe section (18) extending into the duct or wall. A circuit board assembly (32) is mounted within the housing and includes a user interface section (34) within the housing outer section and a sensor section (36) rigidly connected to the user interface section and extending into the housing probe section. The sensor section may be integrated into the circuit board assembly by a rigid connector (42) allowing easy removal and field replacement of the sensor section. A sealing device (56) isolates the housing outer section from the air within the duct or wall. A flexible grounding connection (66) provides electrical grounding of the circuit board assembly and a degree of mechanical support.