PSM Instrumentation has received orders from the Royal Navy for two more tank-gauging systems, which means that to date more than half of the Type 42 frigate fleet will be using the company’s instrumentation. Eventually it is planned to refit all ships.

The systems measure the content of fuel oil tanks. For trim stability the ships’ fuel tanks are maintained full, achieved by replacing fuel with seawater as it is used. Several tanks are linked and a seawater header tank exerts a pressure on the whole “chain” ensuring that oil, followed by seawater, is pushed from one tank to the next. The actual fuel level in the tank is determined by differential SG principle.

A PSM sensor is installed in the base of each tank, and the shallow depth and small specific gravity differential demanded an instrument with high sensitivity. Furthermore the arduous installed conditions and shock immunity called for an extremely rugged device.

Access to tanks is not possible in service so reliability, long-term stability and routine maintenance intervals of four years minimum were design briefs. The transmitter output is fed into a PSM acquisition and display system which processes the information, based on tank tables and the grade of oil in use, to provide a volume display at repeater indicators all over the ship, each connected to a two-wire RS485 data highway.