A-Length refers to the insertion length of the sensor. This dimension is measured from the base of the PG13.5 mounting threads to the tip of the electrode. For refillable sensors 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 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.