A-Length refers to the insertion length of a process sensor. For a traditional 12mm sensor, this dimension is measured from the base of the PG13.5 mounting threads to the tip of the electrode. Refillable sensors are measured a bit differently. The A-length is measured from the base of the refillable chamber to the tip of the electrode.
Example – An Arc pH sensor with 325mm A-Length (top), a refillable Chemotrode pH sensor with a 250mm A-Length (bottom)
The A-Length affects the installation of the sensor. If a Flexifit, Retractex or other holder is being used to install the sensor into the process then the holder will require a specific A-Length to ensure the sensor is at the proper insertion depth into the process.