.....Lastly, to the person talking about tank shapes. It does not matter what shape your tank is a sender will only ever tell you how far up the tank the liquid comes. On a boat it would be essential to know the shape of your tank so that you can tie up what the gauge is telling you compared to what is in the tank. e.g. If the tank reads 1/2 then you might only have 1/3 volume because of the shape of the tank. The only two accurate readings you can absolutely rely on (on boats like this anyway) is FULL and EMPTY.
If you look at what I wrote, you will see that is exactly what I said, the problem is that the read out shows what is in fact the level in the tank, but then displays it in a manner where it is easy to think that it refers to the volume in the tank, 1/4, 1/2, 3/4 full etc., when this is clearly not the case. A large part of the problem being that average users of the gauge only want to pay the minimum price for an instrument and are happy enough (it would seem) to misunderstand the read out. In your first post above, what you were looking for was an inexpensive instrument.
What I also said is that the level can be correlated to the volume, but that would involve carefully measuring or calculating the volume of the tank at different depths and to provide some form of computing ability for the level to be translated to volume. Inevitably that would add to the price of such an instrument.
Such instruments are around, and have been used on ships carrying liquids in bulk for a number of years. In addition to taking into account the ship shapes of each individual tank, they also take into account the volume within each tank that is occupied by the ship's internal structure. Accuracy can be to the nearest tiny fraction of a litre, but cheap they are not. The value of some of those liquids would take your breath away, so that accuracy is paramount. Years back we only had instruments that would measure the height of the "ullage" space (Height from the surface of the liquid to the calibration point at the top of the tank) and from that and with the use of "Ullage Tables" (supplied by the ship builder), we were able to calculate the volume of liquid remaining in the tank. Such tables are still in use and would be there in order for the automated gauges to be checked periodically for accuracy. The tables also contained temperature corrections, because as you know, the volume that a given amount of liquid occupies is highly dependent on temperature, and atmospheric pressure as well, but that's just an aside.
+1 to GeoffV posting no 8, that is the best solution.
+1 also to J.e.R at no 7
