The HS-55 has been retired. It was a gas purge system that measured water level using non-submersible pressure transducer (purchased separately). The HS-55 output air bubbles (60 to 80 bubbles/minute) but did not use a nitrogen gas bubble supply. The system required a 12 Vdc, 38 Ahr battery, minimum (user-supplied). It did not include a datalogger or measurement electronics.