Bentley turbo fuel gauge

[Perkypig]

Hi all, shame about dreadful weather it makes it much less fun working on cars. I have an 89 bentley turbo and the fuel gauge always reads full, would be paradise if true but I know it's not. Any idea what the resistance range is for these senders and gauges. Is it that open circuit reads full? This I suspect as I get a full reading even when engine off and battery isolated.

Many thanks

Stuart.