We have a buried 1000 gal (3785 l) propane tank used for heating.
When it was filled up recently the gauge went from about 49% to 88%. This would mean 490 gal to 880 gals, or an addition of 390 gals.
We were charged for 434 gals, which is an excess of 44 gals or about 11% extra. Both gauge readings were within about 24 hours, there's no way we consumed that much propane in that short of a time.
Are we interpreting this situation correctly? Could the gauge readings have as much as 11% error in them?
Note that this is the first time we have taken photos but we think this situation has happened before. So if the gauge is error prone would that error consistently under-represent the fill level? Usually measurement error goes both ways.
The supplier has said that they take readings from their truck, and that our gauge is really measuring pressure and not volume. I'm not sure if we should trust that or not. Our gauge is in gallons after all, not PSI or something pressure-related.
Here are before and after pictures of the gauge on the tank itself:
Before fill up:
After fill up:
FYI the tank is buried but there is a lid you open to view the gauge.
Best Answer
Your gauge is certainly not reading pressure.
The only thing pressure tells you is what temperature the propane is at (LPG is "liquified propane gas" and reflects the fact that propane is easily transported in bulk because it becomes a dense liquid rather than a gas a relatively low pressure. That pressure varies with temperature.)
So, there's probably a float in the tank reading the liquid level. Such things are not uncommonly in error - if you think about the shape of a typical buried LPG tank (a horizontal cylinder), there's a complex relationship between the distance the float travels and the percentage of the tank that is full, which is being "interpreted" by a mechanical linkage.
If you have a local authority in charge of weights and measures, they (probably) check on the metering of the supplier. You could inquire if they do and (if so) when the last test was, and what it showed.