Abstract: This invention resides in a wet-tap sensor assembly and related method for use with containers, such as, pressurized pipes of flowing liquids to measure the contents therein. In use, the housing is securely attached to over a hole in an external wall of the container, and a sensor cartridge of the assembly movably inserts and retracts a sensor probe relative to the contents. With the sensor probe in place, whether inserted or retracted, the assembly has means to prevent the contents from escaping. The sensor assembly includes a locking mechanism that locks the sensor cartridge in the retracted position allowing for removal of the sensor probe while preventing against inadvertent displacement of the sensor cartridge, and preferably includes a cap positioned over the sensor cartridge that prohibited removal of the sensor probe without first retracting the sensor cartridge and that disengages the locking mechanism.